With all the flooding the United States has experienced, we now have to be extremely a ware of wood decay and mold and fungi.
Wood decay is caused by certain fungi that can utilize wood for food. These fungi, like the higher plants, require air, warmth, food, and moisture for growth. Early stages of decay caused by these fungi may be accompanied by a discoloration of the wood. Paint also may become discolored where the underlying wood is rotting. Advanced decay is easily recognized because the wood has by then undergone definite changes in properties and appearance. In advanced stages of building decay, the affected wood generally is brown and crumbly, but sometimes may be rather white and spongy. These changes may not be apparent on the surface. The loss of sound wood inside, however, is often indicated by sunken areas on the surface or by a hollow sound when the wood is tapped with a hammer. Where the surrounding atmosphere is very damp, the decay fungus may grow out on the surface, appearing as white or brownish growths in patches or strands or in special cases as vine like structures.
Fungi grow most rapidly at temperatures of about 70 degrees to 85 degree F. Elevated temperatures such as those used in kiln-drying of lumber kill fungi. Low temperatures, even far below zero, merely cause them to remain dormant.
Moisture requirements of fungi are within definite limits. Wood-destroying fungi cannot grow in dry wood. A moisture content of 20% or less is safe. Moisture contents greater than this are practically never reached in wood that is sheltered against rain and protected, if necessary, against wetting by condensation or fog. Decay can be permanently stopped by simply taking measures to dry out the infected wood and to keep it dry. Brown crumbly decay, in the dry condition, is sometimes called dry rot, but this is a misnomer. Such wood must be damp if rotting is to occur.
The presence of molds or of fungus stains should serve as a warning that conditions are or have been suitable for decay fungi. Heavily molded or stained lumber, therefore, should be examined for evidence of decay. Such discolored wood is not entirely satisfactory for exterior millwork because it has greater water absorptions than bright wood.
The natural decay resistance of all common native species of wood lies in the heartwood. The sapwood of all species, when untreated, has low resistance to decay and usually has a short life under decay-producing conditions. Of the wood species commonly used in house construction, the heartwood of bald cypress, redwood, and the cedars is classified as being highest in decay resistance. All heartwood, quality lumber is becoming more and more difficult to obtain, however, as increasing amounts of timber are cut from the smaller trees of second-growth stands. In general, when substantial decay resistance is needed in load bearing members that are difficult and expensive to replace, preservative-treated wood is recommended.