APA Engineered Wodd Handbooks Episode 11 pps

9.2 PRESERVATIVE TREATMENTS Wood may be preservatively treated through various processes such as applied or dip applications, addition of preservatives into the product during man-ufactu

CHAPTER NINE TREATMENTS AND FINISHES

Finishes such as paints and stains help protect wood surfaces from ultravioletlight and rapid fluctuations in moisture content Although many finishes containadditives to provide some degree of protection from surface molds and fungi such

as mildew, these additives provide little protection from wood-destroying fungi orinsects Various types of chemical treatments are available, however, which canprovide protection from these organisms The type and extent of treatment neededwill vary depending on the severity of exposure and associated risk factors Addi-tionally, not all treated wood products are paintable Therefore, if a product is to

be finished, it is important to specify the use of a paintable wood preservative.Finally, awareness is increasing of environmental and health concerns associatedwith the use of some chemicals for treating wood The U.S Environmental Protec-tion Agency (EPA) requires the registration of all preservatives used in productsmanufactured for treating wood They also publish guidelines for the handling anddisposal of some types of treated wood Further guidance on this subject is availablefrom the American Wood-Preservers’ Association

9.2 PRESERVATIVE TREATMENTS

Wood may be preservatively treated through various processes such as applied or dip applications, addition of preservatives into the product during man-ufacture, or pressure treatment after manufacture To select the most suitablemethod, one must consider various factors such as code requirements (if applicable),

brush-the severity of exposure, level of protection required, cost, availability, and formance expectations.

per-9.2.1 Dips or Surface Applications

Nonpressure treatments such as water-repellent preservatives or other preservativesare available that can be applied by brush or dip treatment The latter method isgenerally more effective when practical These types of treatments provide limitedprotection in lower decay risk applications or in nonstructural situations They aretopical and provide little penetration on wood surfaces, although they are readilyabsorbed by the end grain If brushed on, they should be liberally applied andworked into any open voids Dip treatments are most effective when the product isimmersed for several minutes Follow the manufacturer’s recommendations Surfacetreatments are not as effective as pressure-preservative treatments in protectingwood Not all water repellents contain preservatives and many are not paintable.Therefore, it is important to specify a product suitable for the intended end use.Other types of treatments, such as copper naphthenate and copper-8-quinolinolate,are also available for surface applications Copper-8-quinolinolate may be used inaccordance with FDA and / or USDA regulations where food contact with the treatedwood is possible

9.2.2 Blended Treatments

There are currently no existing treating standards for OSB, LVL, or I-joists ever, some manufacturers have evaluated the use of preservatives incorporated intothe furnish of OSB Furnish may be defined as wood-based material, such as flakes

How-or strands, including applied resin, wax, and other additives, as the primary stituent of wood-based panels such as waferboard or oriented strand board (OSB).These blended preservatives are intended to provide protection from wood-destroy-ing organisms Check with the manufacturers for additional information Otherproducts, such as LVL, may be treatable with pressure processes or brush-on ap-plications or dip treatments, subject to agreement between buyer and seller Thesuitability of any of these processes will depend on many factors, such as devel-oping technology, the effect of the treating process and / or treatment chemicals onthe durability of the glue bond, the effectiveness and penetration of the preservative,and the level of decay hazard

con-9.2.3 Pressure-Treated Plywood

Most construction applications don’t require preservative-treated wood But in someuses, and in certain climates, treated wood is recommended or required by localbuilding codes to protect against decay, fungi, termites, carpenter ants, or otherwood-destroying insects, or from marine borers in seawater exposure Preservative-treated plywood is impregnated with preservatives by a pressure process The re-sulting deep penetration of preservative provides protection against decay and insectattack See below, Standards and Use Recommendations, for a summary list ofAWPA standards for plywood

Pressure-preservative-treated plywood is used in a variety of applications:

TREATMENTS AND FINISHES FOR WOOD 9.3

• Electrical transformer vaults

• Decks, docks, piers, and floats

• Tanks

• Liquid manure storage tanks

• Planters

• Food transport, processing, and storage

• Roof, wall, and floor sheathing in tropical regions or applications where resistance

to termites or decay is required

Preservative-treated plywood maintains stiffness and strength, thermal properties,workability, light weight, and economy

Wood preservatives and wood treated with these chemicals should be used andhandled appropriately (see below, Standards and Use Recommendations and Pre-cautions for Use and Handling) Always follow the recommendations provided bythe preservative manufacturer or wood-treating company

Acceptances and Availability. Preservative treatments are recognized in modelbuilding codes, federal and military specifications, and American Association ofState Highway and Transportation Officials (AASHTO) specifications

Preservative-treated plywood is generally available in most metropolitan areas.For availability, contact local building materials suppliers or consult the YellowPages of the telephone directory under ‘‘Lumber Treating.’’

Some panel grades, thicknesses, span ratings, or species may be difficult toobtain in some areas Check with your supplier for availability or include an alter-native panel in the specifications

Organic Preservatives. Organic preservatives used for preservative treating of wood include creosote, pentachlorophenol (penta), and copper-8-quinolinolate.Creosote is a coal tar product that is dissolved in a distilled solution or petroleumoil It is an effective preservative in commercial, industrial, or marine applicationswhen there is severe exposure to decay or insect attack, or marine borers in saltwaterenvironments Creosote-treated plywood has an odor and a dark, oiled surface ap-pearance and therefore is not recommended where painting is required

ply-Penta is commonly dissolved in light petroleum oil or solvent or in a petroleumsolvent / water solution It is suitable as a preservative for ground contact or above-ground uses Plywood treated with penta has an odor, and oil-borne penta has anoiled surface appearance and thus is not recommended for painting However, ply-wood treated with solventborne or waterborne penta, including penta in LPG, has

an oil-free surface and natural wood appearance and should be specified where

staining or painting is desired Note: Penta dissolved in methylene chloride should

TABLE 9.1 Chemical Names of Common Waterborne

Preservatives

aThese preservatives are highly leach-resistant.

not be used to treat plywood containing synthetic repairs in face veneers, since the solvent can damage the repairs.

Plywood treated with copper-8-quinolinolate preservative can be used in cations where food is harvested, transported, or stored The preservative is dissolved

appli-in liquid petroleum gas or light hydrocarbon solvents so that the treated plywoodhas a clean surface and is free of solvent odor Check with the company providingthe treatment service regarding applicable FDA and USDA acceptances

Inorganic Preservatives. Inorganic preservatives are the most popular and monly available types used for preservative treating of plywood They includeleach-resistant waterborne arsenical preservatives such as CCA, ACA, ACZA, andACQ-B, which are highly effective in preventing decay and attack by termites,carpenter ants, and marine organisms (see Table 9.1 for chemical names of commonwaterborne preservatives) These preservatives are forced into plywood under pres-sure and become insoluble or fixed in the wood cells after impregnation and drying.Waterborne preservatives are recommended where clean, odorless, and paintableproducts are required Wood treated with such preservatives may be used insideresidences or commercial and industrial buildings, providing sawdust and construc-tion debris are cleaned up and disposed of after construction

com-Note: Treatment of tongue-and-groove (T&G ) panels with waterborne vatives may result in difficulty in mating T&G edges Using square-edge panels or milling T&G edges after treatment should be considered.

preser-Standards and Use Recommendations. Table 9.2 lists applicable AmericanWood-Preservers’ Association (AWPA) preservative-treating standards for plywood,according to the intended end use Table 9.3 summarizes plywood applications andrecommended preservative treatments for these uses, in accordance with AWPAStandards

Preservative Penetration and Retention. AWPA standards specify preservativeretention in terms of pounds of retained preservative per cubic foot of wood Spec-ified retention levels vary according to the type of preservative and the severity ofexposure (see Table 9.3) For preservative-treated plywood, AWPA standards requirethat all veneers must be penetrated by preservative The net retention of preservative

in plywood is high—up to 25 lb/ft3in some treatments—because of plywood’s highratio of surface area to volume Therefore, preservative-treated plywood may be cut

in the field without loss of preservative protection at cut edges

TREATMENTS AND FINISHES FOR WOOD 9.5

TABLE 9.2 Preservative-Treating Standards for Plywooda

Use

Preservative types AWPA

quinolinolate

Copper-8-Waterborne preservatives

Gluability. Plywood can be glued when treated with most of the waterborne servatives and with some penta treatments using light petroleum oil or solvent,liquid petroleum gas, or petroleum solvent / water as a carrier Consult treating firms

pre-or their trade associations fpre-or specific recommendations

Structural Properties. Preservative treatments applied under AWPA standards do

not affect plywood strength and stiffness See APA Plywood Design Specification1(Form Y510) or APA Technical Note N3752 for allowable working stresses andsection properties or design capacities for plywood

Finishing Recommendations. Waterborne preservatives are readily paintablewhen dried after treatment Finishing recommendations are generally the same asthose for untreated plywood Some slight surface degradation is possible in sandedplywood after drying because of surface checking and / or discoloration For thisreason, Medium Density Overlay plywood gives best results where treated paintablesurfaces are required Unsanded grades of plywood do not require further finishingbut can be finished with two-coat acrylic paints or opaque stains Stain finishes ortwo-coat acrylic paints (stain-blocking primer and topcoat) are recommended fortextured plywood

Painting plywood treated with creosote or oil-borne preservatives such as tachlorophenol is not recommended Painting can be done only with difficulty and

Minimum preservative treatmentb

(lb/ft 3 by assay) Creosote

phenold

Pentachloro- quinolinolatef

Copper-8-Waterborne preservativesc

Contact with sea

(Type A, B or C)

or ACQ (Type B)

sheathing and other uses such as fences, exterior siding, exposed structural units such as stressed-skin panels and box beams, reservoir roofs, splash boards in pole-type buildings.

transport and storage containers requiring low human toxicity fungicide.

Not recommended Not recommended

Copper-8-quinolinolate:

empty-cell 0.20

Not recommended

aSee applicable EPA-approved Consumer Information Sheet (CIS) for precautions involving uses and handling of

creosote, pentachlorophenol or inorganic arsenical pressure-treated wood.

bRecommended minimums from the American Wood-Preservers’ Association Standards.

cBased on dry preservative per cubic foot, oxide basis, full-cell treatment.

dOil, gas or other solvent-borne.

ePlywood marked PS 1 or PS 2, or APA Standard PRP-108 is required for the Permanent Wood Foundation system.

fVolatile petroleum (AWPA P9, Type B) or light hydrocarbon (AWPA P9, Type C) solvents only.

gFor dry above-ground applications not subjected to liquid water or Formosan termites.

Minimum preservative treatmentb

(lb/ft 3 by assay) Creosote

phenold

Pentachloro- quinolinolatef

Copper-8-Waterborne preservativesc

TABLE 9.4 Use Precautions for Pressure-Treated Wooda

Application

Organic preservatives Creosote Pentachlorophenol

Inorganic preservatives Arsenicals

3 For industrial and farm buildings,

interior components which are in

ground contact and subject to decay or

insect attack Also see application 5.

4 Laminated beams for commercial or

industrial buildings

5 Interiors of farm buildings when

animals can crib (bite) or lick the

7 Applications where preservative may

become component of food or animal

feed, such as structures or containers

for storing silage or food.

8 Cutting boards or countertops for

preparing food.

9 Decks, patios and walkways if surface

is visibly clean and free from residues.

10 Portions of beehives which may come

into contact with honey.

11 Applications where treated wood can

come into direct or indirect contact

with drinking water for public or

animal consumption.

aBased on EPA-approved Consumer Information Sheets.

bMust be painted with recommended sealer (two coats).

cExcept for glued laminated beams or for building components which are in ground contact and are subject to decay or insect infestation and where two coats of an appropriate sealer are applied.

requires extensive care using an aluminum base paint Paintable pentachlorophenoltreatments are available (See discussion under Organic Preservatives.)

For certain interior applications in commercial, industrial, or farm buildings,creosote- or pentachlorophenol-treated wood may be used if exposed surfaces aresealed by painting with two coats of urethane or epoxy paint or shellac (varnishmay also be used for pentachlorophenol-treated wood) For guidelines on use pre-cautions in these cases, refer to Table 9.4 and the EPA-approved Consumer Infor-mation Sheet (CIS) for the applicable preservative treatment

Precautions for Use and Handling. The chemical formulations used for vative treatment of plywood are registered with the EPA, which has approved guide-lines for the use of pressure-treated wood to ensure safe handling and avoid envi-

preser-TREATMENTS AND FINISHES FOR WOOD 9.9

ronmental or health hazards The use precautions for creosote, pentachlorophenol,and inorganic arsenical preservative-treated wood are published in EPA-approvedConsumer Information Sheets (CISs) for these treatments, available from treaters,and are briefly summarized in Table 9.4

• Avoid frequent or prolonged inhalation of sawdust from treated wood Whensawing and machining treated wood, wear a dust mask Whenever possible, theseoperations should be performed outdoors to avoid indoor accumulations of air-borne sawdust from treated wood

• When power sawing and machining, wear goggles to protect eyes from flyingparticles

• Avoid frequent or prolonged skin contact with pentachlorophenol- or treated wood; when handling wood treated with these chemicals, wear long-sleeved shirts and long pants and use gloves

creosote-• After working with treated wood, and before eating, drinking, or using tobaccoproducts, wash exposed skin areas thoroughly

• If preservatives or sawdust accumulates on clothes, launder before reuse Washwork clothes separately from other household clothing

9.2.4 Preservative Treatment of Glued Laminated Timber

Structural glued laminated timbers (glulams) bearing the APA EWS trademark areproduced by members of Engineered Wood Systems (EWS), a related corporation

of APA—The Engineered Wood Association These glulams are manufactured to

conform with AITC / ANSI Standard A190.1, American National Standard for Structural Glued Laminated Timber.3

Although glulams do not require preservative treatment for most uses, certainapplications may present environmental conditions conducive to decay or insect ormarine borer attack Conditions that favor such attack are the presence of moisture(20% or greater moisture content of the wood) accompanied by temperatures rang-ing from 50–90⬚F Decay progresses more slowly at temperatures outside this rangeand virtually ceases at temperatures below 35 or above 100⬚F

These hazards are typically controlled through recognized design principles andconstruction techniques such as use of overhangs, flashings, ventilation, and properjoint connection details such as shown in Chapter 12 Elimination of potential decayhazards through effective design detailing is the preferred method of controllingdecay When hazards of decay or insect attack cannot be avoided, glulams must bepressure-preservative treated or a naturally durable species must be used Examples

of construction where such hazards may exist include direct exposure to weather,ground contact (including direct contact with concrete foundations and footings),contact with fresh water or seawater, and exposure to excessive condensation.Outdoor uses of preservative-treated glulams include bridges, utility structures,marine applications, highway noise barriers, and decks Indoor uses that may re-

quire pressure treatment include environments subject to high humidity or sation, such as indoor swimming pools or greenhouses, where moisture content ofthe wood may exceed 20% Indoor applications such as post and beam construction

conden-in some farm buildconden-ings may also conden-involve ground contact and thus require vative treatment for those members in contact with the ground Table 9.5 listscommon preservatives used to pressure-treat glulams and provides a summary ofrelevant considerations

preser-Applicable Standards. Applicable standards for preservative treatment of glulamsinclude American Wood-Preservers’ Association Standards C284(and all other stan-dards referenced therein) and M4.5Related specifications include American Forestand Paper Association National Design Specification;6American National StandardsInstitute, Inc ANSI A190.1; Federal Specification TT-W-571,7and American As-

sociation of State Highway and Transportation Officials (AASHTO) (Standard Specification for Preservative Pressure Treatment Process for Timber).8

Preservatives. Pressure preservative treatments listed in American Wood ers’ Association (AWPA) Standard C28 for glulams include creosote, pentachloro-phenol, copper naphthenate, and water-borne inorganic arsenicals

Preserv-Other treatments and processes specified should be agreed to by purchaser, seller,and governing code body Required retention and penetration levels depend on enduse and exposure according to AWPA or other applicable specifications

Organic Preservatives Organic preservatives listed in AWPA Standard C28

include creosote and pentachlorophenol and are the primary treatments used inglulams manufactured from western species See Section 9.2.3 under Organic Pre-servatives for a discussion of these organic preservatives

Inorganic Preservatives Inorganic preservatives are waterborne treatments

such as ammoniacal copper arsenate (ACA) and chromated copper arsenate (CCA).While these are not recommended for use with western species, they may be used

to treat southern pine laminations prior to gluing of the finished product See tion 9.2.3 under Inorganic Preservatives for a further discussion of these inorganicpreservatives

Sec-Treatment Recommendations. Glulams are available in custom and stock sizes.Stock sizes are typically cut to length at a distribution center or on the job site.Most glulams to be pressure-treated will be custom sizes and should be ordered toexact dimensions, when possible, to avoid field cuts, which must be retreated Inaddition, all fabrication, cutting, and predrilling of holes for fasteners is recom-mended prior to pressure treating

Glulams may be treated after gluing, or the individual laminations may be treatedprior to gluing, depending on the wood species and treatment specified Treatmentssuch as creosote or pentachlorophenol (penta) in oil are typically only specified fortreatment of the finished member Penta in light hydrocarbon solvents may be spec-ified for the laminations prior to gluing or for the finished member Waterborne salttreatments may be specified for laminations prior to gluing However, glulam man-ufactured using pretreated laminations is not available from all glulam manufac-turers and for use with all species and availability should be verified prior to spec-ifying

The use of waterborne preservatives for the treatment of finished glulam bers is not recommended The only waterborne preservative listed in AmericanWood-Preservers’ Association Standard C28 for use after gluing is ACZA, and it

mem-is limited to coastal region Douglas-fir, western hemlock, and hem-fir It mem-is

Fresh water, ground contact, aboveground uses including docks, bridges, towers, and beams

Saltwater or fresh water applications and ground contact May be used indoors provided sawdust and construction debris are cleaned up and removed.

Appearance Dark, oily, odor Oily, may be blotchy, may

have odor

Varies from natural appearance

of wood to some darkening

of wood.

Green to brown, depending on chemicals used and exposure to light.

repellent or oil-based semitransparent stain

Can be stained or painted when surface is dry and prepared in accordance with coating manufacturer’s recommendations.

residential interiors.

May be used in industrial interiors when two coats of effective sealer are applied.

May be used in residential, industrial, or commercial interior when two coats of effective sealer are applied.

May develop greenish discoloration

of finish Stain-blocking primer will help to minimize discoloration.

tant to note that waterborne types of treatments may cause dimensional changessuch as warping and twisting or lead to excessive checking, splitting, or raised graindue to the posttreatment drying process resulting in a finished product having anunacceptable appearance They may also result in discoloration of the wood, pos-sible raised grain, excessive checking, or warping of the member.

Glulams that are to be preservative treated must be bonded together with use adhesives conforming to AITC / ANSI A190.1 Table 9.6 provides a summary

wet-of treatment recommendations

Species. Softwood species listed in AWPA Standard C28 for preservative ment include coastal Douglas-fir, western hemlock, hem-fir, and southern pine.Listed hardwood species include red oak, red maple, and yellow poplar whentreated after gluing The most commonly available West Coast species are Douglas-fir and hem-fir Other species of glulams may also be available for pressure treat-ment, subject to agreement by the seller and purchaser and approval by the gov-erning code body

treat-Incising is recommended for Douglas-fir, western hemlock, hem-fir, red maple,and yellow poplar Such incising is normally performed after gluing of the finishedglulam If laminating lumber is to be incised prior to gluing, the mating faces to

be glued should not be incised Incising is not considered to have a detrimentaleffect on the strength of glulam However, the effects of incising on appearanceshould be considered when ordering glulams where aesthetics are important Lack

of incising, if specified, may cause difficulties in meeting the specified treatmentretention and penetration levels and should only be considered with caution

Retention and Penetration Levels. Retention and penetration levels are specified

in AWPA Standards in pounds of retained preservative per cubic foot of wood anddepth of penetration in inches Specified retention and penetration levels vary ac-cording to the type of preservative and the level of exposure Table 9.7 lists stan-dards referenced in AWPA Standard C28 for specified retention and penetrationlevels according to the intended end use

Field Cuts. It is recommended that all fabrication, trimming, and boring of lams be performed prior to pressure treating If there is any field fabrication orsurface damage to the glulams, all cuts, holes, or damaged areas must be field-treated to protect the exposed wood material Copper naphthenate may be used toreseal exposed areas of glulams treated with creosote or pentachlorophenol It mayleave a greenish coloration Field treatments should be applied to saturation bydipping, brushing, spraying, soaking, or coating in accordance with AWPA StandardM4

glu-Fasteners. Fasteners used to connect preservative-treated glulams should be rosion resistant to withstand the effects of the high moisture environment to whichthese members are typically exposed

cor-Corrosion of fasteners is influenced by the amount of moisture present, ature, wood pH, extractives, chemicals in the treatment, and environmental factorssuch as chlorine, salt, and pollutants Oil-borne treatments are generally not cor-rosive whereas the waterborne arsenical treatments can be highly corrosive, de-pending upon environmental conditions Hot-dipped galvanized connectors are typ-ically adequate, but other materials, such as stainless steel or monel, may berequired in certain applications

Treatment type

Western species Glulam

treated prior to laminating

Glulam treated after laminating

Southern pine Glulam

treated prior to laminating

Glulam treated after laminating

Hardwoods Glulam

treated prior to laminating

Glulam treated after laminating

aFor aboveground use only.

bException when penta in light hydrocarbon solvents is used.

cNot specifically provided in AWPA Standard C28 If specified for treatment after laminating, these treatments can

result in discoloration, excessive checking, raised grain, twisting, and warping.

dAlthough not recommended, this treatment is permitted by AWPA Standard C28.

TABLE 9.7 Preservative Retention and Penetration Specifications

Use

AWPA standard

TABLE 9.8 EPA Recommended Sealers for Treated Glulams

suitable for wood block flooring.

Pentachlorophenol Urethane, shellac, latex epoxy enamel, varnish.

Structural Properties. Most building codes generally recognize design values forglulams as specified in the latest edition of the National Design Specification(NDS) Although the NDS does not specify reductions in the dry design values forglulams preservatively treated according to AWPA Standards, it does specify thatwet-use design values shall be used whenever the moisture content in service is16% or more

Use and Handling Precautions. The EPA requires registration of pesticides used

in pressure treatments They have approved use and handling precautions for treatedwood as published in Consumer Information Sheets These sheets also list recom-mended sealers for treated wood used in certain indoor applications These sheetsare available from treaters and should accompany each shipment of treated wood.They can also be obtained from the American Wood Preservers Institute or APA—The Engineered Wood Association Use precautions are summarized in Table 9.4and appropriate sealers are listed in Table 9.8 Also see handling tips in Section9.2.3 under Handling Tips See Consumer Information Sheets for complete infor-mation

Finishing. Creosote or pentachlorophenol in oil is not paintable on a practicalbasis Pentachlorophenol in light solvents can be finished with natural finishes such

as a clear water repellent or an oil-based semitransparent stain Clear film-formingfinishes such as lacquers, varnishes, or urethanes are not recommended for glulamsused outdoors because they have a short service life and require extensive surfacepreparation prior to refinishing

Glulams that have been treated with waterborne preservatives such as CCA can

be finished with clear water repellents, oil-based semitransparent stains, or forming finishes such as solid-color stains or paint systems However, the treatmentmay leave a green or brown color on the glulam surface that can affect the colorand appearance of the finish

film-If an opaque coating is desired, the most durable finish is a top-quality paintsystem such as a stain-blocking acrylic latex primer followed by two all-acrylic

TREATMENTS AND FINISHES FOR WOOD 9.15

latex topcoats, preferably from the same manufacturer A stain-blocking primer mayalso be required under light-colored acrylic latex solid-color stains to help minimizediscoloration of the finish Always follow the coating manufacturer’s recommen-dations

If treated wood is to be used indoors, follow the EPA recommendations forappropriate sealing of the wood

10 minutes, as compared to 30 minutes for FRT plywood

There are two broad types of fire-retardant paints One is a pigmented typedesigned as a masking finish similar to ordinary paint systems Most of these paintstypically intumesce or swell to a thick insulating layer when exposed to rapidlyrising temperatures These types of paints are designed for their fire-retardant prop-erties and may not provide the full range of colors and decorative effects of con-ventional finishing systems Generally, they are flat or nongloss types, althoughsemigloss formulations are also available Some are more durable under washingthan others, and most are designed for interior use only A few companies manu-facture fire-retardant paint with proprietary topcoat finishes designed for exterioruse

The second type of fire-retardant paint is a clear finish, similar in appearance to

a lacquer or clear varnish, that is used for high appearance paneling such as finehardwoods Clear fire-retardant finishes may develop a characteristic bloom or milk-iness when subjected to high humidity or direct exposure to the weather duringconstruction

9.3.2 Fire-Resistive Overlaid Panels

These panels have proprietary overlays and are tested for fire performance alent to fire-retardant-treated wood as defined in model building codes They have

equiv-a mequiv-aximum flequiv-ame spreequiv-ad requiv-ating of 25 tested with the flequiv-ame font 101⁄2ft from thecenter of the burner They must develop no progressive combustion as determined

in a 30-minute test according to ASTM E84

9.3.3 Pressure Impregnated

In any projected use of fire-retardant treatments (FRT), thorough investigationshould first determine that it is the best overall solution in view of long-term in-surance costs and adequate fire protection at lowest construction cost FRT woodproducts are more expensive than untreated wood products, which in most casesperform satisfactorily in regard to both life safety and protection of property.FRT is not recommended for use with glulam (see Chapter 4), structural com-posite lumber (see Chapter 6) or prefabricated wood I-joists (see Chapter 5) due tounknown effects of the treatment on strength and glue bond durability Such dele-terious effects may compromise the integrity of structural members

Fire-retardant treating is a secondary process to plywood manufacturing FRTwood is pressure-impregnated with chemicals in water solution to inhibit combus-tion This qualifies it for lower flame spread (at least as low as gypsum wallboard)and smoke generation ratings and reduces its fire hazard classification When it isidentified as such by a code-recognized testing agency label, it is rated on a paritywith noncombustible constructions by many insurance rating bureaus

Precisely defined, FRT plywood has been impregnated with fire-retardant icals in accordance with American Wood-Preservers’ Association Standard AWPAC27.10 When tested for 30 minutes under ASTM Standard E-84 (the tunnel test),

chem-it has a flame spread not over 25 and shows no evidence of significant progressivecombustion The use of fire-retardant-treated wood products is primarily dictated

by building code and insurance agencies

Function of FRT. The function of fire retardants is to reduce the flame spread of

a material Most commercial fire retardants involve salts that become acidic underelevated temperatures Fire-retardant chemicals directly alter the thermal degrada-tion of wood such that the thermal decomposition of the wood occurs at tempera-tures lower than ignition point The chemicals increase the production of char andreduce the production of volatile combustion vapors

Design recommendations for FRT wood products recognize that the treatmentsreduce the strength of wood products due to the treatment and redrying Prior to

1985, design guides for lumber and plywood included stress adjustments based onpast research Subsequent field problems related to corrosion of fasteners and grossdegradation of fire-retardant-treated structural wood panels led APA to remove sress

adjustments in the Plywood Design Specification A note was added to the

Speci-fication indicating that the provider of the treatment and redrying service should becontacted for information regarding the effect of the treatment Currently, structuraldesign information is made available only from companies that manufacture fire-retardant treatments

When It Is Used. Fire-retardant-treated wood is used in applications where firesafety is a concern The use of FRT plywood is affected by those model codes thatare adopted for a geographical area of the United States One of the most commonapplications for FRT plywood is for roof sheathing as an alternative to a parapet

or noncombustible deck in townhouse roofs This alternative was first accepted bycode in 1979 when adopted by the Building Officials and Code Administrators

TREATMENTS AND FINISHES FOR WOOD 9.17

International (BOCA) for the National Building Code Details on code provisionsand standards are provided in Chapter 10

Field History. Commercial fire-retardant treatment of wood dates back to 1895,when the U.S Navy published specifications for the use of FRT wood in shipconstruction In 1899, New York City passed a requirement that wood trim usedabove the 12th floor of high-rise buildings must be fire-retardant treated In the1920s the use of ammonium phosphates and borax became common for treatingcellulosic materials such as wood.11 Shortages of metal during World War II in-creased the demand for nonflammable building materials During this period thedemand for FRT wood soared

Since World War II, the primary market for FRT wood has been in multifamily,commercial, and industrial buildings where the use is primarily driven by regulatoryconsiderations The demand for FRT plywood has generally depended on perform-ance and the cost of the product compared to alternative materials

Fire-retardant treating of wood products improves fire resistance However, formance problems with FRT products were reported in the 1980s related to sideeffects of the chemicals such as fastener corrosion and strength reductions of wood.Recent developments in phosphate chemistry and flame-retardant research have led

per-to new formulations.12

Corrosion of Fasteners The salts used in commercial fire-retardant treatments

have a natural affinity for water This hygroscopic nature can lead to corrosion ofmetal fasteners when FRT wood is exposed to high relative humidities In suchcases, FRT wood can become saturated with water such that the treatment chemicalsdrip in a solution from the wood.13

When the U.S Navy started using FRT wood in the late 1800s, they encounteredproblems due to the hygroscopic and corrosive nature of the treatments The cor-rosive effect the chemicals had on metal fasteners was a factor in their discontin-uation of FRT wood use in 1902.13

Hygroscopicity and corrosion problems from the use of FRT wood continued to

be reported in the early 1980s Concern over the hygroscopic and corrosive nature

of these treatments led to development of standards for less hygroscopic treatments.The modification in AWPA standards to reflect new treatments was drafted in 1982.Three major treatment companies, Hoover, Koppers, and Osmose, all introducedInterior Type A (low-hygroscopic) treatments in 1981 or 1982

Strength In 1986, an increasing trend of strength-related complaints

concern-ing interior-type FRT plywood was noticed by APA While it had been recognizedthat overdrying after treating may lead to strength loss and brittleness, these newreported concerns generally arose after installation The plywood was usually re-ported charred in appearance and brittle with low strength In many instances, re-placement of the FRT plywood was deemed necessary due to low strength and / orrelated sagging of the panels

Generally, the performance problems that were reported were related to lowhygroscopic Interior Type A formulations Subsequent research by the treating com-panies led to reformulated treatments to address these problems Treating companiesnow submit data on FRT products to building code agencies to obtain an evaluationreport Because particular formulations represent proprietary information of thetreater not subject to public disclosure, specific information such as design rec-ommendations concerning a treated panel product must be obtained from the treater

Treating Standards. Fire-retardant treating practices are covered by standardswritten by the American Wood-Preservers’ Association (AWPA) The AWPA is anassociation of treatment companies, chemical suppliers, wood industry representa-

tives, government and regulatory agencies, and academia Standards written byAWPA address acceptable treating practices and performance levels for fire-retardant-treated wood products Standards developed by American Society of Test-ing and Materials (ASTM) are also applicable to aspects of FRT plywood Thestandards written by the ASTM are consensus standards developed by committeesmade up of a balanced membership of producers, users and general interest mem-bers The following AWPA and ASTM standards may be consulted for specificdetails on FRT treatment and analysis of fire-retardant-treated wood.

• C20, Structural Lumber-Fire-Retardant Treatment by Pressure Process17

• C27, Plywood-Fire-Retardant Treatment by Pressure Process10

• E6, Standard Method for Determining the Equilibrium Moisture Content of Treated Wood18

• P17, Fire Retardant Formulations19

• D4502, Test Method for Heat and Moisture Resistance of Wood-Adhesive Joints22

• D5516, Test Methods for Evaluating the Mechanical Properties of Fire-Retardant Treated Softwood Plywood Exposed to Elevated Temperatures23

• D5664, Test Method for Evaluating the Effects of Fire-Retardant Treatments and Elevated Temperatures on Strength Properties of Fire-Retardant Treated Lumber24

• D6305, Practice for Calculating Bending Strength Design Adjustment Factors for Fire-Retardant-Treated Plywood Wood Sheathing25

Finishing recommendations provided in this chapter are intended as general lines and may or may not be applicable for all wood-based products For specificrecommendations, check with the product manufacturer and paint suppliers or man-ufacturers The finish is the final touch on any building, so it’s important to do itright the first time

guide-For best performance and appearance, wood must be properly finished Theprimary functions of a finish are to protect the wood from the weather and enhanceits appearance To select the best finish for a particular application, weigh theaesthetic considerations against durability and maintenance requirements

TREATMENTS AND FINISHES FOR WOOD 9.19

Some finishes, such as semitransparent stains, accent the natural beauty of wood.These types of finishes, however, generally require frequent maintenance Otherfinishes, such as acrylic latex house paints, are less natural in appearance but typ-ically offer superior durability

Another consideration is the type of product to be finished and its surface acteristics For example, the use of semitransparent stains is not appropriate forsidings that contain numerous surface repairs

char-9.4.1 Common Performance Problems

There are many performance problems that can develop during the service life ofwood products exposed to outdoor weathering This discussion does not attempt toaddress all of them but instead focuses on some of the more common problems.Additional discussion is also provided in the sections dealing with specific products

Mildew Discoloration of Wood Siding. Mildew is a living organism, a fungus,which is normally observed as a black or dull grey deposit on exterior or interiorsurfaces It can easily be mistaken for dirt Mildew develops from microscopicspores, which are carried in the air to a surface where they feed and grow Fungiare plants that contain no green chlorophyll and thus cannot make their own food.They must live on material that possesses organic matter Mildew will grow onalmost any surface where conditions are favorable For example, mildew will grow

on metal, feeding not on the metal itself, but on a thin film of organic material thathas accumulated on the metal In addition to a food source, warmth, moisture, and

a shady location all encourage the growth of mildew Mildew is a surface enon that is a problem principally because of its appearance It does not affect thestrength or other important properties of wood Naturally, light-colored finishesshow mildew more readily than dark-colored ones Mildew may grow on paintedsidings, but proper construction practices, maintenance, and the use of finishescontaining mildewcides will help minimize its growth and often eliminate it.Black spotted areas on siding are quite frequently thought to be dirt, when inreality they are mildew Identification is important because the treatment is differentthan for dirt A test for mildew can be made by applying a drop of 5% sodiumhypochlorite solution (common household bleaching solution) to the dark area.Mildew will usually bleach within one or two minutes Dark areas that do not bleach

phenom-are probably dirt Note: It is important to use fresh bleach solution because, after standing six months or longer, bleach deteriorates and may no longer be sufficiently potent.

Mildew can be removed with commercial mildew removers, following facturer’s directions, or with a solution of one part household bleach (5% sodiumhypochlorite) mixed in three parts by volume of warm water On some finishes amilky film may develop from the bleaching treatment Thorough rinsing of theentire area with clean water will help alleviate this condition

manu-When using bleach, avoid breathing the vapors and contact with the skin andeyes Wear rubber gloves to protect hands and use goggles to prevent eye damage.Shrubs and other plant life should also be protected Children and pets should bekept away from these products

Many mildew problems require only the bleach treatment mentioned above.However, if the finish has deteriorated to the point where a new finish is required,mildew must be killed and the surface cleaned before refinishing Otherwise it maycontinue to grow up through a newly applied finish