Basics of Wood Inspection: Considerations for Historic Preservation

W OODPractice PointsNUMBER03Basics of Wood Inspection:Considerations for Historic PreservationRONALD W. ANTHONYIntroductionWood performs well in structures, often for hundreds ofyears or more, when it is kept dry and protected fromthe deleterious effects of moisture and biological deterioration.The open construction typical of historic structures,which provides air infiltration into the structure,makes it possible for the wood to dry quickly if it getswet. Often this type of construction is criticized for permittingdrafts through walls and around openings. On theother hand, this characteristic means that older buildingscan breathe in such a way that if wood gets wet, ithas a chance to dry. Such openness, of course, is notvery energy efficient and has been largely eliminated inmodern construction.However, sometimes moisture does get trapped, orthe strength requirements of members change. Alterationsmay have compromised the ability of the wood todry quickly or may have overloaded the structural members.Architects, engineers, and owners need to knowwhen deterioration has occurred and whether the currentstrength of the structural members, even when they areoversized and provided more strength than required forthe original use, is adequate for implementing a preservationplan for the structure. Wood inspection providesthe means to acquire that information.There are three primary reasons to conduct a woodinspection: concerns about moisture and its effects,deterioration (both physical and biological), and a needto determine material properties. Wood behavior is highlyvariable, due to different wood species, rate of treegrowth (typically measured in growth rings per inch), ageof the tree, how the lumber was cut from the log, thepresence of defects (such as knots), and end-use conditions(interior or exterior use). It is that variability relativeto the use of wood that those working on such structuresmust understand. This article provides the readerwith a brief description of how to conduct a basic woodinspection in a historic structure and why such aninspection may need to be done. A list of referenceswith additional information on each topic is provided.The Need for Wood InspectionConcerns about moisture. Prolonged exposure to moisturecan produce undesirable conditions and long-termmaintenance issues for wood in a structure, includingmoisture stains, peeling paint, and warping of lumberand timber. Stains can be the result of a single wettingor of periodic wetting and drying. A leak that wasrepaired many years ago may have resulted in a stainthat has not affected the wood in any substantive way.Such stains are of no consequence structurally and canbe ignored, unless aesthetics warrant a repair. In othercases, stains may be the result of periodic leaks in roofsor walls, which may lead to more serious problems, suchas decay or warping. Therefore, it is important to determinewhether a stain is the result of an isolated historicevent or the result of active leaks and ongoing moistureintrusion (Fig. 1). Decay and insect attack are significantproblems associated with periodic leaks or moistureintrusion. Measurements of moisture content can identifywood that has moisture levels favorable for the growth ofwood-decay fungi.Fig. 1.Moisture stain ona sill plate on top of amasonry dome. All photographsby the author.

Fig. 2.Measuring the slopeof a drying check on apainted column by using anacetate grid.Fig. 3.Failure of a joist in asilver-mining mill.Concerns about deterioration. Deterioration of woodcan be the result of physical processes (weathering,failure due to overload, mechanical damage, or shrinkage)or biological processes (decay and insect attack).Structural timber will typically have checks on one ortwo faces, which are due to differential shrinkage of thetimber and are part of the natural process as the wooddries (Fig. 2). A check is a separation of wood fibers ina piece of lumber, post, or timber, typically along thelength of the piece, that results from the drying of thewood after processing or installation in a structure.Checks are not a defect and do not reduce the performanceof the piece structurally, unless two checks onopposite faces join to form a through split. However, ifthe timber has split through the entire thickness, amore detailed investigation is necessary to determinewhether the split is a failure resulting from overload ormechanical damage or whether it is associated withshrinkage around connectors (typically bolts) (Figs. 3and 4). Differential shrinkage in mortise-and-tenonjoints can result in failure of the joint that is restrictedby the treenail (wooden peg).Weathering of wood results from cyclic wetting anddrying of the wood, exposure to ultraviolet light, and erosionby wind-blown debris, a process similar to sandblasting.Unlike decay or insect attack, weathering istypically not a significant factor in the failure of woodcomponents and the collapse of a structure. Weatheringwill change the appearance of wood, but theprocess is so slow that failure of components due todecay generally occurs long before weathering becomesa major factor in the failure. Weathering seldom damagesthe wood enough to require replacement, with theexception of shingles and cladding. Weathered wood isoften considered aesthetically pleasing (Fig. 5).Biological deterioration is generally caused by fungalor insect attack. Bacteria can degrade wood but aregenerally not a concern with historic structures becausedecay fungi and insects tend to impact material propertiesmore rapidly than bacteria. Thus, the focus of awood inspection for biological deterioration is typicallyfungi or insect activity.All wood is subject to a variety of deterioration mechanisms,the most prevalent being wood-decay fungi,which can ultimately lead to the inability of structuralmembers to perform. Depending on wood species, largetimbers will retain moisture internally, frequently causinginterior rot with no visible sign of the deterioration onthe surface of the wood. Moisture absorption thoughend grain, checks, or holes provides a highly favorableenvironment for decay fungi to attack the heartwood atthe center of a large timber. The heartwood (the innergrowth rings of the tree) typically has more decay resistancethan the sapwood (the outer growth rings of thetree). However, even the heartwood of naturally durablespecies such as chestnut will decay when exposed toenough moisture. Deterioration is a particular concernwhere the wood is in contact with the ground or withother materials, such as porous masonry, that mayallow for moisture to be absorbed into the wood.Fungi associated with wood include mildew and stainand decay fungi. Fungi propagate from spores presentin the air. Mildew grows on the surface of wood andpaint and does not affect the strength of the wood.Stain fungi (not to be confused with moisture stains)penetrate the surface of the wood but do not reduce itsstrength. Decay fungi, however, break down wood componentsover time. All types of decay fungi — brown rot,white rot, and dry rot — affect the ability of wood toperform its intended function. Although identifying thespecific fungus during wood inspection is not essential,identifying the location and extent of deterioration dueto decay fungi is important.Generally, if the moisture content of the wood is lessthan 20 percent, fungi are unable to grow. Areas withmoisture contents between 20 and 30 percent can supportthe growth of fungi, but the moisture may not besufficient to support long-term active decay. Moisturecontents between 30 and 40 percent are highly favorablefor active fungal growth and are often an indicationof advanced decay, with symptoms that may includeinternal voids and surface deterioration. Highly saturatedwood (with more than 60 to 80 percent moisturecontent) typically has insufficient oxygen for fungus togrow; therefore, the decay may be inactive. Insects generallyrequire that moisture be greater than 10 percentto be active and deteriorate the wood. In addition tofavorable moisture content, both fungi and insectsrequire suitable temperature and oxygen, as well as thewood as a food source. However, moisture is the controllingfactor for decay in historic structures.The early stage of decay, known as incipient decay, ischaracterized by discoloration and an initial loss of2 PRACTICE POINTS 03

integrity of the wood. No voids are present. Probing withan awl or a screwdriver may reveal that the surface ofthe wood is soft or punky. As the decay progresses, thewood integrity deteriorates until small voids develop.This stage is termed intermediate decay. The smallvoids continue to extend primarily along the wood grain,where it is easier for moisture to move through thewood, but can also extend across the grain. Larger voidsdevelop where the decay originated, and the boundariesof the decay continue to extend, reducing the integrity ofthe wood and compromising its structural capacity (Fig.6). At this advanced stage, termed advanced decay, simpleprobing with an awl or a screwdriver is unlikely todetect the hidden deterioration in internal voids. Anincrement borer or a portable hand drill may be used toexamine wood removed from the interior of larger timbers.However, more advanced techniques, such asresistance drilling, are able to quantify the extent ofdeterioration rather than simply identify its presence.Termites and wood-boring insects reduce the crosssection of a wood member by either digesting or tunnelingthrough the wood. Subterranean and drywood termitesdigest the wood as they move below the surfaceof the wood. Termites can often be detected through thepresence of mud tubes on the exterior of either thestructure or the individual wood members. The tubesallow the termites to maintain a favorable moisture environmentas they move towards a new food source.Wood-boring beetles create holes that are packed withfrass (the byproduct of the tunneling process). Carpenterants and bees leave large clean tunnels in affectedwood.With decay, there is a definite progression from soundwood to punky wood to a total loss of wood fiber, i.e., avoid. Unlike decay, insect damage tends to have anabrupt transition between affected and unaffected areasof the wood. Wood that has not been penetrated byinsects retains its structural integrity, although there canbe a loss of cross section. The loss of cross sectiondirectly relates to the load-carrying capacity of the memberby reducing the volume of wood available to carryloads. The reduction in cross section, if known, can betaken into account by engineers to determine whetherthe affected member can still carry the required loads orneeds to be reinforced or replaced. For both types ofdeterioration, moisture is generally required, and theresult is a loss of integrity of the wood member, as wellas a loss of cross section (Fig. 7).Concerns about material properties. Material propertiesare important when wood components carry loads. Todetermine whether the existing wood can carry therequired structural loads, it is important to know theappropriate strength and stiffness properties of thewood. Identification of the wood species is also animportant factor, not only to calculate its strength andstiffness but also to determine whether there will be significantdifferential shrinkage if other woods are used forrepairs.Although some individuals can identify wood speciesin the field using a hand lens, the most reliable meansto accurately identify species is done by examininganatomical features of the wood under a microscope.Individuals who want to learn how to identify woodspecies themselves can purchase wood samples andidentification handbooks. Wood scientists and wood conservatorswill identify wood species for a fee. The U.S.Forest Products Laboratory in Madison, Wisconsin, willidentify up to five samples without charge, but it typicallytakes several weeks to get the results.For new wood construction, structural engineers relyon design values referenced in building codes to determinean acceptable species, size, and grade for a particularload condition. For existing buildings, engineersoften rely on current building codes and standards todetermine adequacy of the wood members. However,current standards are generally based on lower-qualitymaterial than what is found in many historic structures.Since many older buildings were constructed beforebuilding codes or design values for wood products wereestablished (and, thus, before grade stamps were used),engineers inexperienced with historic structures or materialsare often in a quandary when determining whatdesign values are appropriate. Frequently a species andgrade are assumed, leading to wood members beingdeclared structurally deficient. The result is often anoverly conservative estimate of design values andunnecessary replacement, repair, and retrofit decisions,with the associated unnecessary project costs.Although estimates of strength and stiffness can bemade by knowing the species, establishing the grade,and therefore the design properties, is more involved. Insitugrading should not be conducted unless the individualis familiar with the appropriate standards and hasFig. 4.Split between boltscaused by wood shrinkage.Fig. 5.Weathered cladding,considered aestheticallypleasing.PRACTICE POINTS 03 3

Fig. 6.Decayed beam supportinga timber column.Fig. 7.Insect damage on a porchbeam, as evidenced by theyellow frass from powderpost beetles.Fig. 8.Awl inserted to probe theinterface between a timberbeam and a masonry wall.the knowledge of how grades are established within thedesign codes.Tools for Basic Wood InspectionThere are three “tools” for a basic wood inspection:visual inspection, a sharp probe, and a moisture meter.Equipment used for nondestructive evaluation can givemuch more information about wood condition, but theuse of such tools should be reserved for situationswhere a basic inspection cannot sufficiently answer thequestions of the architect, engineer, or owner. An individualexperienced in wood inspection may also use ahammer for sounding or a portable hand drill to gaininformation about the relative condition of the wood,although neither of these methods allows for quantifyingthe extent of deterioration: they are best suited foridentifying locations that warrant further investigation.A sharp probe will also not allow for quantifying theextent of deterioration in larger members, but it easilydetects areas of surface deterioration and should beincluded in any wood-inspection tool kit.A visual inspection allows for identifying componentsthat are missing, broken, or in an advanced state ofdeterioration. Missing components are those that havebeen removed or have fallen away, frequently due toextensive deterioration. If missing components wereintended to provide structural support or protectionfrom the elements (e.g., to prevent moisture intrusion),their replacement may be essential to prevent long-termdamage to the structure. This problem often manifestsas roof leaks in historic buildings. A small mirror with atelescoping handle and a flashlight are useful wheninspecting relatively inaccessible areas.Visual inspection also allows for the detection ofpast or current moisture problems as evidenced bymoisture stains on the exposed surface of the wood.Further, visual inspection enables detection of externalwood-decay fungi or insect activity as determined by thepresence of decay fruiting bodies, fungal growth, insectbore holes, mud tubes, or wood removed by wooddestroyinginsects. Visual inspection provides a rapidmeans of identifying areas that may need further investigation.Probing the wood with a sharp pick or an awl enablesrapid detection of voids in the wood that may not bevisible on the surface. Internal decay is often maskedby the lack of evidence on the exposed surface of thewood. For advanced decay where large internal voidsare present near the surface, probing can detect evidenceof potentially serious deterioration. For internalvoids in large timbers more advanced inspection methodsare generally required to detect the void. Even forthe early stage of decay, probing can reveal areas thathave experienced sufficient deterioration due to decayfungi by allowing for easy entry of a sharp probe, althoughno void is yet present. Wood without incipientdecay tends to offer more resistance to probing, due toits higher density and more intact internal wood structure(Fig. 8).The true moisture content of wood can be determinedonly by oven drying a sample removed from astructure. However, portable moisture meters usingelectrical capacitance or electrical conductance can displaythe approximate moisture content of wood.Capacitance-type meters measure the electrical fieldwithin a small area of a piece of wood (Fig. 9). They donot require penetration of probes into the wood andgenerally provide the average moisture content throughouta certain depth, typically less than an inch; however,a wet surface (e.g., rain on a window sill) can dramaticallyaffect the reading. These meters are particularlyuseful for measuring the moisture content of interiorand exterior woodwork (doors, windows, trim, etc.) anddimension lumber.For thicker material, such as structural timber, a conductancemeter, often called a resistance moisture4 PRACTICE POINTS 03

meter, will provide a better indication of the internalmoisture content. A conductance-type meter conductselectric current through wood between two probes (Fig.10). The probes, which come in different lengths, can beinserted into the wood to various depths, thus allowingfor determining the moisture content at different depthsof larger timbers. This technique is useful in determiningwhether wood is drying or taking up moisture.Advanced Investigative TechniquesIn the interest of saving the maximum amount of historicfabric while not altering or scarring the materialsduring investigative probes, preservationists often lookto nondestructive testing methods to answer questionsabout the evaluation and identification of materials, conditions,and alterations made to structures over time.However, they are not part of a basic wood inspectionand are not generally needed for most wood assessments.When the information revealed from a basicinspection is insufficient to make informed decisions,then advanced investigative techniques should be considered.Only a few nondestructive techniques haveproved useful in historic-preservation projects. The mostcommonly used methods are:Fig. 9.Capacitance-type moisturemeter being used on windowsill.Fig. 10.Conductance-type moisturemeter being used on a timbercolumn in contact withthe ground.Fig. 11.Timber sill and wood claddingin contact with stonefoundation.• stress-wave analysis, used to locate advanceddecay• resistance drilling, used to quantify the loss ofmaterial due to decay or insect damage• digital radioscopy, used to view hidden conditionsand constructionWhere to LookDetermining the condition of wood components is themost common reason for conducting an inspection.Knowing what parts of a structure to inspect and whattools to use depends on the goal of the inspection. Theinspection should begin with looking for problems wherethey are most likely to occur in a structure. Missing orfailed components, moisture stains, the presence of fungalfruiting bodies, decayed wood, insect bore holes,mud tubes, or frass are indicators that need closerinvestigation. An inspection should focus on likely problemareas, such as:• wood in contact with the ground• wood that exhibits moisture stains• wood with visible decay• roof penetrations, such as around chimneys and vents• attic sheathing, framing lumber, and timbers• sill beams and wall plates, particularly when they arein contact with masonry• floor joists and girders, particularly where they rest onexterior walls• openings (doors and windows)• material interfaces, such as wood and masonry,particularly beam pockets (Fig. 11)• exterior woodwork, including cladding, shingles, andsoffits• porches• crawl spaces and basements• areas of the structure that have been alteredIt is essential to remember that the purpose of theinspection is to provide data that can be used to answerquestions raised by the architect, engineer, or ownerabout the condition of the wood. If the wood has moisturestains, are the stains recent, as indicated by highmoisture-content readings, or is the wood sufficiently drythat the stain likely occurred long ago? If decay is present,can it be active, as indicated by moisture-contentreading greater than 20 percent, or is the decay fungusdormant? Are splits due to normal drying checks, or is itan indication of failure of that component? If so, wasthe failure due to loads exceeding the capacity overtime, or could it be due to a one-time occurrence? Theinspector should ask these types of questions. Soundtechnical data about the current condition of the woodare necessary for effective repair and replacementPRACTICE POINTS 03 5

decisions to be made. Such data are the result of athorough wood inspection.RONALD W. ANTHONY, wood scientist for Anthony andAssociates, Inc., received M.S. and B.S. degrees in woodscience and technology from Colorado State University.He focuses on assessment of timber structures. In 2002he received the James Marston Fitch Foundation Grant forhis approach to evaluating wood in historic buildings.Additional ReadingAnthony, Ronald W. “Condition Assessment of Timber UsingResistance Drilling and Digital Radioscopy.” APT Bulletin 35, no. 4(2004): 21–26.American Society of Civil Engineers. Evaluation, Maintenance andUpgrading of Wood Structures: A Guide and Commentary. New York:American Society of Civil Engineers, 1982.American Society of Civil Engineers. Guideline for StructuralCondition Assessment of Existing Buildings. Reston, Va: AmericanSociety of Civil Engineers, 2000.Beckman, P. Structural Aspects of Building Conservation. London:McGraw-Hill Book Co., 1994.Charles, F. W. B., and M. Charles. Conservation of Timber Buildings.Dorset, England: Donhead Publishing Ltd., 1984.Collings, J. Old House Care and Repair. Lower Coombe, England:Donhead Publishing Ltd., 2002Evers, C. The Old-House Doctor. Woodstock, N.Y.: Overlook Press,1986.Feilden, B. M. Conservation of Historic Buildings. Oxford andBoston: Reed Educational and Professional Publishing Ltd., 1994.Forest Products Laboratory, Forest Service and U.S. Dept. ofAgriculture. Wood Handbook: Wood As An Engineering Material.Washington, D.C.: U.S. Government Printing Office, 2000.Friedman, D. The Investigation of Buildings: A Guide for Architects,Engineers, and Owners. New York: W. W. Norton and Co., 2000.Harris, S. Y. Building Pathology: Deterioration, Diagnostics, andIntervention. New York: John Wiley and Sons, 2001.Hoadley, R. B. Identifying Wood: Accurate Results with Simple Tools.Newtown, Ct.: Taunton Press, 1990.Hoadley, R. B. Understanding Wood: A Craftsman's Guide to WoodTechnology. Newtown, Ct.: Taunton Press, 2000.Hutchins, N. Restoring Wooden Houses. Buffalo: Firefly Books,1999.Larsen, K. E., and N. Marstein. Conservation of Historic TimberStructures: An Ecological Approach. Oxford: Butterworth-Heinemann,2000.Nash, G. Renovating Old Houses. Newtown, Ct.: Taunton Press,1992.Newman, A. Structural Renovation of Buildings: Methods, Detailsand Design Examples. New York: McGraw-Hill, 2001.Oliver, A., J. Douglas, and J. S. Stirling. Dampness in Buildings.2nd ed. Oxford: Blackwell Science Ltd., 1997.Panshin, A. J., and C. de Zeeuw. Textbook of Wood Technology,Vol. 1. New York: McGraw-Hill Book Co., 1970.Pellerin, R. F., and R. J. Ross. Nondestructive Evaluation of Wood.Madison, Wisc.: Forest Products Society, 2002.Ridout, B. Timber Decay in Buildings: The Conservation Approach toTreatment. London: E. and F. N. Spon, 2000.Robson, P. Structural Appraisal of Traditional Buildings. Hants,England: Gower Technical, 1991.Robson, P. Structural Repair of Traditional Buildings. Dorset,England: Donhead Publishing Ltd., 1999.Ross, P. Appraisal and Repair of Timber Structures. London: ThomasTelford Ltd., 2002.Ross, R. J., B. K. Brashaw, X. Wang, R. H. White, and R. F. Pellerin.Wood and Timber Condition Assessment Manual. Madison, Wisc.,Forest Products Society, 2004.Singh, J., ed. Building Mycology: Management of Decay and Healthin Buildings. London: E. and F. N. Spon, 1994.Watt, D. S. Building Pathology Principles and Practice. Oxford:Blackwell Science, 1999.Watt, D., and P. Swallow. Surveying Historic Buildings. LowerCoombe, England: Donhead Publishing Ltd., 1996.Weaver, M. E., and F. G. Matero. Conserving Buildings: Guide toTechniques and Materials, Revised Edition. New York: John Wileyand Sons, 1997.Williams, R. S., M. T. Knaebe, and W. C. Feist. Finishes for ExteriorWood: A Comprehensive Guide to the Painting/Staining andMaintenance of Homes, Decks, Log Structures and More. Madison,Wisc.: U. S. Dept. of Agriculture, Forest Products Laboratory, 1996.Practice Points presents basic information on technical topicsrelated to preservation practice as an introduction forthose new to the field or as a refresher for more seasonedprofessionals.© 2007 by the Association for Preservation TechnologyInternational. This Practice Point originally appeared inVol. XXXVIII, No. 2-3, of the APT Bulletin, The Journal of PreservationTechnology. Reprint requests should be submittedin writing to the Association for Preservation TechnologyInternational, 3085 Stevenson Drive, Suite 200, Springfield,IL 62703, or to info@apti.org.The Association for Preservation TechnologyInternational3085 Stevenson Drive, Suite 200Springfield, IL 62703217.529.9039fax (toll free) 888.723.4242administration@apti.orgwww.apti.org