Funny thing about rotten wood: Everybody knows it when they see it, but not many people understand how it happens. For instance, how many times have you heard the term "dry rot"? Well, where wood's concerned, there's no such thing. Dry wood will not rot. That's why archaeologists are still finding intact wood containers in Egyptian tombs. That wood's dry, bubba, and as long as it stays dry, it'll last until the sun goes Red Giant and sets it ablaze.

Wet wood, on the other hand, can rot really fast. This comes as a surprise to a lot of people. I know this, because once every two or three years, a customer will call up and complain that we didn't say anything about the rotten floor under his bathroom. The conversation usually goes like this:

"My plumber found a leak in our shower, and it rotted out the floor under the bathtub. He says it was an old leak, and you guys should've found it."

To which I respond, "How does he know it was an old leak?"

"The wood is all black and soft."

"Uh-huh," I say. "And how many weeks, months, or years does he think it took for the wood to get that way?"

"Aw, c'mon. Anybody can look at that rot and tell it's old."

Understand, I have much sympathy for folks with rotten bathroom floors. I've had rot trouble myself, and fixing it is an ugly chore. I also have sympathy for plumbers and handymen who look at wood that's all black and rotten, and assume it's been that way for a long time.

But I'm here to tell you: Unless you run across a plumber or handyman who's an expert in wood biology, you probably don't want to rely on his opinion about how long your wood's been rotten.

Nobody knows how fast rot grows.

You don't have to take my word for it. About a year ago, I asked the wood experts at the U.S. Department of Agriculture's Forest Products Laboratory (FPL) if there's any way to know how long it takes for a piece of wood to rot.

Leslie A. Ferge, a biological technician specializing in the biodeterioration of wood, responded, "There is no way even to crudely estimate the rate of wood decay or its age. Wood decay is a complex biological phenomenon subject to many physical and environmental factors, such as moisture level, temperature, wood species, fungus species, geographic locality, and climate. Because of the varying influence of these factors, decay may progress very rapidly or extremely slowly, depending on the particular set of conditions present in each individual case."

Here's the skinny: It's fungus, not water that causes wood to rot. Fungi are primitive little organisms. They need only air, moisture, a food source (like wood), and temperatures between about 50 and 100 degrees F to thrive. All those ingredients are present in your average Southern crawl space.

Fungus spores can colonize wood that has about 20-percent moisture content. That's just damp enough to feel wet to the touch. And because the spores of wood-destroying fungi float around in the air and live in the soil, this colonization can happen in a day's time. If a house has an intermittent leak (say, at a shower that hasn't been used since the last teenager went away to college), the fungus in the bathroom floor can go dormant, then start growing again when the wood gets back up to about 30-percent moisture content (say, when new owners move in and start using the shower every day).

Then it's off to the rot races: "It's a vicious circle and a remodeler's nightmare," writes Terry Amburgey, professor of forest products at Mississippi State University, in the February/March 1992 issue of Fine Homebuilding. "Moisture promotes fungi, fungi increase wood permeability, permeability allows moisture to enter the wood, and that encourages the growth of more decay fungi."

EIFS not funny anymore

Speaking of nightmares, here's the latest on wood rot in EIFS (synthetic stucco) houses: Some of my EIFS-testing colleagues are telling me that as they see more and more houses where the EIFS cladding is being torn off, they're seeing rotten framing and sheathing that's perfectly dry. Apparently, some EIFS houses have had intermittent or one-time leaks (such as during a wind-driven rain) that let in enough water to get a good fungus crop growing.

The nightmare factor is that rotten-but-dry areas might not show up, even with invasive testing. Since the troubles with EIFS first became common knowledge in 1995-96, invasive probe testing has been considered the best test for finding structural rot in EIFS houses. As a general rule, any area with wood or wood products having moisture content above 20 percent was thought to need repairs, such as caulking. Areas with moisture content above 30 percent were thought to be saturated, and probably needed at least some structural repairs.

If it turns out that the probe testing is unreliable, EIFS homeowners and potential-EIFS homebuyers will be left with no good way to learn how much damage there is behind EIFS cladding, short of tearing off the cladding. Tearing off and replacing cladding on a whole EIFS house can cost tens of thousands of dollars.