Surface molds or mildew fungi discolor the surface of wood, but do not weaken it. They [Molds and Fungus] are generally green, black, or orange and powdery in appearance.

The various building codes allow the use of framing lumber with surface molds or mildew, providing that the wood is dry and not decayed. Spores (or seeds) of surface molds or mildew fungi grow quickly on moist wood or on wood in very humid conditions. They can grow on wood before it is seasoned, when it is in the supplier's yard or on the building site, or even in a finished house.

When the wood dries, the fungi die or become dormant, but they do not change their appearance. Thus, wherever surface molds or mildew and fungi are observed on wood in a building, it is a warning sign that at some time the wood was moist or humidity was high.

Surface molds and mildew fungi are controlled by eliminating the source of high humidity or excess moisture; for example by repairing leaks, improving ventilation in attics or crawl spaces, or installing soil covers. Before taking corrective action the source of the moisture that allowed fungus growth must be determined. If the wood is dry and the sources of moisture are no longer present, no corrective action is necessary..

Sapstain or blue stain fungi are similar to surface molds, except that the discoloration goes deep into the wood. They color the wood blue, black, or gray and do not weaken it. They grow quickly on moist wood and do not change their appearance when they die or become dormant. They usually occur in the living tree or before the wood is seasoned, but sometimes they grow in the supplier's yard, on the building site, or in a finished house.

In the latter case, they are normally associated with rain seepage or leaks. Stain fungi are a warning sign that at some time the wood was moist. Control is the same as for surface molds or mildew fungi.