Basics of Wood Inspection: Considerations for Historic Preservation

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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
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Basics of Wood Inspection: Considerations for Historic Preservation

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