Q. I’m confused by the term "dry rot." It seems contradictory since rot occurs when wood is wet. Or is there a kind of rot that happens to wood that is too dry?

A.Preservation consultant John Leeke responds: You are right; rot does indeed require water. Wood rots, or decays, when fungus organisms eat it. Three conditions must be present for the fungus to thrive: temperatures between about 40°F and 100°F, food (which is the wood itself), and a wood moisture content above 20-25%.

The presence of the fungus is a given because fungus spores are everywhere, carried about by the wind. Two common types of wood-eating fungus are "cubic brown rot" and "white rot." Cubic brown rot eats the cellulose component of wood, leaving the darker brown lignin component, which shrinks into characteristic blocky formations. White rot eats the lignin, leaving the light gray cellulose and covering the wood surface with a white mat of fungal fibers. Both types of rot are found throughout most of North America.

Rotten wood is often found in a dry condition and so is called "dry rot." But the wood had to have once been wet for the decay to occur. Another possible confusion arises with "water syphoning fungus." This fungus spreads by forming tubes through which it carries water to wood that is too dry. Syphoning fungus is common in the tropics and sometimes appears in the southern states along the Gulf Coast.