Whether selecting building materials from a supplier, or installing materials that have been stored on site, it is important for all building professionals to avoid materials showing visible signs of water damage or mold. Installers on site should be especially vigilant to store building materials properly at the construction site and to avoid choosing any moldy or water-damaged materials.
Water damage. Wood products swell when they take on moisture. This change in dimension can create problems when installing the materials. Pieces may not align well when swollen with water, and when they dry out after being installed wet, the material will shrink, leaving wide gaps. Uneven wetting and drying will cause wood to warp. Rapidly drying materials can crack, and fasteners that were tight when the materials were wet, can loosen when the materials dry (Reitz 1978). Products like OSB and plywood that have become saturated and then dry may not return to their specified (dry) dimensions, creating problems when the finish roofing, siding, and flooring materials are installed over them (Fisette 2005). Water can also cause the adhesive bonds to weaken, causing delamination of plywood that destroys the structural integrity of the panel.
Generally, installing finish materials over any wet substrate will cause problems. The moisture from a damp substrate will migrate into the cover materials, compromising adhesive bonds, and causing more delicate finish materials to swell, crack, and warp (Forest Products Laboratory 2010).
Mold. When lumber and other water-absorbing building materials are allowed to get wet, and cannot dry quickly, mold will often grow on the surface. Mold is a problem first and foremost because it is as an indicator of high moisture content. In addition, mold may present an indoor air quality problem. Some molds produce mycotoxins, which are compounds that may be irritating to people with allergies, inducing respiratory discomfort (including asthma) and fever. Mycotoxins are not volatile and cannot "off-gas" into indoor air (Robbins and Morrell 2006).
Owing to the legal attention that so-called "toxic molds" have received, installing moldy materials in a building can also expose building professionals to legal liabilities that they will want to avoid.
Storing Materials to Avoid Mold Growth
Dry building materials won't stay dry and mold-free unless they are properly stored. When stored outside or in a damp basement or garage, building materials can absorb water from the ground and from damp air. To avoid problems, follow these recommendations when handling and storing building materials on job sites:
- If possible, do not unload the lumber in the rain. This may mean rescheduling deliveries during times of wet weather, or taking other precautions to avoid materials getting wet when they are delivered to the building site.
- Avoid storing materials on the ground. Keep stored building materials elevated off the ground with pallets or stickers (i.e., long pieces of lumber). This will prevent materials from absorbing water from the ground, and will allow air to circulate, promoting drying.
- Elevate the materials on pallets or stickers even when storing them on a concrete slab. The moisture content of concrete is usually much higher than other building materials, and the concrete will transfer this moisture to any materials directly in contact with it.
- On large construction sites where bound bundles may be offloaded from a roller truck, it is sometimes difficult to avoid materials being unloaded onto the ground. Efforts should be made to move the bundles with a forklift to a protected location on pallets or stickers. Or, break apart the bundles and restack.
- Protect materials from the weather. Store lumber in a garage or other covered area, if possible. If the materials are outdoors, cover them with tarps or poly to protect them from rain and snow. Leave the ends of the covered pile open to allow air to circulate through the materials.
- Inspect bundles that are delivered with pre-applied protective coverings. Any tears or openings in the covering will exposed the material to water, and water trapped inside the protective covering can accelerate moisture problems.
- If possible, schedule deliveries to limit the amount of time that building materials must be stored on the job site prior to their installation (Reitz 1978).
Figure 1 - Elevate Stored Lumber. To prevent lumber from absorbing moisture from the ground or a concrete slab, elevate the bundles on stickers (i.e., long pieces of lumber).
Figure 2 - Cover Lumber Stored Outdoors. Lumber that must be stored in the open should be covered to protect it from rain and snow. Leave the ends of the lumber pile open to allow air to circulate under the protective covering.
Visible Inspection of Building Materials
The most effective method of identifying water damage and mold on building materials is by visibly inspecting it (Robbins and Morrell 2006).
Avoid mold. Do not select lumber or other wood products that have a black, powdery stain on the surface (see photo below).
Figure 3 - Lumber with Visible Mold Should Be Avoided. If mold is found on lumber at the lumber yard, do not purchase the product. If the mold grows on lumber stored on site, due to improper storage, it may be cleaned off with a mild detergent and warm water.
Some "stain molds" may go deeper than surface molds, but often these will have only affected the living tree and are no longer actively growing. Blue stain, for example (see photo below), is common but it does not adversely affect the wood; it is not an indicator of high moisture content and may still be selected as a suitable framing material.
Figure 4 - Blue Stain. The board in the center of the photograph shows blue stain. Not to be confused with lumber mold, this discoloration of the wood was caused by a fungus affecting the living tree, and the fungus did not harm the structural integrity of the wood. The fungus is no longer actively growing and does not necessarily indicate high moisture content. Therefore, it may be used as a suitable framing material.
Look for mold on other wood-based building materials, such as plywood, OSB, and millwork. Any wood product, such as the OSB sheathing shown below, is susceptible to growing mold when moisture levels of the material remain high for extended periods of time without drying.
Figure 5 - Mold on OSB Sheathing. The mold growing on the inside face of this OSB sheathing is a prime indicator of an ongoing moisture problem caused by a siding failure.
Always protect drywall from moisture. Wet drywall is especially susceptible to growing mold. Even the moisture-resistant drywall product commonly referred to as "green board" may support mold growth.
Figure 6 - Mold on Drywall. If it is allowed to stay wet long enough, the paper facing on drywall provides a ready food source for mold.
Avoid water damaged materials. Look for other visible signs of water damage such as water stains or separation of wood fibers. Pay particular attention when selecting plywood panels to avoid panels that have delaminated. If plywood becomes soaked, the moisture can affect the adhesive bonds between plies, effectively destroying the structural integrity of the panel.
Figure 7 - Delaminated Plywood. A sheet of plywood that was soaked by rainwater has delaminated, rendering it unusable for building.
If materials do grow mold, small amounts may be cleaned using a mild detergent and a warm-water solution. Use the gentlest cleaning method that effectively removes the mold. Materials should be dry and visibly free from mold before installing them (NYCDHMH 2008).
Non-porous materials (such as metals, glass, and hard plastics) usually can be cleaned. Semi-porous and porous structural materials, such as wood and concrete, can be cleaned if they are structurally sound. Porous materials (such as ceiling tiles, drywall, and insulation with more than a small area of mold growth) cannot usually be cleaned and should be discarded (NYCDHMH 2008).
Do not use bleach when trying to remove mold. Bleach will not render the spores inactive, and bleach is a lung and skin irritant, which presents an unnecessary risk to the workers using it (NYCDHMH 2008).
If efforts to remove the mold are unsuccessful, discard the materials. Do not take the chance of installing it.