After a flood, the normal response is to clean up and rebuild as soon as possible.

“However, rebuilding too quickly after a flood can cause continuing problems, such as mold growth, which is a health hazard, and deterioration of wood and wall coverings,” warns Ken Hellevang, North Dakota State University Extension Service agricultural engineer and flooding expert.

Hellevang says one important thing for people to remember is that cleanup, including removing all porous materials (such as carpet, drywall and ceiling tile), cleaning all nonporous materials (such as metal and glass) and thoroughly cleaning and drying the structure, should be done soon. However, they need to delay rebuilding until wood has dried to 15 percent moisture content or less.

Wood submerged in water will absorb a large amount of water, so drying may take weeks. Ventilating the structure with outdoor air and using fans to circulate air across surfaces reduces the drying time.

A common problem with rebuilding too quickly is mold will grow in closed wall cavities, such as on the back of drywall (Sheetrock). Air moves from wall cavities, basements and crawl spaces into the living space, so mold in these spaces is a health hazard.

Mold can trigger allergic reactions, asthma episodes and other respiratory problems. People have reported suffering an increase in respiratory problems several months after floodwaters subsided.

Building materials may be wet even though they appear dry on the surface, so checking the moisture content by using a moisture meter before rebuilding is important, Hellevang says.

Reducing the moisture content in wood to 15 percent or less will minimize the potential for mold growth. While wood may not decay until exceeding a moisture content of about 20 percent, mold growth likely will occur in wall cavities at moisture levels below decay-causing levels.

Most county NDSU Extension Service offices have moisture meters that people can borrow to determine if they adequately dried the parts of their homes and other buildings that flooded before they start rebuilding. Typically, the counties charge a $100 deposit, which they refund when the meter is returned.

Meters calibrated for measuring wood moisture content also can provide a relative moisture measurement of other materials such as drywall. However, the meters only measure moisture content; they do not determine if mold is present.

Meters are not available to detect mold growth. Testing for mold requires a trained professional collecting samples with special equipment and a technician with training in mold identification evaluating the samples. This testing is very expensive.

Test kits purchased from local stores or off the Internet do not provide accurate information, according to Hellevang, so he generally does not recommend homeowners test for mold.

“If mold is visible or you detect a musty smell, remove the moldy material using accepted procedures such as described in NDSU publication ‘Remove Mold for a Healthy Home,’” he advises. “Focus on removing wet materials and drying the structure.”

Moisture will continue coming through concrete in the basement until the soil around the basement is dry, so homeowners may have to wait months before the basement will be dry enough to rebuild.

Hellevang recommends checking for moisture coming through concrete walls and the floor by taping a 3- by 3-foot plastic sheet to the floor or walls and watching for moisture accumulating behind the plastic or the concrete to darken in a few days.

The NDSU Extension Service’s flood website, http://www.ag.ndsu.edu/flood, provides information on flood recovery, including videos on restoring flooded buildings and fact sheets on topics such as cleaning flooded or water-damaged homes and drying out before rebuilding.

NDSU Agriculture Communication - July 26, 2011