Professional landscapers define drainage as a two-stage process—the flow of water across the ground according to grade and the subsequent seepage of the water into the soil. In the first stage, rainwater can create problems in several ways: It can erode steep ground; it may flow to low areas and leave them soggy long after the rain has stopped; or it may pool around a house and perhaps find its way through the foundation.
If a high water table or some other unseen problem is causing a wet basement, drainage professionals will have to be called in—but first check to see if the situation is simply a result of faulty surface drainage. In a 10-foot-wide zone around the house, the grade should drop at least 1 vertical inch for every horizontal foot.
Correct any insufficiency in the grade, and at the same time, use flexible plastic pipe to extend your gutter downspouts so that rainwater is channeled away from the house. Depending on the slope of your yard, the extension can end either in an underground dry well that traps and slowly disperses the water (opposite) or in a simple culvert that drains it away.
To keep rainwater from collecting at the base of a gentle slope, divert the flow by constructing berms and swales—low earthen dams and shallow trenches (page 30). For steeper slopes, the solution may be to terrace the land and build a retaining wall (pages 31-35).
Checking the grade
Drive a stake next to the house and another one 10 feet away from the foundation. Tie a string between them and level it with a line level. Measure from the string to the ground at 1-foot intervals to calculate the grade. Move the stakes and repeat at other points along one side of the house. In any area where the grade drops less than 1 vertical inch for each horizonal foot, strip the sod (page 26) and remove any shrubs (page 88). Dig a trench for the downspout extension; the trench should be 8 inches wide, a minimum of 10 feet long, and at least 6 inches deep at the downspout. It should also slope 1 inch per foot (dashed lines, above).
Extending the downspout
Attach an adapter to the end of the downspout. Lay flexible nonperforated drainpipe in the trench and connect it to the adapter. The pipe must lie flat along the bottom of the trench without any dips or humps. Remove or add dirt under the pipe as necessary.
Making a dry well
At the end of the drainpipe trench, skim the sod from a 2½-foot-square area and set it aside. Then dig a hole about 3 feet deep. Pull the flexible drainpipe so that the lip protrudes a few inches over the hole. Fill in the trench with topsoil and tamp it down. Fill the hole with gravel to a point about 1 inch above the top of the pipe. Add topsoil and replace the sod (above). If the trench ends on a slope, lead the pipe out of the hill and onto a splash block (inset). The block will prevent erosion at the outlet point.
Correcting the grade
When the drainpipe extension is complete, correct any improper grade around the house. If you are piling dirt higher against the foundation, first treat the masonry with a waterproofing sealant. The soil level must remain at least 6 inches below wooden siding in order to keep termites out. Use a tamper to pack the soil firmly.
Creating berms and swales
Dig a trench, or swale, about 3 inches deep and at least twice that wide across the slope above the area you wish to protect. Create a berm by piling the leftover soil into a gently rounded mound below the swale, then tamp it down. Lay sod (page 68) on the berm and the swale or plant a ground cover (page 69).