Mold on Lumber: Mold on Lumber During Construction

While framing lumber is designed to take a lot of abuse, significant wetness can deteriorate the wood by causing mold and mildew. Mold on lumber 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.

Rain or condensation can cause dry lumber to get wet. Sometimes wet boards are wrapped in the middle of bundles which creates a mold-friendly environment. . See What is Moisture?

How can mold on lumber be prevented?

Fungicidal Coatings

Every year builders buy millions of feet of unseasoned  or green lumber . Their plan is to allow the wood to dry out in open air during the framing phase of building. Often mills will apply anti-stain treatments which are coatings of fungicides intended to resist mold on lumber. The fungicides are applied by spray or by dipping bundles of wood into a fungicidal bath.

The treatments provide protection against mold on lumber for up to six months. The fungicides are quite mild so workers can handle the boards without gloves — they are not a long-term protectant against mold or mildew.

mold on lumberInspection for mold on lumber 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

Framing lumber can stand up to a good amount of moisture. It is usually wet by rain or wet snowfall and dries out as the weather conditions dry out. It is essential though to give it enough time to adequately dry before closing it in with walls and other structures that restrict the drying air flow.

Considering the usual sequence of building it should not be an issue to allow framing to dry. It may be exposed to enough rain to cause substantial wetting at the framing stage. After this stage though the roof goes on and it is sheathed and sided well before it is closed in with insulation and drywall. Generally this would allow several weeks for drying.

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 it will remain free of mold on lumber. Lumber that is exposed to moisture after it has been dried will support mold growth.

Removing Mold from Lumber

There are quite a few commercial products available for removing mold on lumber.  The U.S. Environmental Protection Agency recommends wet vacuuming the area, scrubbing with a solution of water and mild detergent, then drying and vacuuming with a vacuum equipped with ahigh-efficiency particulate air (HEPA) filter.

For the most part the mold we see on framing lumber consists of a spattering of spore colonies over the surface. The EPA method of scrubbing is usually quite adequate to clean the mold from the lumber. Scrubbing can however cause spores to become airborne. To prevent this we suggest gently spraying the surface to wet it and then scrubbing. The wetness tends to help prevent spores from taking flight.

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. There is much debate about the using chlorine bleach to kill mold. We advise against chorine bleach and encourage the use of  Oxygen bleach instead.

Removing mold on lumber is a simple process for small infestations. Mold removal more of a problem when there are heavy infestations. When this is the case you may want to call in a professional mold remediation company.


  1. joe hernandez says

    Is this mold a danger to a adults health? Inhaled dried green mold from bundle of wood.

    Second hand contact with an infant?

    Thank you for your time