While framing lumber is designed to take a lot of abuse, significant wetness can deteriorate the wood by causing mold and mildew. Mold can develop in wood within 48 hours of becoming wet. Once the rain stops, it is important to dry out the wood and look for signs of mold or mildew development immediately. Mold must be removed before you proceed with applying finishing materials.
Dry lumber can become wet through direct sources, such as rainfall or condensation. Even dry lumber contains some moisture, so wet pieces inside wrapped bundles of lumber could create conditions for mold growth. Exposing the bundle to direct sunlight, for example, could heat the lumber and the wrapping may trap the evaporating moisture. This trapped moisture can be sufficient to support mold growth. See What is Moisture?
How can mold on lumber be prevented?
Each year, billions of board feet of lumber are sold as unseasoned, or green products and are allowed to dry naturally, usually during the framing stages of building a house. Many mills reduce the risk of mold and stain on green lumber by applying antistain, or sap stain treatments, which are thin coatings of fungicides on the wood surface. These fungicides are applied by dipping entire bundles of lumber into a treatment solution or by spraying all four surfaces of individual boards.
These chemicals are designed to provide a microscopic barrier against fungal attack that lasts for three to six months, depending on the chemical, the concentration used, the wood species and the climatic conditions. The chemicals used for preventing mold and stain are usually very mild and include many used on food crops as well as in shampoos and paints. They are not designed for long-term protection of the wood.
Inspection on Delivery
Since mold can grow on construction materials like wood and drywall, the question of mold first arises when these materials show up on the site. At this phase of the process, a builder or his contractors can inspect material shipments like framing packages to ensure that they don’t present a mold problem.
Whenever it’s possible, materials like wood framing should be stored under roof. Since this is often not feasible, the next best thing is to store materials with clearance above the ground to avoid wetting from storm runoff and permit air circulation from below. An example would be storing a stack of OSB on 3 lengths of scrap 4″x4″ lumber (or paired 2″x4″ lengths) to keep the material off the ground. This stack should also be covered with a tarp or plastic sheeting to protect the wood from rainwater. While the tarp should be weighted down on top to prevent it from blowing away, it should not be tight against the sides of the stack because this can reduce circulation and hold moisture inside the sheeting. One alternative is to stake the sides of the tarp off to the side of the stack.
Wetting During Construction
Reasonable amounts of wetting can be endured by framing, which will simply dry out again once outdoor conditions allow. Allow for a suitable amount of drying time following wetness before “closing in” building components. Closing in is considered the point at which components are covered up with additional materials that restrict their ability to dry.
Given typical sequencing of work at a site, this shouldn’t be a problem. For instance, a building may be exposed to several days of rain during the framing stage, resulting in moisture uptake of wood members like wall studs. However, as a typical construction sequence would progress, the house would be put under roof, sheathed, sided, and roughed-in (mechanicals) before the studs would be covered up with insulation and drywall (e.g., closed in). This would permit at least a few weeks to dry. What’s more, the studs would not be exposed to further wetting from rain once the house was under roof, sheathed, and sided. At this stage, the interiors of homes are protected from bulk water and moisture-sensitive components like drywall, insulation, and finish products can be installed.
How to Dry Out Framing Lumber
Squeegee any standing water off your lumber. If necessary, plug in fans throughout your construction site. Drying lumber reduces the likelihood of mold formation but it does not guarantee the wood will remain free of mold. Lumber that is exposed to moisture after it has been dried will support mold growth.
Removing Mold from Lumber
For any mold clean up that may generate large amounts of dust, basic personal protection equipment such as rubber gloves, eye protection and a high-quality pollen or dust mask should be worn. Clean-up of small spots or areas of mold generally does not require any special protective equipment.
There are a number of products on the market, from commercial mildewcides to common bleach, which are promoted for removing mold from wood. However, the U.S. Environmental Protection Agency suggests using mild detergent and water for most mold clean up. For cleaning wood surfaces, the EPA recommends wet vacuuming the area, wiping or scrubbing the mold with detergent and water and, after drying, vacuuming with a high-efficiency particulate air (HEPA) vacuum.
The molds seen on lumber are largely a collection of fungal spores on the surface of the wood. As such, wet wiping or scrubbing the lumber will remove the mold.
Simply wiping the wood, however, can release those spores into the surrounding air. A better approach is to gently spray or wet down the mold prior to removal. Once the mold has been wetted, it can be removed by wet-wiping the surfaces with a water and detergent solution, scrubbing if necessary.
If commercial products are used for cleaning mold, be sure to follow the manufacturer’s instructions for use. Common bleach also can be used, particularly to clean the discoloration caused by mold fungi. The U.S. Centers for Disease Control (CDC) recommends using a solution of 10 parts water to one part chlorine bleach to clean mold from surfaces. When using bleach and other cleaning chemicals indoors, make sure there is adequate ventilation and wear personal protection equipment outlined previously. Never mix bleach with ammonia.
Removing small amounts of mold from wood is relatively straightforward. Mold removal becomes more complex when there are heavy amounts of growth on a majority of the lumber or if the building has been in service for some time and the mold originated from leaks into the building cavity. In these instances, the mold clean up should be done by a professional cleaning and restoration company.