This publication is part of the U.S.Forest Service information programdesigned to make forestry researchon protection of wood in use moreunderstandable and usable forpractitioners and laymen.

The information and illustrationsin this booklet relate research findingsat the Southern Forest ExperimentStation and at the Forest ProductsLaboratory to practical problemsof wood protection.

Rodney C. DeGroot, former plantpathologist with the Southern ForestExperiment Station, is now associatedwith the U.S. Forest ProductsLaboratory, Madison, Wisconsin.

For sale by the Superintendent of Documents, U.S. Government Printing OfficeWashington , D .C .

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How�sYour House?

SOME HOUSES built of woodendure for centuries; yet othersdevelop decay problems soon afterconstruction. Why? Because wood isa biological material. Used properly,it doesn’t deteriorate. Misused, itsuccumbs to the same biologicalprocesses that decompose deadtrees in the forest. In other words,it may be rotted by fungi or eaten bytermites and other insects. In theforest, decomposition serves aworthwhile purpose, but to thehomeowner it means a repair bill.

This booklet explains why wooddecays. It will alert you to the conditionsthat create decay hazards in buildingsso you can prevent future damagein your current home or plannedconstruction. As you will find, simpleprocedures often provide remarkableprotection. Other times, moredrastic repairs are needed. Butwhatever the damage. it willonly get worse until youfind the fault and correct it.

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What is Wood?

Wood is composed of many thick-walled cells, cemented together.

What is Decay?

First, it’s important to know that the wood and attack its thick cellwater and the elements do not decaywood by any direct action on their

walls. Given the necessary environmentof moisture and air to thrive, these

part. Decay is caused by microscopic,thread-like fungi which move into

fungi in time will destroy enoughcell walls to weaken the wood.

Some fungi cause a brownish discoloration, in whichcase the wood readily breaks into small cubes. Thisis called BROWN ROT.

Other fungi produce a bleached appearance whichmay be surrounded by very dark lines of discoloration.This is known as WHITE ROT.

What is Decay? Fungi NeedWater and Oxygento Survive.

continued

When decay is well advanced,the fungi produce fruiting bodieson wood. These are the repro-ductive bodies. They producespores (like seeds of floweringplants) which, when released,are blown about by the wind. Ifthey alight on moist wood, theymay germinate and start anotherdecay process.

Decay fungi need at least a traceof water on the surface of wood cellwalls to grow, without water, decaycan’t occur. Thus, buildings shouldbe designed to minimize the wettingof wood or to expedite the dryingof wood when wetted by rain.

The cliff dwelling ruins at Kiet Sielin northern Arizona (below) date backto the 13th century. Dwellings in thissemi-arid region are further shelteredfrom occasional precipitation by cliffsoverhead. Wood beams and super-structure have remained sound forcenturies because they have always

If Wood Can�tBe Kept Dry

The best way to prevent wooddecay in houses is to protect thewood from excessive moisture. Butsometimes wood must be used whereit will be continually wetted. In suchcases, use wood that resists decay:

Preservative-treated wood.Pressure treatments force woodpreservative chemicals into the wood.Such treatments are designed eitherfor use in the ground or for useabove ground. It’s important thatyou specify the right treatment foryour specific need. In addition, insistthat the treatment be of certifiedquality and that the treated woodis labeled accordingly.

For some uses, pressure treatmentis not necessary. Assembled millworkitems such as window frames andsash are usually sold already treatedwith a water repellent preservative.This treatment doesn’t penetratedeeply into the wood, but it preventsdecay by retarding water absorption.The repellent also contains a smallamount of preservative to preventfungal growth.

Naturally decay-resistant wood.Heartwood (wood in the centralpart of the tree trunk) of severaltree species is naturally resistantto some decay fungi, and is suitablefor some uses. California redwoodand western red cedar are theprincipal decay-resistant woods usedin construction today.

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8 Chief Sources of MoistureLeading to Wood Decay

SOIL CONTACT

SPLASHINGRAIN

FREQUENT RAIN SEEPAGE WATER FLOWINGRAINS OFF THE ROOF

WATERCOLLECTING

AGAINST WOOD

PLUMBINGLEAKS

CONDENSEDMOISTURE

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Rain Seepage

Decay sometimes occurs only where boards or beams arejoined together (end to end), while the rest of the woodstructure remains sound. One reason is that water gets trappedin the joints. Also, the ends of boards or beams absorb watermuch more rapidly than do the sides. This happens becauseof the arrangement of wood cells.

In the living tree, these cells conducted water up the stem.Because they were oriented lengthwise in the stem, they aresimilarly oriented lengthwise within boards and timbers cut

Water running around roofedges and onto wood belowis absorbed most rapidly atcut ends of wood trim andbeams. Hence, paint failureand decay begin at thesepoints.

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C r a c k s t h a t o p e n a s w o o ddries out permit serious rain-wetting of central portions ofl a r g e b e a m s . I n r e g i o n s o fh i g h d e c a y h a z a r d , b e a m sthat are exposed to weathers h o u l d b e p r e s e r v a t i v e -treated.

Rain Seepagecontinued

from the tree stems. Each of the many thousands of thesecells, which are exposed at the cut ends of boards, functionssomewhat like a soda straw. Water moves much more rapidlyup through the length of each cell than it does through thethick walls. Hence, water is absorbed rapidly from the endsof cut timbers, but only slowly through the sides.

In thin wood materials such as exterior siding or facia boardsat the edges of roofs, water is absorbed mostly through theends. In larger beams, however, water is absorbed two ways:through the ends and through small checks in the upper,weathered surface which collect water during each rain. Asthe wood wets, it swells and the checks close; hence dryingis retarded. Similarly. exposed beams, comprised of several

A small metal cap on top oft h i s e x p o s e d b e a m d i d n o tprovide adequate protect ionf r o m w o o d d e c a y i n a h i g hr a i n f a l l a r e a . R a i n w a s a b -sorbed both through the endsand be tween the ind i v idua lt i m b e r s t h a t c o m p r i s e t h eb e a m . S e r i o u s d e c a y i s e v i -dent by fruiting bodies of de-c a y f u n g i g r o w i n g f r o m t h eend of the beam.

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layers of smaller timbers nailed together, may be wetted bywater that creeps in between each layer. This moisture, whichdries slowly, contributes to a decay hazard. Large, load-bearingtimbers that are exposed to weather should be preservative-treated. Their top surfaces should be flashed to prevent waterentering through checks.

Absorption of water at the ends of siding is most importantin the high decay hazard zones, especially where storms comepredominantly from one direction. Butting siding against avertical trim provides maximum opportunity for rain seepageinto the siding. Capping the ends of wood siding with metalcorners provides excellent protection against end-grainedabsorption.

Caps over the siding at build-ing corners (B) provide great-e r p r o t e c t i o n a g a i n s t w a t e rabsorption than mitered (A)or butt joints against a verti-ca l edg ing s t r i p (C) , wh ichp e r m i t e x c e s s i v e a b s o r p t i o nof water.

S a s h , d o o r a n d w i n d o wf r a m e s t h a t a r e m a d e o f adecay-resis tant wood or pre-s e r v a t i v e - t r e a t e d w o o d a r ec o m m e r c i a l l y a v a i l a b l e .C h e c k c a r e f u l l y b e f o r e y o ubuy.

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Water FlowingOff the Roof

Many wood decay problems are caused by roof constructionthat doesn’t allow rain to move off the roof and away fromthe house.

In some houses, water flow over the roof is interrupted bychimneys. dormers, or adjoining walls. Such areas ofconcentration need flashing to make the roof watertight.Even slight leaks can produce serious decay problems becausea large supply of water is available with each rain, and woodwetted by the leak has little opportunity to dry rapidly. Adequate D e c a y r e s u l t s w h e n w a t e r

flashing should be installed in such areas.from low-pitched roof drainsover wood below. Decay ap-

Wood trim at the edge of the roof is continually exposed tomoisture and is prone to decay, particularly in the area of

p e a r s f i r s t i n p l a c e s w h e r ewood is joined together, dueto end-grained absorption.

R u n o f f f r o m t h e l o w e r r o o fflowed against the joists tar-row) suppor t ing th i s porchroof. Water creeping betweenthe individual t imbers of thejois t promoted a serious de-cay problem which extendedin to the house p roper , e venaffecting the ceiling over theb e d r o o m b e h i n d t h e f r o n twall.

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I f sh ing l e s don’ t ex t end j a renough beyond facia board,w a t e r t h a t c u r l s u n d e r t h eshingle wil l drain over woodtrim at roof edge. This leadsto paint failure and decay.

Water FlowingOff the Roof continued

high decay hazard. Water tends to curl under the shingles andflow back towards the trim. Thus, shingles must be extendedfar enough over the edge and metal edgings must be carefullypositioned so that water dripping from the roof will clear thetrim. Leaking and overflowing rain gutters further wet the trim.Homeowners should keep rain gutters open and in goodrepair. In the high decay hazard areas, use of decay-resistantor preservative-treated wood for roof trim is recommended.This is especially true for roofs with low pitch.

Several approaches can min-imize this problem:

U s e p r e s e r v a t i v e - t r e a t e dwood in roof trim.

I n s t a l l f a c i a b o a r d a t r i g h tangles to slope of roof.

Extend shingles jar enoughbeyond roof edge and instal lmetal edging so that drip linefrom roof will clear wood trim.

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Plumbing Leaks

Obviously, wood that is wetted continuously by faultyplumbing will decay. Spillage behind a washing machine, leaksat the top of a built-in tub, or leaks in a shower stall may gounnoticed for a long time. If so, they can lead to a seriousdecay problem in the floor and lower parts of interior walls.Annual inspections and minor home maintenance can preventthis type of problem.

C o n d e n s a t i o n o n j o i s t s o ff l o o r s o v e r e n c l o s e d c r a w ls p a c e c a n b e c o n t r o l l e d b ycovering the soil with a vaporbarrier.

Condensed Moisture

Many houses are built over an enclosed crawl space. Ifventilation is poor or if the soil is particularly moist, condensationproblems can develop in the floor. This is particularly true insome regions (see map). The hazard is accentuated whenvents in the perimeter walls are closed in the winter. Duringthe cooler months, moisture from the soil beneath the housecondenses on subfloor timbers, which become wet enough todecay. If condensation is left unchecked, sills and joists canrot out in 5 to 15 years. But this problem can be preventedby placing a vapor resistant covering, such as heavy roll roofingor a polyethylene sheet, over the soil in the crawl-space area.Where soil covers have been used, closing the crawl-spacevents to conserve heat has not created condensation problems.

Not all houses need a soil cover, nor will such a cover beeffective against improper surface drainage. A soil cover doeslittle good under a house if water collects on top of themembrane.

Climatic conditions are apt tocause condensat ion dur ingw i n t e r m o n t h s i n s o m e s e c -t ions of the country. Groundcovers are recommended forhouses wi th enc lo sed c rawls p a c e s i n t h e d a r k s e c t i o n sshown here.

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Does Painting ProtectWood from Decay?

K e e p i n g t h e p a i n t s e a l a twood junctures intact is al l-i m p o r t a n t . T h e s e a r e t h epoints where moisture creepsinto the wood most readily.

Coatings that form a moisture-imperviousfilm are effective in preventing decay ifthe seal where wood units are joinedtogether is kept intact. A well-maintainedfilm of paint over the wood and goodpaint seals in the joints shed water, thusprotect wood by keeping it dry. But ifpaint seals permit entry of water, moisture-impervious coatings contribute to decay.Water seeping through the broken sealswhere wood is joined together will be

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rapidly absorbed at the ends of the woodpieces. Its loss by evaporation from thesides of the wood member is retarded bythe impervious, intact paint film over therest of the wood. This keeps the woodmoist and favors decay, at least near thejoints. Thus, the effectiveness of a moisture-impervious paint in protecting wood fromdecay depends upon the quality of theseal in the joints.

Wood Fences

The same principles of decay that areapplicable to buildings also apply towooden fences.

The greatest decay hazard exists at theground line. For posts, either wood pressuretreated for ground contact or naturallydecay-resistant wood may be used in theregion of low decay hazard. Elsewhere,pressure-treated wood approved for usein the ground is recommended.

A lesser, but still important, decay

hazard exists where horizontal rails arejoined together and where vertical boardsfasten to the rails. Both of these locationscollect water and dry slowly. Use naturallydecay-resistant or preservative-treatedwood for horizontal rails.

Keep vertical boards off the ground. Ina high decay hazard area, use pressure-treated wood or naturally decay-resistantwood.

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For over 75 years, the U.S. ForestService through its 8 regional researchstations has overseen forestry researchprograms aimed at protecting, managing,using, and replenishing our forestresources.

Besides such research, the ForestService cooperates with state agenciesand private forest owners to stimulateproper management practices and toprotect the 440 million acres of theirforests against fires, insects, and diseases.

The Forest Service also manages 187million acres of public land, including154 National Forests and 18 NationalGrasslands in 43 states and Puerto Rico.Each National Forest resource - timber,water, forage, fish and wildlife, andrecreation - is managed to produce aharmonious environment along withtangible benefits for all of America’speople.

Photo credits, Photographs of the ParsonCapen, Henry Grady, Baldwin and JonathanAshley houses were loaned by the AmericanWood Council. Washington, D.C. Printsof the Fairbanks House, and the MarrsLog House were provided by theLibrary of Congress. Photograph of the KietSiel cliff dwellings is property of Laboratoryof Tree-Ring Research, The University ofArizona. The National Maritime Museum,Stockholm, Sweden provided the photographof the WASA. Photograph of subfloor decayon Page 8 was furnished by Small HomesCouncil, University of Illinois at Urbana-Champaign.

Rear coverphoto: Front door-w a y t o J o n a t h a n A s h l e yHouse, Deerfield, Massachu-setts, built in 1730.