APA Engineered Wodd Handbooks Episode 14 pdf

The roofing system, whether pitched or low-slope, is made up of a number ofdifferent components: roof sheathing, underlayment, roofing material, roof inter-sections, flashing, and ventil

be virtually eliminated by proper design, construction, and routine maintenance.Every element of the structure must be protected from each of the hazards men-tioned above Traditional framing and construction techniques do a good job ofproviding protection for most building elements Many elements of traditional con-struction that we take for granted are there for the express purpose of providingthe permanence that we have come to expect in wood structures Roof overhangs,for example, keep rainwater and direct sunlight off the walls of the structure Theroof overhangs also minimize the amount of water that enters the ground adjacent

to basement or crawl spaces

However, designers often are unaware of the real purpose of such traditionalbuilding elements and treat them as mere architectural features Once they areperceived as an architectural feature, they may be eliminated at the whim of thedesigner In the case of the example given above, once the roof overhang is removed

to save money or alter the look of the structure, the protection it would haveprovided is also eliminated

Some of the consequences of the seemingly insignificant act of reducing orremoving the roof overhang are:

• The siding has less protection from UV rays

• The siding has greater exposure to direct rain

• The siding undergoes greater extremes of wetting and drying

• The useful life of the finish on the wood elements of the exterior is reduced.

leaks

• The basement or crawl spaces are more susceptible to water intrusion

Another example of unintended consequences resulting from a simple designchange is the decision to replace a trussed roof with a conventionally framed vaultedceiling A conventionally framed vaulted ceiling is traditionally formed by using

2⫻lumber framing supported at one end by the walls of the structure and supported

at the other by a beam at the peak The roof sheathing is attached to the top of therafters with the drywall attached to the bottom Insulation is normally placed be-tween the rafters The problem occurs with trying to provide adequate ventilationbetween the top of the insulation and the bottom of the sheathing While thesetypes of roofs have traditionally performed well, given the amounts of insulationrequired in most parts of the United States today, it is very difficult to create anair space to provide for the passage of air unless the rafters are specifically oversizedfor this application

In addition, because each space between the rafters is not connected to any otherspace, each space must be ventilated independently The use of skylights interruptsthese air passages, if any The use of ceiling penetrations allows both air and watervapor to be drawn up into the poorly ventilated spaces between the rafters Anyleaks in the roofing due to flashing errors, wind-driven rain, or the formation of icedams can result in the roof sheathing and lumber framing getting wet The lack ofventilation will keep the wood from drying out and could result in the decay of theroof framing system Thus, the designer must take special care in the design of thistype of roof to prevent these problems

Of course, any architectural feature can be successfully designed and detailedonce the principles for designing for permanence are understood In the case of thevaulted ceiling, the use of a scissors truss can impart the same effect without any

of the unintended consequences Wood I-joists could also be used, as described inChapter 5, since they can be purchased deep enough to provide room for insulationand allow sufficient space for ventilation The knockouts provided in the I-joist canalso be used to provide ventilation above the insulation and around roof penetrationssuch as skylights See Fig 5.45

There are numerous factors that impact the long-term performance or nence of wood building components, ranging from ultraviolet degradation due toexposure to the sunlight to predation of the wood structural elements by insects orfungi

perma-The next sections will cover a number of these factors in practical terms, withspecific recommendations for the designing and detailing of roof, walls, and foun-dations for permanence

12.1 FACTORS CAUSING DEGENERATION OF

WOOD STRUCTURES

There are several factors that can significantly shorten the life of a wood structure,including weathering, decay, insect attack, and excessive moisture intrusion

12.1.1 Weathering

Natural weathering of wood and finishes involves a complex process that is difficult

to duplicate in laboratory tests Without protective treatment, all wood productsexposed to the outdoor environment are subject to a number of physical changes.The most obvious, common, and benign is the change of color caused by ultravioletlight Light woods become darker, and dark woods become lighter, both converging

on the color gray Hardwoods change color more slowly than soft woods The graycolor, made up of partially degraded cellulose fiber and microorganisms, does notextend very far into the surface of the wood and if left unaltered actually protectsthe wood from further degradation The wind, rain, and wind-borne sand and debriscontinually abrade this fragile coating, allowing more degradation to occur Theerosion rate is about1⁄4in per 100 years

Unprotected outdoor exposure causes other, more damaging alterations to woodproducts that must be carefully considered when designing the structure, such assplits and checks in exposed wood products These splits and checks can causefinishes to fail, permit insects and decay organisms to get beneath preservativetreatments, and adversely impact the strength of the wood element

Environmental factors causing degradation to sidings and other exposed woodproducts, glued engineered or solid sawn, include solar radiation, heat and cold,water, normal air contaminants, and wind The importance of a particular weath-ering factor varies with the geographical location in which the product is located.For example, solar radiation is an important factor in degrading organic polymers

in finishes and is usually more severe in the southern part of the United States.Water and temperature variations can also assist in the degradation of finishes andare more likely to occur in the northern states

12.1.2 Fungi

As evidenced by buildings worldwide, wood construction can provide centuries ofservice life However, as a natural, organic material, wood is susceptible to deg-radation by organisms under certain conditions The most common organisms thatmust be taken into consideration during building design are fungi

Fungi are low forms of plant life that derive their nutrition by using other organicmaterials as food rather than producing it themselves, as green plants do For prac-tical purposes, fungi can be separated into decay fungi and nondecay fungi Decayfungi are probably the most significant organisms responsible for degradation ofwood Degradation by fungi is commonly referred to as rot, decay, brown rot, ordry rot Nondecay fungi include stains and molds

Decay Fungi. Decay fungi are spread by microscopic spores, which are produced

by the fruiting bodies of fungi Spores are always present in the atmosphere butneed proper conditions to begin their growth Under suitable conditions, the fungispores grow into thread-like hyphae that spread throughout the host material andmay ultimately produce fruiting bodies, which produce spores for further propa-gation

As with all organisms, certain threshold conditions are necessary for survivaland propagation of decay fungi These conditions can be categorized into temper-ature, food, oxygen, and moisture In most wood structures, decay can be besteliminated by controlling the moisture content of the wood

Moisture Control Decay growth in wood requires prolonged conditions where

wood moisture content is in excess of 20–25% The moisture content of woodcomponents is a function of:

• Humidity: Eventually, wood will equilibrate to approximately 6–12 percent

mois-ture content during service in most geographical locations if un-wetted by rain

or condensation The exact moisture content at equilibrium is primarily a function

of relative humidity The time it takes for wood to equilibrate is a function ofmember size and can be substantial for large sawn timbers and glued-laminatedtimbers

• Direct wetting: Exposure to direct wetting leads to elevated surface moisture

content over the short term and high moisture content throughout the entire woodmember if exposure is prolonged As an example, due to the large exposed surfacearea, wood structural panels continuously exposed to several days of rain wettingmay lead to a moisture content of 50% or more

• Condensation: Most wood components used in construction are ultimately

pro-tected from direct exposure to weather However, some components may be ject to wetting from condensation (see Section 12.1.4)

sub-• Climate: Wood products exposed to weather vary considerably in moisture

con-tent as they are always seeking equilibrium with changing humidity or undergoingmoisture cycling from wetting due to rain or snow Climate conditions affectdecay potential of wood used above ground and exposed to weather In areas ofhigh rainfall or high humidity, the moisture content may be elevated For suchcases, good design and construction practices combined with the use of preser-vative-treated or naturally durable woods will minimize risk of decay and helpassure good performance As the exterior exposure conditions and moisture con-tent are a function of climate, a decay probability map can be developed as shown

in Fig 12.1

Decay Control in Wood Construction The primary method of preventing decay

fungi in wood construction involves keeping the wood below the threshold moisturecontent needed for decay The following discussion provides an overview of properdesign, storage, construction, and maintenance details that minimize the potential

of reaching this moisture level

Floors Since floors are enclosed by the building envelope, they are generally

at low risk of decay except in circumstances where they are over a damp soil crawlspace or where plumbing leaks lead to localized wet spots Adherence to the fol-lowing provisions will help ensure good performance

• Crawl space ventilation: Model building codes require a ratio of 1 ft2of net freeventilation for every 150 ft2of floor area The ventilation requirements can bereduced to 1 ft2for every 1,500 square feet of floor area when a vapor retarderground cover is placed over exposed soil in a crawl space See Section 12.1.4under Ventilation Requirements for more information

• Distance between grade and nearest untreated wood: Codes typically require a

distance of at least 6 in between the grade and nearest untreated wood All wood

in contact with the ground or below grade should be preservative-treated

• Treated sill plates: Wood members in contact with concrete foundations should

be preservative-treated or of naturally durable species

Notes:

Lines defining areas are approximate only.

Local conditions may be more or less severe than indicated by the region classification.

Roofs Wood roof members may be exposed to moisture from leaks, from

mois-ture introduced at the time of construction, and from moismois-ture generated from densation Attention to design and construction details can significantly reduce thesemoisture hazards as noted below

con-• Attic area ventilation / vapor retarders: Model building codes typically require 1

ft2of net free ventilation for every 150 ft2 of attic area This provision can bereduced to 1 ft2for every 300 ft2when a ceiling vapor retarder is used The samereduction applies for sloped (pitched) roofs when at least 50% of the requiredvent area is located in the upper portion of the space to be ventilated and is atleast 3 ft above eave vents See Section 12.1.4 under Ventilation Requirementsmore information

• Low-slope roofs: It is often impractical to ventilate low-slope roofs since they

generally do not contain ventable attic space Experience has shown that slope roofs in commercial buildings can perform adequately even with minimalventilation These unique cases demand attention to other details to minimizeentrapment or accumulation of moisture in the roof cavity Attention to the fol-lowing provisions minimizes decay hazard in low slope roofs

low-Penetrations in flat or low-slope roofs such as skylights, roof accesses, andHVAC duct openings pose special problems because they form isolated pocketswithin the roof framing Often the installed vents do not draw from these pocketsand can result in unventilated areas where moisture can build up When usingI-joists in the roof framing, the factory-installed knockouts can be removed fromthe joists forming these isolated pockets The joists can then be installed with the

knockouts on the top and sized such that the knockouts provide a path to anadjacent vented space See Chapter 5 for more information on I-joists.

• Moisture content of wood components: The limited size of the roof cavity (located

between the roofing membrane and the lower ceiling or insulation membrane),especially in conventionally constructed roofs, increases sensitivity to entrappedmoisture introduced during construction It is therefore important to specify theuse of air- or kiln-dried lumber and to allow a period of drying after roofing andprior to installation of the insulation and ceiling if the wood structural panelswere wetted during construction

• Moisture accumulation due to condensation: Condensation in roofs is primarily

dependent upon two factors: the roof deck temperature which can cause a densation surface on the back side if it drops below the dew point, and the amount

con-of moisture vapor accumulation in the rocon-of cavity The potential for moistureaccumulation in the roof cavity depends upon entrapped moisture discussed aboveand interior humidity conditions Interior moisture sources include such things ashuman occupancy, moisture generated from building use such as manufacturingoperations, and moisture infiltration into the building In some cases, additionalventing to the outside of the building and use of vapor retarders are needed toavoid accumulated moisture that can lead to condensation (See Section 12.1.4for further information.)

• After installation: Once installed, protect wood structural panels as soon as

pos-sible with roofing felt or finish roofing material If panels were wetted prior toroofing installation, allow some time for drying prior to installing the insulationand / or vapor retarder under the roof deck Handheld moisture content meters areavailable for making a quick assessment of moisture conditions of the woodcomponents Target moisture content is 19% or lower

Walls Wood components used in walls may be subjected to moisture generated

from condensation or moisture intrusion Proper design, construction, and nance prevent these causes of moisture problems See Section 12.2.2 for moredetailed information

mainte-• Vapor retarders: Condensation may occur in the inside surface of exterior wall

sheathing in cold winter climates when moist air comes in contact with a coolersurface Such moisture can come from sources inside the house such as cooking,clothes dryers, and showers Installing exhaust fans over cooking stoves and inhigh-humidity areas such as bathrooms or laundry rooms can help vent excessmoisture to the outside Clothes dryers should also exhaust to the outside.Additionally, a vapor retarder should be installed on the warm side of the wall.Failure to protect the wall cavity from water vapor can result in condensationand elevated moisture content of the wall sheathing and studs See Section 12.1.4for more information on condensation

• Moisture intrusion: Performance problems with walls can arise when the weather

resistance of the exterior finish system degrades and allows moisture intrusion.Virtually any exterior finish system, whether it is wood siding, vinyl siding, stucco

or proprietary systems such as exterior insulated finish systems (EIFS), can loseits moisture resistance, resulting in wall moisture problems Often these moistureproblems can be attributed to installation shortcomings such as lack of buildingpaper and inadequate flashing around doors and windows Wood structural panelsheathing as well as wall framing require protection from exposure to permanentmoisture when used in wall systems

Decay Control in Exposed Applications Exposed products, such as siding, are

often fully exposed to weather and thus have increased susceptibility to elevatedmoisture conditions Although siding products will often experience moisture con-tents above the threshold value needed to support decay on an intermittent basis,wood-based siding products have a good history of performance due to the factthat they dry below this threshold value before decay can take hold Proper archi-tectural detailing, use of flashing and caulking, and adherence to the manufacturer’sinstallation recommendations are essential for proper performance For example, iftrim is improperly installed around siding, it may trap moisture and / or reduce thedrying ability of the siding This can lead to long-term moisture accumulation thatcauses decay

Exposed end grain of wood products warrants special consideration such asflashings or other means of protection, since the high capillarity of end grain in-creases water absorption Use sealants and protective flashing on the end grain ofwood members exposed to the elements

Preservative Treatment Preservative-treated wood products, which are

pres-sure-impregnated in accordance with standards of the American Wood-Preservers’Association,2should be specified for applications that involve high decay hazard.See Chapter 9 for further information

Mildew / Mold Fungi. Mildew / mold fungi, also known as nondecay fungi, are lowforms of plant life similar to decay fungi The major difference is that mildew andmold fungi do not cause the structural degradation of wood products

Mold and mildew can be found both indoors and outdoors Mold and mildew

are terms commonly used interchangeably, although mold is often applied to black,blue, green, and red fungal growths and mildew to whitish growths The presence

of high concentrations of mold and mildew in buildings is an indication of moisture conditions that may be detrimental to the structure Mold and mildew mayalso cause health problems This section provides basic information on mold andmildew and methods to minimize the high moisture conditions that can lead to high

high-levels of mold growth in structures In this section, the term mold will be used to

cover both mold and mildew unless otherwise stated

Effect of Mold and Mildew Growth on Wood Components As organic

materi-als, mold and mildew can readily grow on wood if suitable moisture is present.Mold can grow on wood if exposed to water or prolonged humidity in excess of70%

When mold or mildew occurs on wood products during construction, it should

be cleaned as specified below and then allowed to dry before being closed in.When mold or mildew occurs on surfaces of wood products in a finished struc-ture, the products should be similarly cleaned In addition, the source of excessivemoisture must be determined and rectified

Control of mold and mildew in wood structures Since mold and mildew require

high-moisture conditions, proper moisture design, construction, and maintenance ofstructures are necessary to maintain moisture levels below the threshold for moldgrowth

There are many sources of occupancy moisture that can lead to elevated interiorhumidity such that mold can grow Following is a short list of interior moistureloads that can be anticipated in a residential structure:

• Cooking dinner 1.2 pints (plus 1.6 pints if gas cooking) per family

of four

Cleaning Mold and Mildew Mold or mildew is often mistaken as dirt A simple

test for mold or mildew is to apply a few drops of 5% solution of household bleach.(It is important to use fresh bleach since bleach deteriorates in potency when olderthan six months.)

Mold or mildew will usually bleach within one to two minutes Areas that don’tbleach are probably dirt

Mold and mildew can be removed with commercial mold / mildew removers,following the manufacturer’s directions, or with a solution of one part householdbleach (5% sodium hypochlorite) mixed in three parts by volume warm water.When using bleach, avoid breathing the vapors and contact with skin and eyes.Children and pets should be kept away from these products

Floods Flooding represents an extremely high mold and mildew hazard level.

Mold will start growing within 48 hours after floodwaters recede Because of thehigh levels that may be encountered, flood-damaged structures present an extreme

risk for mold The American Red Cross Publication 4477, Repairing your Flooded Home,3and Institute of Inspection, Cleaning and Restoration Certification Standard

S500-94, Standard and Reference Guide for Professional Water Damage tion4provide guidance on dealing with flood reclamation

Restora-Health Issues with Mold and Mildew Excessive mold and mildew growth can

pose a potential health risk The health aspects of molds are beyond the scope ofthis publication The American Lung Association5 is a source of information onhealth aspects of mold

12.1.3 Termite Protection for Wood-Framed Construction

Termites occur in every state of the United States except Alaska The presence orabundance of termites is determined by their environmental requirements such astemperature, humidity, soil moisture, and availability of food Termite damage can

be controlled with proper building practices and preventative measures

Termite Species. Based on their habitat and mode of attack, termites found in theUnited States can be grouped in three classes: subterranean termites, drywood ter-mites, and dampwood termites For more information see Chapters 1 and 9

Termite Protection. Techniques for termite protection involve prevention of access

to wood or moisture required for termite existence

Job-Site Sanitation Houses built on land cleared of trees and brush are

prob-ably in the midst of subterranean termite colonies in those geographic areas wheresubterranean termites are known to exist In these areas job-site sanitation is critical.Proper job-site cleanup includes removal or burning of all debris, lumber, logs,limbs and stumps The presence of buried wood attracts termites and can lead toinfestation of the house Lumber scraps should be removed from the site prior toenclosing with the wood or concrete floor

TABLE 12.1 Minimum Clearance for Untreated Wood to Grade (based on 2000 International Building Code, 9 Section 2304.11)

Outside grade

• To framing and sheathing 8 in.

• To wood siding 6 in.

Inside grade (crawl space)

• To floor joists or sheathing 18 in.

• To floor girders 12 in.

Construction Where termites are prevalent, the best protection is to build using

techniques that prevent their gaining access to the building Foundations may beconstructed using the Permanent Wood Foundation (PWF), poured concrete or ma-sonry block with a poured concrete cap through which the termites cannot penetrate.Crawl space and attic vents must be screened to prevent access of winged termitesduring mating season

Required minimum clearances between the ground surface and any untreatedwood in the building are presented in Table 12.1 Lesser clearances are also ac-ceptable provided such wood is pressure preservative-treated

With less than 18 in of clearance under floor framing or less than 12 in underfloor girders, the shallow under-floor space is generally inaccessible for inspection

In such cases any wood that is at or below the level of the floor sheathing (includingthe floor sheathing itself, the floor framing, girders, posts, rim joists and blocking,and PWF, if used) must be pressure preservative-treated

Proper ventilation and use of vapor barriers on the ground in the crawl spacewill help prevent the moist conditions that subterranean and dampwood termitesfavor

Soil Treatment / Wood Treatment In regions where a termite hazard exists, treat

the soil outside of foundation walls, along the inside of crawl space foundationwalls, under basement floors or slabs, and at other points of ground contact withtermiticides For under-floor plenum heating / cooling systems use only termiticidesthat have been approved for plenum applications when treating soil inside the ple-num

If soil treatment is not used in termite hazard regions, preservative-treated woodshould be considered for the subfloor sheathing, floor framing, and supports Thefoundation walls and underside of the floor structure should be inspected periodi-cally for evidence of termite infestation, especially if untreated materials are used

in the floor sheathing, framing, and supports

Plywood should be treated in accordance with American Wood-Preservers’

As-sociation Standards C9 ( Plywood Preservative Treatment by Pressure Process)6and

C15 (Wood for Commercial-Residential Construction, Preservative Treatment by Pressure Processes)7or the American Wood Preservers Bureau FDN Standard (for

and quality control requirements and should be marked by an approved inspectionagency certified to inspect preservative-treated wood, indicating compliance withthese requirements All such treated wood should be dried to moisture content of19% (18% for plywood) or less after treatment to minimize subsequent shrinkage

12.1.4 Reduction of Moisture through Condensation Control

Whenever moist air comes into contact with a cooler surface, condensation is likely

to occur The cool surface may be the underside of roof sheathing or the inside ofexterior wall sheathing in winter, or the underside of a subfloor in summer whenthe building is air-conditioned

The only requirements for condensation are moist air and a cool surface In thewinter, the moisture content of the indoor air (usually measured as relative humidity

or vapor pressure) is important, as is the temperature of the surface on which thismoisture could condense The amount of moisture in the air outdoors is also some-times a factor

Condensation can be controlled in three ways: (1) reduce the amount of moistureinitially in the air; (2) prevent the moisture from reaching a cold surface by intro-ducing a vapor retarder; or (3) carry it away by ventilation

The Language of Condensation Control. Water stays in the air as vapor as long

as the temperature of the air and the amount of water vapor are such that the aircan hold it The amount of water in the air, relative to the amount that the air canhold, is called relative humidity Warm air can hold more water vapor than coldair Thus, as air with a given amount of water vapor in it is cooled, the relativehumidity will rise until a temperature known as the dew point is reached At thispoint relative humidity becomes 100%, and some of the moisture will condense asdew If moist air contacts a surface at or below its dew point temperature, conden-sation will occur on that surface

Water vapor in the air produces vapor pressure, which is a measure of moisturevapor concentration Air with high vapor pressure tries to escape to or seek equi-librium with air of lower vapor pressure The vapor can escape either with a flow

of air through cracks or openings in the building shell or without it by directpenetration of building materials if a differential vapor pressure exists across thematerial

Vapor permeance is a measure of the ease with which vapor can penetrate solidbuilding materials Materials with low permeance are rated as vapor barriers or,more properly, vapor retarders

Changes in construction due to energy-saving features have tended to increasemoisture levels within today’s homes Washers, dryers, cooking, showers, indoorsteam rooms, and swimming pools are sources of water vapor within houses Inolder houses, air infiltration around doors and windows, and often directly throughcracks in the walls, more or less automatically eliminated condensation With thetighter, energy-efficient houses being built today, control of condensation must beplanned

Condensation Control. The first step in the control of condensation involves ducing excess moisture inside the home

re-• Vent clothes dryers to the outside and not into the attic or crawl space

• Install range hoods over cooking stoves and operate them when any appreciableamount of steam is being generated

• Install exhaust fans in bathrooms and vent them to the outside, not to the attic(consider wiring the fan so that it goes on automatically with the bathroom light)

Methods of moisture control vary with location in the house For attics andenclosed cathedral ceilings, the simplest form of control involves ventilation Aceiling vapor retarder is recommended in conjunction with ventilation for cathedralceilings With today’s ever-increasing amounts of insulation and tighter construc-tion, a ceiling vapor retarder may not be as necessary for attics when adequateventilation is provided Its omission would allow vapor to travel more easily throughthe ceiling and out through the attic vents It is important, however, to seal or avoidpenetrations for electrical ceiling fixtures, which can allow mass movement of moistair into the roof cavity or attic.

For walls, ventilation is impractical, and condensation control will generally takethe form of vapor retarders Vapor retarders in walls and at other locations shouldalways be on or nearest the winter warm side in order to block vapor before itreaches a portion of the construction with a temperature below the dew point (Inhot, humid climates, a wall vapor retarder is sometimes omitted or even placed onthe exterior wall Check local practice in these areas.) If vapor is allowed to pen-etrate a wall and temperature reaches the dew point within the wall, the vapor maycondense and cause trouble

Wood floors are seldom so cool as to cause surface condensation of vapor fromwithin the house Structural panel floors bonded with exterior adhesives have suf-ficiently low vapor permeance (1 perm or less) to prevent excessive indoor moisturefrom escaping into the crawl space when penetrations or openings are adequatelysealed This is particularly important when insulation is applied to the under-floorarea

Use a vapor-retarder ground cover to prevent introduction of moisture from theground beneath the house to the crawl space or interior This is easy in crawl-spacehouses, where a layer of 6-mil polyethylene over the ground in the crawl space isusually all that is required It is more difficult in basement houses, where vaporretarders should be installed under basement floors and outside foundation walls.Condensation in the crawl space is unlikely in winter when a ground cover isused and adequate drainage is provided around the foundation to prevent moistureaccumulation Thus, foundation vents may be closed during the winter for energysavings Closure of vents in winter for energy saving is particularly effective whenfoundation walls, rather than the underside of floors, are insulated This technique

is also more effective than floor insulation for preventing summer condensation,particularly when the building is air-conditioned

In modern basement houses, ventilation is usually inherent with forced-air ing systems Ventilation and air movement should be given separate considerationwhen heating systems are used that do not provide air circulation, such as baseboardheaters

heat-Ventilation Requirements. Minimum ventilation requirements are usually covered

in building codes The requirements in Table 12.2 are based on the 2000 tional Residential Code (IRC)1and may be used as a guide for residential construc-tion

Interna-The required net free area of vents can be found by multiplying the value in thethird column of the table by the appropriate floor or attic area of the building Notethat these are minimum code requirements, which have been found to be adequateunder most normal residential circumstances However, ventilation in excess ofthese minimums may be necessary when unplanned moisture is introduced by vent-ing an appliance, such as a dryer, into the space (which is not recommended), or

by misdirected surface or rainwater Care should also be taken to provide adequate

TABLE 12.2 Minimum Ventilation Requirements—2000 International Residential Code

Location Construction

Natural ventilation net free area opening

as proportion of floor

or attic area Attic and structural spaces No vapor retarder

Vapor retarder in ceiling

At least 50% and not more than 80%

of required vent area in upper portion of space to be ventilated at least 3 ft above eave or cornice vents

L / 150

L / 300

L / 300

Crawl space No vapor retarder

Vapor-retarder / ground cover and one vent within 3 ft of each corner

L / 150

L / 1500

extra attic vent area when moisture-laden air is introduced to the attic by house fans In such cases, attic vent area should be increased in accordance withmanufacturer’s recommendations Attic ventilation strategy should also considerlocation of vents to minimize dead air spaces

whole-Ventilation in excess of minimums may be necessary in high-occupancy tures or in structures that contain moisture-producing activities, such as commercialkitchens or laundry facilities It is traditionally the responsibility of the buildingdesign professional to determine the amount and location of ventilation to ensuresatisfactory performance

struc-Ventilation Checklist It is sometimes necessary to inspect an existing building

for adequate ventilation where there are signs of unusual moisture When checkingfor ventilation, be sure to note the following information:

1 Area of floor and attic to be ventilated

2 Presence of ground cover and ceiling vapor retarder

3 Signs of moisture accumulation, including decay, water stains, blistered paint,

water standing in crawl space, rusty fasteners, or mold growth

4 Quantity, size, type, location, and condition of roof and foundation vents

Mea-sure vents to be certain of size and check vent manufacturer’s data for their netfree area

The free ventilation area published by vent manufacturers varies slightly, soany calculations regarding vent area provided would be approximate In somecases, the net free area may be marked on the vent When manufacturer’s dataare not available, Table 12.3 may be used to estimate net free vent area

A vent may actually have zero free area and thus may be ineffective, eitherpermanently or intermittently Examples include closed foundation vents, cov-ered roof ventilators, inoperative power vents, and eave vents that are clogged

or blocked by insulation or paint

5 Compare actual ventilation with minimum requirements, as shown in the

follow-ing example

TABLE 12.3 Net Free Area Guidelines for Vents and Screens

Ventilator type Area (in 2 ) Net free area (in 2 )

Roof Screened jacks, button caps

Vent pipe area where

Screens

1 ⁄ 16 in mesh Height ⫻ width Area ⫻ 0.5

1 ⁄ 8 in mesh Height ⫻ width Area ⫻ 0.8

1 ⁄ 16 in mesh with louvers Height ⫻ width Area ⫻ 0.33

1 ⁄ 8 in mesh with louvers Height ⫻ width Area ⫻ 0.44

Example A 30 ⫻ 45 ft house has a vapor-retarder ground cover in the crawl space There are four louver-type foundation vents (46 in 2 free area per vent) Triangular gable vents (155 in 2 free area per vent) are at the top of the gables at each end of the house for natural attic ventilation Does this meet the code minimum ventilation cri- teria?

1 Determine Minimum Required Ventilation Area

Ratio for attic with at least 50% of vent area in upper half of space to be vented

is 1 / 300 Note, however, that in this case 100% of vent area is in the upper half

of the attic, reducing effectiveness of the ventilation and requiring that a ratio of

Ratio for crawl space with vapor-retarder ground cover is 1 / 1500.

Crawl space vent area required ⫽ floor area times ratio

2

⫽ 310 in (free area of attic vents is less than that required)

2 Crawl space vent area ⫽ 46 in /vent ⫻ 4 vents

2

⫽ 184 in (free area of crawl space vents

is more than that required)

3 Conclusion

Crawl space vents meet minimum ventilation requirements, but attic vents are less than 1 ⁄ 4 of the required area Additional attic venting is required, and could be accomplished by using larger gable vents or adding vents along eaves Eave vents are recommended.

12.2 BUILDING DESIGN AND DETAILING FOR

PERMANENCE

In Section 12.1 the factors leading to the premature degradation of wood structureswere discussed The long-term performance of the entire structure is predicatedupon the control of these degrading agents through the proper design and detailing

of the entire building envelope

The building envelope provides a number of different environments in whichthe inhabitants, contents, and structural and nonstructural components exist Whilethe primary purpose of a structure is to provide an appropriate environment for theinhabitants and contents of the structure, a secondary but equally important purpose

is to protect the structure itself from environments that will cause its deterioration.This section will concentrate on providing a building envelope suitable for pro-tecting the building components This is done with the knowledge that providing asuitable environment for the structural elements results in an excellent starting pointfor the protection of a structure’s contents and inhabitants Both must be success-fully accomplished

For purposes of this chapter, the building envelope has been divided into 3components; the roof, walls, and the foundation These three components essentiallydefine the boundaries of the building envelope and are discussed below

12.2.1 The Roof System

One of the most fundamental components of the building envelope is the roofsystem This part of the building envelope provides essentially the first line ofdefense against the greatest source of moisture loading on the building: rainfall Inaddition, properly sized roof overhangs protect the siding of the structure from allbut wind-driven rain and, when equipped with gutters properly designed to dis-charge rainfall away from the foundation of the building, also alleviate many ofthe subsurface moisture problems often faced by building owners While properlydesigned roofs and roof overhangs can also contribute significantly to the longevity

of the exterior wall finishing system and provide a positive impact on energy ings by reducing solar load, the emphasis of this section will be on the prevention

sav-of damage due to water infiltration

Roof Systems—Low-Slope and Pitched Roofs. As far as waterproofing is cerned, roofing systems fall easily into two categories; the first is the near-flat orlow-slope roof, and the second is the pitched roof Each of the roof types uses

con-different waterproofing methods to keep water away from the interior Pitched roofsystems rely on the force of gravity to direct the flow of water downward andoutward These systems rely on a series of overlapping elements—roofing felts,shingles, tiles, and flashing details—to provide the system to accomplish this re-direction of rainfall The pitch of the roof provides the gravity and the detailing ofthe roofing system provides the redirection.

When the size or style of the building dictates the use of a near-flat or low-sloperoof, such as for industrial or light commercial buildings, a different technology isused to provide waterproofing In low-slope roofing systems water is kept outside

of the building envelope by providing a waterproof barrier over the entire roofsystem and around every penetration in that roof system In this case, the force ofgravity actually works against the waterproofing system Instead of providing theredirecting force to channel the water away from the inside of the building envelope,the force of gravity provides the motive force to drive the water into every imper-fection in the roof waterproofing system Because the roof is near flat, or of verylittle slope, the chances for standing water are very high Moderately high windscan force water to pile up in areas that would normally drain adequately Oncestanding water is present, even minor defects can cause major water leaks

Roof Slopes In a pitched roof the force of gravity is large enough to overcome

both the force of wind-driven rain and the surface friction of the roofing material

to permit the rainwater to be directed away from the interior of the building envelop

As the roof pitch gets steeper, the impact of gravity acting over the roof increasesand it becomes easier to weatherproof / waterproof the roof On the other hand, asthe roof slope decreases, the effectiveness of such a system also decreases There

is no ‘‘magic slope’’ where conventional steep-slope waterproofing becomes fective and near-flat or low-slope roof waterproofing must be used This point isimpacted by the roof slope, the wind speed, the surface friction of the roofingmaterial, and other, more esoteric factors, such as the shape of adjacent roof sur-faces and the general topography around the building and the direction of the pre-vailing wind

inef-The roofing industry, however, has decided on a slope of greater than 3 in in

12 in (3:12) for the cutoff between a steeply pitched roof and a low-slope roofsystem This is an arbitrary point, and a designer or builder may be well served tocontact the local building department for guidance when dealing with a specificroof at or near this cutoff point The local building official will know whether localconditions demand special considerations

Low-Slope Roofs Roof slope equal to or less than 3:12 is a low-slope roof.

Outside of the fact that it is almost impossible to construct a truly flat roof, there

is a valid structural reason for not designing one This is to prevent a phenomenon

known as ponding, which may occur when water collects in a depression on a flat

roof As water builds up in the depression, the load on the supporting membersincreases This causes additional deflection, which permits more water to accu-mulate Once initiated, this cycle can continue until a roof failure occurs In fact,

an initial depression is not even necessary to initiate ponding Wind can cause water

to build up in one area of the roof and cause the initial deflection While meltingsnow is another potential culprit, ponding is more prevalent in areas where roofsare designed for low live-to-dead-load ratios Roofs designed for snow loads aretypically stiffer and have greater ability to resist ponding than similar roofs designedonly for relatively light construction loads

Roof framing should be designed for drainage by the application of a slightslope leading to roof drains installed in the lowest portions of the roof The mini-

mum slope recommended is 1⁄4 in in 12 in (1⁄4:12), or about a 2% slope Thisshould be treated as a minimum only It is always good practice to provide as muchslope as can be allowed by the geometry of the building.

Truly flat structural framing should be considered only when some other means

of guaranteeing roof drainage is provided An example of such a system is the use

of foam blocks that are placed over the top of the flat roof deck to provide insulationand, due to their taper, provide for drainage These blocks also provide a foundationfor the roofing system One problem associated with such systems is that duringremodeling at a later date the whole insulation system may be removed and replacedwith an unsloped system

The Roofing System. The proper detailing and installation of each of the elementsdiscussed below is essential to the construction of a sound watertight roofing sys-tem Rather than listing a number of rules and regulations for the installation ofeach component that must be memorized, this section will discuss each of theelements and their interrelationship with the other elements and provide illustra-tions, with the hope that an understanding of the system will make the requireddetails obvious to the reader Because specific information may be required in cer-tain applications, this section provides a list of informational sources for the variousroofing-material types

The roofing system, whether pitched or low-slope, is made up of a number ofdifferent components: roof sheathing, underlayment, roofing material, roof inter-sections, flashing, and ventilation details Each of these components must be cor-rectly installed, in the proper order, and with the proper relationship to each other

in order to make the system work as planned These components are discussedbelow and illustrated in Fig 12.2

Roof Sheathing Roof sheathing forms the structural base or foundation for the

roof system It attaches to the roof framing and provides the nail base for the othercomponents of the roofing system In addition to supporting the roofing system, theroof sheathing is an important part of the building’s structural frame, transferringwater, snow, wind, earthquake, equipment, and construction loads into the structuralframe below This is true for low-slope roofs as well as pitched roofs The mostcommon types of roof sheathing used in residential and wood-framed light com-mercial structures are wood structural panels such as APA Rated Sheathing andStructural I Rated Sheathing

A number of recent hurricanes have emphasized to the design, building, andcode-enforcement communities the importance of designing the roof sheathing forall of the potential loads that may occur In hurricane country, for example, the roofuplift loads can be far greater than the downward-acting traditional roof designloads, and the proper nailing of the roof sheathing is essential to resist the resultingwithdrawal loads for the satisfactory performance of the roof system as well as thestructural performance of the building itself

Since the roof sheathing is installed with small gaps around the perimeter ofeach panel (to facilitate panel expansion), the sheathing itself will provide protectiononly against gross amounts of rainwater If uncovered during a storm, a seriouslevel of leakage can be anticipated Waterproofing is not its job Its purpose, as far

as this discussion is concerned, is to provide the foundation for the rest of the roofsystem

Installation Requirements Chapter 2 gives the minimum nailing

recommen-dations for attaching wood structural panel sheathing for a roof deck In areas ofhigh wind or seismic activity, additional nailing may be required Panels should beinstalled in accordance with the recommendations presented in Chapter 2 Chapter

FIGURE 12.2 Roof system components.

8 also provides information on mechanical fasteners used to attach the sheathing

to the framing

Roofing Underlayment Roofing underlayment, often consisting of building

pa-per or felt, is really the first weatherproofing layer for a pitched roof Propa-perlyinstalled underlayment is placed from the bottom of the pitched roof to the top,such that the upper layer overlaps the lower layer The underlayment provides apath for any water that leaks through the roofing materials along the top of thepaper to the edge of the roof while protecting the wood-based roof sheathing fromthis leakage While wood structural panel roof sheathing is made with waterproofadhesive and has some ability to absorb and dissipate small amounts of water, itmust be protected from prolonged exposure to high moisture contents in order toprevent the start and propagation of decay organisms

In high-wind areas where the roofing materials can be blown off during a storm,the underlayment is often attached to the roof sheathing to prevent it from beingblown off along with the roofing material Small circles—about 2 in in diameter—

of thin sheet metal called ‘‘tin tabs’’ are often used in high-wind areas to securethe underlayment in case the roofing material is lost to the storm The underlayment

is nailed or stapled down through the tin tabs to increase the pull-off resistance ofthe paper

While more common because of first cost, such mechanical devices are not aseffective as a fully adhered system where the underlayment is overlapped and ad-hered to the roof with a waterproof adhesive system The advantage of a fully

Metal perimeter

drip edge flashing

applied over felt

Proper felt underlayment

installation shown with three-tab,

square-butt strip shingles

Underlayment

Ridge

Starter course Flashing

2" min lap

4" min end lap

(a)

FIGURE 12.3 (a) Typical single-layer underlayment installation for steep-slope roofs.

adhered system is that it can easily resist tear-off by not allowing wind pressure tobuild up on its backside

On low-slope roofs, the underlayment, if used, can form a number of differentfunctions, depending on the type of roofing applied over it Unless used as a part

of the roofing material, almost none of these functions are water protection-related

In some systems the underlayment is attached mechanically to the roof sheathingand the roofing material is adhered to the underlayment In this case it has themechanical function of holding down the roofing material If a leak forms in theroofing, the underlayment provides minimal leak protection because of the fastenerpenetrations

In those roofing systems where the first layer is adhered to the roof deck, thedistinction between the underlayment and the roofing material is blurred In someroof systems the underlayment provides a slip surface between the roof sheathingand the roofing to enhance the performance of the roofing under thermal loads.There are single-ply, ballasted roof systems where no underlayment is used

Installation Requirements For sloped roofs underlayments can be installed in a

number of different ways, depending on the roofing material used See Fig 12.3a–cfor examples of proper underlayment installation Note that the underlayment is

18" No 30 Asphalt-saturated felt (shake felt) Space neighboring shakes

Starter course – wood shake

or wood shingle (length

depends upon exposure

specified for the roof)

Ice dam protection membrane or No 30 asphalt-saturated felt underlayment

1 1 / 2 " min overhang at downslope edge or reduce to 1"

with gutter Recommended: metal perimeter drip-edge flashing applied over felt along rakes

(c)

FIGURE 12.3 (b) Underlayment centered in valley (c) Wood shake application (Continued )

always installed in such a way as to channel the water out and down, away fromthe wood structural panel sheathing below.

Roofing Materials The roofing material is the material that can be seen on the

finished roof The purpose of the roofing material, or roofing, is to provide theprimary waterproof barrier for the structure Because of the hostile nature of theroof surface, subjected to extremes of heat and cold, rain, snow, hail, flying debris,ultraviolet light, and foot traffic of maintenance personnel, the roofing material musthave additional durability-related properties in addition to those required for keep-ing water out of the structure’s envelope

For pitched roofs, almost all roofing materials rely on some form of shingling

to provide the weatherproof barrier Like the underlayment, these roofs are installedfrom the bottom up, with successive layers overlapping, both vertically and hori-zontally The roofing material most commonly used for pitched roofs are asphaltshingles, but other materials include slate, clay and concrete tiles, wood shinglesand shakes, and metal shingles The exceptions alluded to above are standing-seamand corrugated metal roofs These roofs are often made up of single-piece elementsthat are full-length from the ridge to the overhang Adjacent panels are connected

to one another with a folded standing seam (the seam is elevated above the surface

of the roof) or by overlapping adjacent corrugations

Low-slope roofs utilize a staggering number of proprietary and nonproprietaryroof systems, ranging from single- to multiple-ply; adhered, mechanically anchored,

or ballasted; hot-mopped or cold-applied (solvent-, urethane-, or epoxy-based) tems; rolled on or poured on; vented or unvented: and any combination thereof

pages on the proper installation of such roofing types, including over 250 pages offlashing details for this vast array of roofing systems

Roof Intersections The majority of roof leaks occur in locations where the

plane of the roof is interrupted by a ridge, another roof intersecting at an angle, awall, or a penetration Even the simplest of rooflines have dozens of potential leaksites due to chimneys, skylights, ridges and valleys, utility vent stacks, kitchen andbathroom ventilation fans, and code-required roof ventilation penetrations Otherthan those caused by natural disasters, it is the improper execution of the requireddetailing around these discontinuities that is the cause of most roof leaks Con-versely, the proper execution of these details is a very important part of the roofsystem

Proper Detailing of Roof Intersections Figures 12.4a–d illustrate examples of

proper roof ridge detailing for asphalt, slate, tile, and wood shake roofing systems,respectively

Figures 12.5a–e illustrate typical valley intersection details Figure 12.5a shows

an example of an open metal valley flashing suitable for all roofing types, and Fig.12.5b shows closed mitered valley flashing for utilization with flat roofing materialssuch as slate or flat tile A closed mitered valley should not be used with a woodroof if leaves or needles can be an impediment to rapid water runoff Figures 12.5cand d are examples of common valley details for asphalt shingles Valley intersec-tion details for corrugated metal roofing are shown in Fig 12.5e

Figures 12.6a–d deal with hip roof intersections Figure 12.6a is provided toillustrate a typical example for flat roof products such as slate or low profile tile.Fig 12.6b shows common hip details for asphalt shingles, and Fig 12.6c illustrates

a roof hip made with high-profile tile Figure 12.6e is an example of a hip ridgedetail designed for corrugated metal roofing

Direction of prevailing wind

First course placed over starter course

Trim off 3" of starter cours

5" 1"

(a)

Copper ridge nails,

or as specified (longer than field nails)

Inject and tool in polyurethane sealant, vertical grade asphalt roof cement, or slater’s cement

in joints

Combing slate

Wood lath

Copper ridge flashing

Field roofing slate

(b)

FIGURE 12.4 (a) Shingle roof ridge details (starter course detailed as shown) (b)

Slate roof ridge details.

Ridge closure (NRCA* suggests mortar for pan and cover tile

systems)

* National Roofing Contractors Association

Pan and cover field tile

Nails or fasteners

as specified

Ridge nailer (preservative treated, cedar,

or other type of decay resistant suggested)

Min two plies No 30 saturated or one ply No 40 asphalt-saturated and coated underlayment

asphalt-Roof deck Course to course

overlap not less than 3"

Ridge nails (longer than nails used in field, if ridge board nailer

is omitted) Wrap nailer with underlayment felt

(c)

If not a vented ridge, the underlayment or interlayment may wrap the ridge for added weather protection

Wood ridge boards Field courses of wood roofing Recommended: asphalt saturated felt ridge covering

(d)

FIGURE 12.4 (c) Pan and cover tile roof ridge detail (b) Wood ridge detail for use with shake or shingle-type roof (Continued )

Bend clip over nail heads.

Lap underlayment 12" min in valley

Valley metal formed from approx 24" wide metal, min

4" extension under tile

Note: Field underlayment not shown for clarity

(a)

FIGURE 12.5 (a) Typical metal open valley flashing.

Figure 12.7 provides the most common detail used for eaves and rakes—the use

of drip-edge material

Flashing Details Flashing is made up of thin sheets of corrosion-resistant

ma-terial used in conjunction with the other elements of the roof system to preventleaks around roof intersections and penetrations discussed above Flashing is nor-mally made up of galvanized steel, copper aluminum, lead, or vinyl Often smallroof penetrations such as vent stacks utilize flanged rubber boots in lieu of moreconventional flashing because of the circular shape of the penetration

In a pitched roof, regardless of the application or the type of flashing used, theprinciple used is always the same The purpose of the flashing is to direct the flow

of the water that leaks into the intersection down and away from the interior of thestructure to the topside of the roofing material In every case shown, the top edge

of the flashing passes underneath the underlayment, the upper pieces of flashingpass over the lower pieces, and the lower edge of the flashing passes over the top

of the roofing material In such a manner, the flashing never directs the flow ofwater to the bottom side of the roofing underlayment, never putting it in contactwith the wood structural panel sheathing

beyond both sides of valley centerline

Individual soft metal valley flashing – (extend 2") upslope from tile to

be overlaid, extend downslope 1 /2" short

of overlying tile)

Note: Field underlayment not shown for clarity

(b)

FIGURE 12.5 (b) Closed mitered valley with interwoven metal valley

flashing—shown with flat tile roofing (Continued )

Proper Flashing Installation Details Some typical flashing details are covered

in Figs 12.8–12.11

Fig 12.8a is an illustration of a common vent stack penetration utilizing a formed rubber of soft metal flashing designed specifically for that use While theillustration shows a high-profile tile being used, the general procedure for properlyinstalling the vent pipe flashing is the same for all roofing material types Fig 12.8bshows a similar stack penetration in a corrugated metal roof

pre-A series of illustrations is presented in Fig 12.9 showing the steps necessary toflash around a masonry chimney Many of the steps shown are common to otherapplications in steeply pitched roof applications

Figures 12.10a and 12.10b are provided to illustrate the flashing details around

a skylight or other similar applications An example of a typical installation forlow-profile roofing such as flat tile, slate, asphalt shingles, or wood shingles isprovided, as well as one showing such an installation with high-profile clay tile.The flashing required in intersections between roofing elements and verticalwalls immediately adjacent are shown in Figs 12.11a–c

Ventilation The purpose of each component of the roof system discussed so

far has been to keep water from penetrating the building envelope It can be ipated that at some time during the life of the roof system some leakage will occur,whether by deterioration of the roofing material, wind-driven rain, ice dams, orother causes A little leakage onto the wood structural panel sheathing in mostcases can be tolerated because of the code-required ventilation underneath the roofsheathing

18" wide strip –

Asphalt roof cement (vertical grade) Corner of shingle trimmed

Ridge

Extend a full shingle at least

12" beyond center of valley

Keep nails 6" min

from valley center Extra nail in end of shingle Ridge

Note: Field underlayment not shown for clarity (d)

FIGURE 12.5 (c) Use of rolled roofing material for open valley

con-struction (d ) Woven valley (Continued )

Roof felt

Metal roof

panel

Roof panel cut ends sealed with plugs cut from a closure strip and

sealed top and bottom with Butyl tape All nails used to attach panels

must be placed up-slope of sealed cut ends.

Valley flashing

Butyl tape

Closure plugs placed square

to panel ribs Fasteners

Drip edge Fasteners

4" min overlap 3" to 4" exposure

Soft metal hip flashing

between each course

Recommended:

Vertical grade asphalt

cement, slater’s cement

or polyurethane sealant along

centerline

Ridge along hip

Double wrap hip with underlayment Recommended: Vertical grade asphalt cement, or slater’s cement

Mitred field (hip) slates Field slate Roof deck

5" exposure Start here

FIGURE 12.6 (b) Asphalt roof hip detail (Continued )

The required ventilation serves two purposes One is to provide a passage forairflow over the lower surface of the sheathing to assist in the drying out of thepanel, which may have gotten wet from leakage of the roofing system or fromcondensation As mentioned before, wood structural panels are manufactured with

a fully waterproof adhesive and can tolerate the kind of wetting and drying ciated with very minor roof leaks The problems occur when the leakage is greatenough that the required ventilation is insufficient to dry out the roof sheathingbetween wettings In such cases, wood products are susceptible to mold and decay.The second benefit of this ventilation air is to lower the temperature of the roofdeck in the summer High roof deck temperatures have been found to adverselyimpact the useful life of some types of fiberglass asphalt shingles The flow of air

asso-Min two plies No 30

asphalt-saturated or one

ply No 40

asphalt-saturated and coated

underlayment

Roof deck Pressure

Pressure treated or cedar wood

nailer (Recommended: Cover with

felt prior to installing hip tiles)

Mortar ridge tile

(c)

FIGURE 12.6 (c) Hip detail for clay or concrete tile (Continued )

on the underside of the sheathing can reduce the temperature of the shingles by20–30⬚

Figure 12.12 is an illustration of roof ventilation in a sloped roof

Special Considerations

Ice Dams. Ice dams are caused when natural heat losses through the roof causethe snow to melt The meltwater flows downward until it hits the roof overhangand then refreezes because this area of the roof is at ambient winter temperature.Given the right set of circumstances, this layer of ice in the roofing material canget thicker and thicker Eventually the meltwater will pond up behind the ice dam

Metal “J”-trim Ridge cap

FIGURE 12.6 (d ) Sealing the hip of a metal roof (Continued )

and start backing up the roof slope As it does this it moves up under the shinglesand, if improperly applied, even the underlayment This can saturate the woodstructural panel sheathing and cause leaks into the habitable space Figure 12.13ashows an example of proper detailing to mitigate the ice dam problem through theuse of a double layer of roofing felt underlayment and, Fig 12.13b shows the use

of self-adhering underlayment for the same application Resistance heaters are stalled in some locations where the problem is especially severe

in-High-Wind Considerations While high-wind considerations were mentioned

previously, some elaboration is warranted to help ensure good performance of theroof system A number of special factors must be considered in designing the roofsystem of a structure in a high-wind area:

de-signing a low-slope roof As previously discussed, high winds can cause water

to build up in areas of a low-slope roof that would normally have adequatedrainage Designing low-slope roofs with a slope greater than the minimum 2%

is recommended

pitched roofs near the minimum slope of 3:12 The minimum slope for a pitchedroof is based on the ability of the slope to channel water down and away fromthe interior of the structure One of the forces that must be opposed by the gravityforces on the water is the wind force On the windward side of the roof, the windtries to force the water up under the shingles This is opposed by the gravityforce on the water due to the slope of the roof As the slope increases, the roofbecomes more resistant to wind-driven rain Avoid roof slopes near the minimumfor a pitched roof in high-wind areas

• Proper selection of roofing material and underlayment Asphalt shingles, in eral, are designed for 65 mph winds If asphalt shingles are used in high-windareas, their loss during a windstorm must be anticipated As such, to preventdamage to the structure and its contents when asphalt shingles are used, theunderlayment must be selected and attached to provide the requisite weather-

drip-edge flashing over

underlayment along rakes

Trim and bend

in to form closure

at corner

Extended perimeter drip-edge flashing below underlayment along downslope perimeter edges

Field felt overlaps flange

Raised edge perimeter flashing

Roof

deck

Tile Roof Option A

Field felt overlaps flange

Raised edge perimeter flashing

Roof deck

Tile Roof Option B

Strip of

No 30 felt

on deck

Raised edge perimeter flashing

Roof deck

Field felt overlaps flange

FIGURE 12.7 Extended drip-edge metal flashing at eaves and rakes for roll roofing and shingles Options A and B are shown for tile roof.

STEP 1

STEP 2

Soil pipe through roof

Underlayment (laps over top of primary flashing flange)

Primary flashing flange Min 6" flange all sides

Metal flashing extends upslope

so a 3" headlap is achieved

Soldered flashing sleeve (copper

or lead)

Primary flashing sleeve below

FIGURE 12.8 Two-stage flashing for sealing plumbing vent stack with pan and cover tile roof.

Wood cricket built on upslope side of chimney (Recommended if chimney is 24" or wider, or roof slope is 6:12 or greater, or ice or snow accumulation is probable.)

Deck

(a)

Asphalt plastic cement

Coat of masonry

primer Underlayment

Apron flashing for downslope portion of masonry chimney

Underlayment shown pulled away from chimney.

Apron flashing applied over shingles and set in asphalt plastic cement Width of chimney 10"

Nail flashing

to deck

Step flashing both sides of chimney

Interlace step flashing with shingles Set step flashing in asphalt

Nail flashing

to deck Cricket

Extend step flashing up chimney and around corner Nail corner

flashing to deck and cricket.

(d)

Cricket flashing extends up chimney at least 6"

Cricket flashing cut to fit over cricket and extend

Nail flashing

to deck

Set step and cricket flashing

in asphalt plastic cement

Place preformed cricket flashing over cricket and corner flashing

Set cricket flashing in asphalt plastic cement.

(e)

FIGURE 12.9 (c) Chimney flashing—Step 3 (d ) Chimney flashing—

Step 4 (e) Chimney flashing—Step 5 (Continued )

Flashing strip extends up chimney at least 6"

Flashing strip cut to contour of ridge in cricket Size to extend up chimney at least 6".

(f)

Counter flashing

Counter flashing

Place counter flashing over step and cricket flashing

Shingle remainder of roof.

Seal joint with Portland

FIGURE 12.9 (i) Counter flashing details (Continued )

Backer flashing extends upslope

under shingles a min of 3 courses

(Where deemed necessary hold

shingles up 1 course and nail high,

depending upon anticipated debris

and/or snow accumulation.)

Skylight

Integral counter flashing

with hemmed drip edge

Counter flashing laps over step flashing approx 2" min.

Apron flashing with lower

edge hemmed under

Raised curb (2"x8"

suggested as min to

attain flashing clearances)

Underlayment turned up curb

Step flashing

(a)

FIGURE 12.10 (a) Flashing around skylight with shingle-type flat roofing.

proofing for when the shingles blow off The practice of relying on the shingleattachment to hold down the underlayment is not an option Fully adhered un-derlayments or even a layer of hot-mopped asphalt roofing may be used Obvi-ously, this is not a consideration when other roofing materials that are designedand properly attached for the design wind load are used

• Often the actual shape of the roof can be a factor in maintaining the tight integrity of the roof The use of hip roofs as opposed to gable-end roofs

weather-Backer flashing extends upslope under tile

approx 24" (Where deemed necessary hold

tile up 1 course, depending upon anticipated

debris and/or snow accumulation.)

Skylight

Integral counter flashing

with hemmed drip-edge

Apron flashing formed

to fit over tiles

(b)

FIGURE 12.10 (b) Flashing around skylight—cover and pan concreter / clay roofing tile (Continued )

can provide better bracing for the exterior walls and reduce wind uplift pressures

on the roofing material Better structural integrity and better performance of theroofing materials result with the use of hip roofs in high-wind areas

• Another reason to use hip roofs in high-wind areas is because of the poor formance of gable-end vents in wind-driven rain situations At hurricane windspeeds large quantities of rain can be driven in through gable end vents Whilethis water is transient in nature and poses little long-term threat to the structurefrom a decay perspective, it can cause a tremendous amount of damage to thecontents and potentially to the occupants of the structure The rainwater soaksthe insulation while weakening the drywall Within a short period of time theceiling falls, destroying most of the contents of the room below The use of hiproofs requires the use of ridge vents, or small surface-mounted roof vents, whichhave better resistance to water infiltration during high winds

per-• As the sheathing forms the foundation for the whole roof system it is imperativethat it be sufficiently attached to resist the kind of loads that can be found inhigh wind situations APA has published nailing schedules for roof sheathingattachment in high-wind areas See Chapter 7 for further information

Cathedral Ceilings In the past cathedral ceilings have been the source of some

problems, due primarily to the difficulty in maintaining a workable ventilation pathbehind the sheathing as required by the building code while maintaining locallyrequired levels of insulation Difficulties arose from trying to get joists of suitabledepths and the inability to maintain ventilation around roof deck penetrations such

Underlayment carried

up onto sidewall 3"

to 4"

Step flashing positioned

over shingle so that next

course of shingles covers

Flashing placed just upslope from exposed edge of shingle –

extends approx 4" over underlying shingle and approx 4"

Approx 2" head lap

Siding/cladding – maintain 2" above the roof surface

Wall cladding/siding serves as counter flashing

and should overlap step flashing a min of 2"

Place nails high, so nails are overlapped

by the next upslope step flashing

Asphalt-saturated felt underlayment turned up vertical walls

approx 3" to 4"

(b)

Note: where building codes require building paper

under siding, the building paper

will extend over the roof step flashing.

With the introduction of engineered wood I-joists, most of the ventilation lems can be easily solved I-joists are readily available in depths that easily permitrequired insulation levels and allow sufficient space for ventilation above the in-