Rotting Wood, Rampant Mold Threaten Homes

New Orleans residents who return to their soggy homes and businesses this week are in for some rude lessons in physics and biology, experts say.

Lesson one will focus on the surprising amount of damage that floodwaters can wreak very quickly upon wood-frame buildings. Lesson two will be an equally difficult tutorial in the astonishing versatility of mold and other fungi, which are poised to enjoy a major population explosion in the steamy microbial incubator that was once the Big Easy.

Wooden structural supports that have become saturated under water will in many cases be warped and are unlikely to straighten out again as they dry, making structures unsafe, according to engineers with flood experience.

Even those structures deemed sound enough to be salvageable will soon come under a major assault from some of Mother Nature's smallest invaders: Health-threatening molds and wood-rot fungi that are very difficult to control and can consume a house from the bottom up.

Beyond the obvious need to remove carpets and floor pads, owners will have to tear out sheetrock and other kinds of porous wallboards, along with any insulation inside walls, which tends not to dry out if left in place. Wiring will have to be replaced, as will many gas lines and plumbing components.

"They're pretty much going to have to strip a lot of these houses down to the studs," said Jon Heintz of the Applied Technology Council, a nonprofit engineering organization in Redwood City, Calif., that will be training many of the inspectors who will soon be swarming into New Orleans to judge which buildings can be salvaged.

"It's pretty simple," said Paul H. Gilbert of Parsons Brinckerhoff, a Seattle-based global engineering firm. "If the water gets above the foundation for any period of time . . . it will likely be cheaper and safer . . . to clear the site and start fresh."

If there is one silver lining, urban entomologists said, it is that the flood probably took a huge toll on New Orleans' famously robust population of termites, which have been chewing up the city's buildings and trees to the tune of $300 million in damage every year.

Federal officials have said little about their plans for assessing the structural damage left in Hurricane Katrina's wake. But with residents of some neighborhoods getting the go-ahead to return to their homes over the weekend, experts warned of physical and medical risks that residents will face as they begin to dig out their homes.

For homes with concrete foundations, residents will first have to see whether those foundations have cracked or shifted. Floodwaters can easily scour beneath the edges of concrete slabs, causing an irreversible settling that can leave the wooden structure above unsound.

And although basements are unusual in southern Louisiana, where flooding is so common that even the dead are interred above ground, any structure that does have a well-sealed cellar will have to be checked to see if it has been displaced.

"Basements that are watertight can pop up out of the ground and float, like boats, when they're surrounded by water," Heintz said.