June 28, 2005
Drainage Around the Home Can Prevent Wet Basements
Controlling water above ground often can prevent water from getting into basements, according to a North Dakota State University engineer.
"Correcting those above-ground problems may prevent structural damage to your home, as well as dry up those basement damp spots," says Ken Hellevang, an agricultural engineer with the NDSU Extension Service.
He notes that saturated soil increases the soil pressure on the basement walls, which can lead to cracks, shifts, collapses and other structural problems.
Start by looking to the roof, he advises. An inch of water on 1,000 square feet of roof amounts to about 623 gallons of water.
"Getting all that water away from the house is a big first step to preventing basement problems," he says. "That's why all eave trough downspouts should have extensions to carry the water several feet from the house. In addition, clean eave troughs and verify that they are functioning.”
Just as the roof is sloped to shed water, the ground around your home should be sloped, too. The soil next to the house frequently settles, so water runs toward the house rather than away from it. A slope away from the house of about 1 inch per foot near the walls usually is adequate. Slope the ground to carry water away from the downspout discharge, as well, and make sure the soil on the surface next to the house has low permeability to reduce infiltration.
“In some cases, it is desirable to place an impermeable material under the soil next to the wall to ensure that the water flows away from the house," Hellevang says.
A drainage system below ground is another important step to keeping your home dry. A properly installed drainage system at the house foundation and under the basement floor will ensure a dry basement and eliminate saturated soil conditions next to the walls. A study of leakage problems showed that more than 90 percent were due to improperly installed drainage systems.
Hellevang says a properly installed foundation drainage system includes drainpipes placed alongside the footing, with the top of the drainpipe below the top of the footing. Install drainpipe on both the inside and outside of the footing. If installing drainpipe only outside the footing, 2-inch-diameter weep holes through the footing are required every 6 feet. The drainpipe should be sloped about 1 inch per 20 feet. The drainpipe can be installed with less slope for houses with perimeters less than 200 feet.
Use several inches of granular material around the drainpipe to improve moisture flow from the soil to the drainpipe. Place a filter fabric between the granular material and the soil to keep soil particles from plugging the granular material and drainpipes. That is essential to keeping the system functioning for the life of the house, Hellevang says.
Place granular backfill or a drainage mat next to basement walls. Using soils that don't drain well can cause pressure on the walls if the soils become saturated. Also, wet soil next to the basement walls increases the potential for moisture flow through the walls into the basement.
The drainage system should include a 4- to 6-inch layer of gravel under the basement floor. The soil under the gravel should slope to drainpipe installed along the inside of the footing. Without this drainage layer, water may seep up through the basement floor if the water table is high.
The granular layer also reduces the potential for moisture or water vapor to enter a home. More than 6 gallons of water vapor per day can enter a home from wet soil around a basement. In certain areas, that moisture can carry minerals (salts) that are detrimental to the concrete. Frequently, the salt is deposited as a fluffy, white material on the floor or walls where the moisture is evaporating. Place a vapor retarder, such as a 6-mil polyethylene film (plastic sheet), on top of the gravel layer and under the concrete floor.
Also construct window wells correctly, with drains linked to the foundation drains. Soil elevation in the window well should be several inches below the bottom of the window and sloped to the drain. Drainpipe filled with gravel is a common way to create the drain.
"The cost of installing the drainage system during new construction is minimal and the benefits are priceless," Hellevang says. "Because so many problems can result from a poorly designed or installed drainage system, it's important to install the system correctly or find a contractor who knows how to do the job.
"In existing houses with wet basements, correcting the problem may be as easy as controlling the water above the ground," he adds. "If that's not successful, then an exterior and interior drainage system may need to be installed."
Installing a gutter system where the basement walls join the floor will collect water coming through a wall, but will not solve the problem of water coming through a floor due to a high water table, he says.
For more information about proper drainage, contact your local NDSU Extension Service office or visit the Web site www.ag.ndsu.nodak.edu/abeng/
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