“Dry Rot” isn't dry
Water is the basis of life. This is true for animals and plants but it is also true for the fungi. The kingdom Fungi includes organisms such as the mushrooms we buy in the grocery store. It also includes the wood-rotters that cause significant damage to wood in buildings. Unlike mold and stain fungi that can discolor but do not weaken wood, wood-rotters – or “decay” fungi – consume parts of the wood and thus destroy its structural integrity. There are many species of decay fungi and they are grouped by the appearance of the wood after attack, for example: brown rot and white rot. Regardless of the type of wood decay, all require that the wood be wet.
Dry rot is a commonly-heard term. Usually when wood decay is identified as “dry rot” it is a misidentification of brown rot. In the advance stages of brown rot, all that remains of the wood is a spongy brown material that can easily be broken by hand. If this dries out after the fungus has run its course, the dusty, crumbly residue will then be both “dry” and “rotten,” but it is still correctly referred to as brown rot.
Most decay fungi feed on wood that is exposed to a steady source of water. This can occur if wood is in contact with the ground, or if a leak in a building traps water in contact with the wood. There are however a few, relatively rare, fungi that can transport the needed water to the wood. These species may be called the “dry rot fungi” but they are actually water-transporting brown rot fungi. Meruliporia incrassata or “Poria” is one such fungus that is found in the southern United States . Poria grows specialized water-conducting tubes called rhizomorphs that can help it to move water from wet areas (e.g. the ground) to nearby wood. Poria is of further concern because it can also decay some woods that other fungi can't: naturally durable wood such as cypress and cedar and wood treated with copper-based preservatives.
Poria is very susceptible to drying, so it is very unlikely to become established on wood that is in a dry environment. Thus, even though Poria can move water to wood that would otherwise be too dry to support decay, prevention of this fungus is similar to that for other wood rotters: Keep the building dry. If, for example, gutters are installed and maintained, crawl spaces are kept dry and leaks are promptly fixed, it is very unlikely that Poria will be a problem.
Wood is a remarkably durable material and wooden buildings exist that are many centuries old. The key to this longevity is keeping the wood dry. This not only prevents attack by “dry rot” but also prevents mold, stain and insect infestation and lessens the effects of weathering.