A variety of fungi can result in mold or mildew on floor joists. Removing the problem depends on the type of fungus responsible and the degree of infestation. In some cases, altering the conditions around the wood can stop growth, but in more severe situations, a visit from a professional is required.
Surface Mold and Mildew
Mold and mildew on your floor joists indicate a moisture problem. They will not cause the wood to decay when they cover only the surface of the wood. Lowering the moisture in the area where mold or mildew is present is enough to stop the growth. In any situation where you have mold, it is wise to call a qualified professional to inspect the mold and verify the air quality is affected by a serious type of fungus.
Blue, black or gray mold on floor joists may indicate sapstain fungi, which are similar to surface mold but go deeper into the wood. They will not weaken the floor joists, but the presence of this fungus is an indication that the wood has been moist or wet for a long period. Once the wood dries out, the fungus stops growing. Adjusting the moisture in the air and drying out the wood will remove the problem, although the staining will still be present.
White or Brown Rot
A white cottony mold on your joists indicates white or brown rot. The fungi that cause this can damage your joists and affect their structural strength. As the wood decays, it darkens and shrinks and then cracks form. Eventually the wood becomes powdery. If this is the type of mold on your floor joists, address the source of moisture in the area the joists are located. If the moisture in the wood is lowered to below 20 percent, the fungi stops growing because there is no water to feed it. Repair leaks and faulty drains and ensure the area has good ventilation to reduce the moisture in the air. You may treat the wood chemically, but without addressing the moisture problem that caused the mold's presence, it will simply return. If the moisture cannot be adjusted enough to stop the growth of the white or brown rot, then the joists must be replaced.
Some types of fungi can conduct water from a source such as soil into dry wood, using root-like strands. These are sometimes called water-conducting fungi. They're commonly found in the southeastern United States and can cause significant damage in less than three years. The wood appears similar to that of brown or white rot. To remove and treat the fungus, you have to eliminate the source of moisture. Joists that are in contact with or near soil are the most vulnerable to this type of mold. If the joists cannot be moved so that they are more than 8 inches from the soil, you can treat it with a borate pesticide.
Regularly check areas such as crawl spaces and basements to ensure moisture levels remain low. Rainfall, ground water, faulty drains and vents, or the lack of vapor barrier in your walls can allow moisture into your home. Close off vents to ensure moisture doesn't seep into your home and use a dehumidifier in areas where you know moisture is excessive.