Wood products used in construction are susceptible to decay (rot). However, if properly maintained, they can easily last for hundreds of years. There are three basic types of fungi that attack wood: stain, mold, and decay. Wood rot is caused by an attack of the decay fungi. Stain and mold fungi mainly grow on the wood surface, causing discoloration. By themselves, they do not weaken the wood; but their presence does indicate a moisture problem and should serve as a warning of conditions favorable to the growth of the decay fungi. The decay fungi are microscopic threadlike plants that grow in the wood and attack its thick cell walls. They break down the walls and feed on the contents of the cells. With the destruction of the cells, the wood disintegrates, and decay becomes evident.
In the early stages, it is difficult to recognize that a section of wood has been attacked by the decay fungus. The wood might merely be discolored. However, the advanced stages of decay are easily recognizable because the wood undergoes changes in properties and appearance. The affected wood might be brownish and crumbly or white and spongy. In either case, the decay greatly reduces the strength and structural value of the wood member. The brown, crumbly rotted sections readily break into small cubes and in the final stage of deterioration are often quite dry. Most people when seeing this condition refer to it as “dry rot.” This is really a misnomer. The actual decay occurred when the wood was wet since decay fungi cannot survive in dry wood.
Sometimes the physical changes in the wood are not apparent on the surface. They can be detected easily, however, by probing the wood with an ice pick or a screwdriver. If the wood is in good condition, the probe will not penetrate much beyond the surface. However, if the wood has deteriorated, the probe will easily penetrate into the wood. The conditions that promote the growth of the decay fungi also promote subterranean-termite and carpenter-ant activity. Consequently, when probing wood-framing members, you might find deteriorated sections that are caused by a combination of insect damage and decay. Under suitable conditions of temperature and humidity, the decay fungus gives rise to a fruiting body that contains enormous numbers of microscopic spores. The spores are the seeds of a new generation of decay fungi and are readily distributed by air currents. The spores are always present in the air and under normal conditions cannot be kept away from wood. The presence of decay-fungi spores on wood is of no concern unless the moisture content of the wood and the temperature are such that the spores will germinate and grow. Figure 8-9 shows the decay hazard zones in the United States.
Decay fungi will grow and develop only when the moisture content of the wood is in excess of 20 percent and the temperature is in a range from 40° F to 115° F. Temperatures above the upper limit kill the decay fungi; temperatures below the lower limit cause the fungi to become dormant. The latter is a condition that readily occurs in the northern states during the winter months. In the spring, when the temperature rises, the decay fungi in infested lumber resumes growth, assuming the moisture content of the wood has not changed. The moisture content of wood is defined as the weight of water in the wood expressed as a percentage of the weight of the wood when oven-dry. The decay fungi thrive when the moderate to severe moisture content is about 25 percent. However, when the wood is saturated with moisture, the decay fungi are inhibited from growing because of the lack of oxygen.
NOTE: Lines defining areas are approximate only-see local FHA offices for
Region no. 2
Region no. 3
Region no. 1
slight to moderate
none to slight
The moisture content of green lumber can be as high as 200 percent; after the lumber is kiln-dried, its moisture content may be as low as 7 to 10 percent. Wood in a house that has been properly constructed and maintained will seldom have a moisture content over 15 percent. However, once the moisture content exceeds 20 percent, the wood becomes vulnerable to deterioration by the decay fungi. If the building design is such that some of the wood must be subjected to damp or wet conditions, those sections should be treated with toxic chemicals to prevent decay or be made from the heartwood of certain species (cypress, cedar, or redwood) that are resistant to rot.