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Mold and Mildew on Wood: Causes and Treatment

May 2000     

Dampness or inadequate ventilation typically results in the growth of mold or mildew on wood surfaces. Mold usually appears as black or greenish-brown patches on surfaces in humid environments. Mildew describes both mold fungi and their discoloration to unfinished wood. Besides causing an unsightly appearance, spores from mold fungi can cause allergies and other health concerns.

Mold Fungi Growing on Wood BeamsMold fungi thrive in high humidity and are often seen on the northern exposure of wooden structures and on ceilings, joists, walls, surfaces in damp basements, and other areas of high humidity such as bathrooms.

Mold fungi cause no damage to the wood other than unsightly discoloration and are typified by their colored spores (usually green or black). Differentiating between mold fungi and decay fungi is important because decay fungi actually weaken the wood structure. Brown rot, the most destructive type of deterioration caused by decay fungi, is typified by darkening and shrinking of the wood, with eventual crumbling in advanced stages of decay.

How can mold growth be discouraged?

  • In enclosed areas, increase ventilation with fans and vents.
  • Run a dehumidifier.
  • Look for leaks in pipes and basements. Leaks may not be obvious; but continuously exposing wood to moisture encourages more destructive decay fungi to colonize the wood, possibly leading to structural problems.

When the humidity problem is eliminated, mold and mildew can be removed by cleaning the wood surface with one part household bleach mixed with four parts water. This removes most of the surface mold spores while renewing the look of the wood. Rinse thoroughly with fresh water and be sure to dry the area after cleaning.

Prepared for
U.S. Department of Housing and Urban Development
Office of Policy Development and Research

Prepared by:
Forest Products Laboratory