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To reduce basement water problems, the most important portion of the lot to be graded is the lawn that's within 10 feet (3 meters) of the basement wall or foundation. If this critical area is properly graded, so that no water can pool within 10 feet (3 meters) of the basement, the water will not soak back through the soil and enter the basement.
Expert advice varies regarding the slope needed around the basement to prevent water entry. For example, a major home inspection company, HouseMaster of America, recommends that the slope or grade of the soil should be 3 inches per foot for the first 3 feet (7.5 centimeters per 0.3 meters for the first meter), or a 15 angle. But water can soak a long distance in very loose or sandy soil, so I prefer to extend the slope so it drops 1 inch or more per foot of run (2.5 centimeters per 30 centimeters of run), for a distance of 10 feet (3 meters) away from the basement wall. This means that the grade from the basement wall outward would have a slope of 10 inches in 10 feet (25 centimeters in 3 meters).
To check your grade or slope, use a 6-foot (2-meter) straight edge such as a straight 2 x 4. With the 2 x 4 sitting on its narrow edge, use masking tape to tape a 24-inch (50-centimeter) carpenter’s level to the top of the 2 x 4.
Now, set the 2 x 4 on the ground with one end against the wall. Does the 2 x 4 touch the ground along its entire length, or are there ridges and dips along the bottom edge? The slope should be continuous, with out any low spots or dips where water can puddle and soak back to the basement wall. If the slope is not continuous, fill the low spots with dirt to eliminate the possibility of water puddling.
To check the grade, leave the 2 x 4 end that's against the basement wall sitting on the ground. Lift the outside end of the 2 x 4 until the bubble is centered in the level. Now use a ruler or measuring tape to measure the distance from the ground up to the bottom edge of the 2 x 4. If the straight edge is 10 feet (3 meters) long, the distance should measure 10 inches (25 centimeters). If the slope is less, add dirt and contour it so the slope is 10 inches (25 centimeters) over 10 feet (3 meters) of run.
Keep in mind that any improvement in the slope of the lawn is better than none at all, and even a slope of ½ inch (1 centimeters) per foot of run will permit water to run away, providing the soil is not cultivated as it would be in a flower bed.
However, before adding soil, check the distance from the bottom edge of the exterior wall siding down to the ground. You should maintain a distance of at least 8 inches (20 centimeters) — more distance is preferable, especially in snow country — between the ground and the bottom edge of the siding. If you add enough dirt at the basement wall to raise the slope to 1 inch (2.5 centimeters) of drop per foot of run, can you maintain at least 8 inches (20 centimeters) of space between the bottom of the siding and the ground?If you can't add more dirt at the basement wall, you can call in a landscape contractor to use a device called a transit to accurately shoot or determine a grade. He or she will drive grade stakes, then use a small tractor to cut away the soil so the grade slopes away from the basement wall. This will leave a slight drainage ditch, called a swale, 10 feet (3 meters) away from the basement wall. Water then can't stand or puddle within 10 feet (3 meters) of the basement wall, which will ensure a dry basement.
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