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Swept Under the Rug

The duration and extent of a water loss can be established by studying the lowly carpet tack strip.

May 25, 2010 Photo
Carpet tack strips are among the most humble of building products, yet they offer one of the most effective tools for determining the duration and extent of a water loss. Research using carpet tack strips that were exposed to either continuous or cyclical wet-dry periods of moisture exposure revealed visual and microbial clues that prove effective in differentiating the duration of a loss and establishing whether the loss originated from a single event or repeated ones. A study of carpet tack strips will tell the water-release history of a damaged structure and guide adjusters to more precise and defensible evaluations of water-related claims.

The duration of loss is a key question on claims when the policy's language states that coverage is limited to a duration of less than two weeks. Since only a few materials are consistently wetted after a water release, an adjuster must use those limited resources to establish a time frame. Wood flooring and sheathing, carpet and padding, sheetrock, cabinet particleboard and carpet tack strips offer opportunities to evaluate absorption patterns and sequences of deterioration when a water loss occurs.

An experiment on water-treated carpet tack strips revealed that nail stains, wood discoloration and microbial growth occurred within a few days of exposure, while dark discolorations, composite wood delamination and the emergence of different fungal species occurred later. The experiment catalogued the response of carpet tack materials to moisture exposure over a period of 100 days.

The Experiment
Carpet tack strips are wood laminates composed of multiple plies (usually an odd-numbered total), with the outer plies oriented parallel to the length or long dimension of the panel. The alternation of grain direction in adjacent plies provides dimensional stability across the width. Tack strips are most commonly made of birch, fir or poplar. Tack strips are cut into 7/8-, 1-, and 1¼-inch widths with the wider strips constructed with three rows of coated pins. The support nails are made of hardened high-carbon steel (10-14 gauge) to penetrate wood and concrete floors and are coated to minimize rust stains.

The research conducted obtained observations on the gradual deterioration of two different types of carpet tack strips under continuous moisture exposure and cyclical moisture exposure. One set of 12 samples—six manufactured by Halex (made of White Birch) and six made by Roberts (made of Douglas Fir)—was exposed to continuous moisture, while the second set (also six Halex and six Roberts) experienced cyclical wet-dry cycles—seven days wet and seven days dry—over a total test period for both sample sets of 100 days.

During the study, the moisture content of the carpet tack strips ranged from 7% to 16% for the dry cycle and 23% to 38% for the wet cycle. The relative humidity was maintained between 37% and 45% for the dry cycle and 63% and 84% for the wet cycle. The temperature was held relatively constant for both, between 73°F and 76°F. The changing appearance of the test strips was photographed and compared to controls (no moisture). Moisture measurements were obtained using a Tramex Penetrating Moisture Meter. Sections of Olefin carpet covered the test strips, and a cotton towel was placed beneath the strips with tap water added to maintain moisture. During the dry cycle, the tack strips were removed from their covered plastic container and allowed to air dry.
Four elements of deterioration were documented. A summary of the results is presented in Table 1 and Table 2.

The Halex brand (birch) carpet tack strips that were exposed to continuous moisture exhibited gradual darkening from the initial appearance to near black after 100 days. The Roberts brand (fir) carpet tack strips responded similarly (see Figure 1). Both sets experienced moisture saturation and gradual darkening from moisture exposure, followed by a gradual increase in microbial growth. In comparison, carpet tack strips exposed to wet-dry cycles showed less discoloration after 100 days (see Figure 2).

Rust on the pins and support nails was observed after the first day, and rust on the adjacent wood the second day. In both exposure scenarios (continuous and cyclical), the nails exhibited gradual oxidation, rust stains and darkening to a black appearance after 100 days.

Visible microbial growth (spotting) was observed after three days of moisture exposure (see Table 1). The initial microbial growth consisted of mycelia. That growth was followed by the development of Aspergillus/Penicillium-like spores. These fungal species predominated for the first 28 days. Sometime between Day 28 and Day 35, Chaetomium spores became evident and were the predominant fungal specie thereafter. Carpet tack strips exposed to the wet-dry cycles responded similarly (see Table 2). The dark discoloration observed at the end of the test period was primarily attributed to dense microbial growth.

Separation between the layers of wood laminate was a key distinction between the effects of the continuous and cyclical exposures. Carpet tack strips exposed to continuous moisture did not separate, but carpet tack strips exposed to four or more wet-dry cycles did (Photo 1).
Delamination Understood
Wood is a hygroscopic material: It absorbs water from humidity in the air as well as from contact with free water. Under continuous moisture conditions, the adhesive durability and strength of the plies are affected not only by the surface adhesion, but, more important, by the dimensional strains that occur as the moisture content increases and the wood expands.

The swelling of wood fibers expands the wood in all directions. Similarly, when wood dries, it contracts. Both expansion and contraction cause stress and strain along the bond lines between the adhesive and the wood. The expansion of wood creates tensile (expansion) strains between cellulose fibers located on the outside that are swelling as opposed to fibers that have not yet become wet. Similarly, when the wood begins to dry, compressive stresses are created. Separation begins to occur when the outside wood fibers experience drying and begin to contract while those fibers that remain wet maintain their expanded size. Repeated wet-dry cycles eventually tear the fibers apart, damaging the wood and the adhesive bond.

What to Remember When Evaluating a Claim
  • Carpet tack strips that get wet for a short period of time (one day) will show evidence of rust on or near the pin or nail.
  • Carpet tack strips exhibit a progressive and predictive darkening the longer they are exposed to moisture.
  • Carpet tack strips exposed to one long-term moisture release (up to 100 days) did not exhibit laminate separation. However, carpet tack strips exposed to repeated wet-dry cycles (repeated events) will show delamination after completion of a minimum of four wet-dry cycles.
  • Visible microbial growth can be observed within two to three days after continuous moisture contact.
  • The first fungi to be observed are Aspergillus/Penicillium-like. After one month, Chaetomium will be detected and become the predominant specie.
  • The two test brands (Halex and Roberts) responded similarly to continuous and cyclical moisture exposures.
Applying What You Find
The appearance and condition of carpet tack strips provide one of several prongs in the adjustment of a water damage claim. Interviews with the insured, examination of the source and the extent of damage, review of appropriate service contractor invoices, and other details related to the loss are essential to make a comprehensive and competent assessment of the duration and extent of the water loss.

Comparing findings to the results of this experiment should give an adjuster a baseline to measure against to substantiate or challenge the claim on the issue of exposure duration.
Don Nehrig CIAQP, EI, CIEC (dnehrig@hsa-env.com) is a senior building scientist at HSA Engineers & Scientists in Tampa, Fla. Ralph Moon (rmoon@hsa-env.com) is director of Building Sciences at HSA. The authors would like to acknowledge the helpful comments from Chin S. Yang, Ph.D., Nicholas Albergo, P.E., DEE and Robert Braun, P.E.

Forest Products Laboratory, USDA Forest Service Research Note FPL-0226, "Moisture Content of Wood in Use," 6 pages.

Hofferber, B. M., Kolodka, E., Brandon, R., Moon. R.J., Frihart, C.R., 2006. "Effects of Swelling Forces on the Durability of Wood Adhesive Bonds." In: Proceedings of the 29th Annual Meeting of The Adhesion Society, Inc., Pages 19-22.

Hann, R.A., Black, J.M., R.F. Blomquist, 1963. "How Durable is Particleboard? Part II, The effect of Temperature and Humidity." Forest Products Journal, May, Page 169-174.

Simpson, W.T., "Drying and Control of Moisture Content and Dimensional Changes." 1999. Wood Handbook – Wood as an Engineering Material. Chapter 12, Gen. Tech. FPL-GTR-113. Madison, Wis.: U.S. Department of Agriculture, Forest Service. Forest Product Laboratory, 463 pages.

Southern Cross Building Products, 3461 High Ridge Road, Boynton Beach, Fla. 33426.

Youngquist, J. A., "Wood-based Composites and Panel Products." 1999. Wood Handbook – Wood as an Engineering Material. Chapter 10, Gen. Tech. FPL-GTR-113. Madison, Wis.: U.S. Department of Agriculture, Forest Service. Forest Product Laboratory, 463 pages.
About The Authors
Ralph E. Moon, Ph.D., CHMM, CIAQP

Ralph E. Moon, Ph.D., CHMM, CIAQP, is with GHD, a worldwide consulting firm. He has been a CLM Fellow since 2013 and can be reached at www.ghd.com.  rmoon@hsa-env.com

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