Residential Grading and Drainage

Rough Grading

• Rough grading sets the drainage pattern for the lot, leaving the ground ready for the addition of the finish-landscape materials, such as concrete and topsoil. The ground is sloped away from structures, and runoff is channeled to an acceptable outlet, discharged to approved storm drains and open grassed areas. The work should meet with existing grades at the edge of the property if possible; it's standard practice that the drainage for adjacent properties should not be disrupted.

Swales and Slopes

• Typically, drainage is channeled into swales -- shallow depressions. Grassed swales help runoff infiltrate into the soil, reducing the amount of stormwater runoff discharging to the municipal storm sewer system. The minimum slope for drainage from unpaved surfaces is 2 percent -- a rise of 10 feet for every 100-foot rise -- and that's best for exterior pavement as well. In flood-prone and excessively flat areas, higher slopes are recommended at the foundation.

Critical Areas

• The ground must slope so that runoff is directed away from the foundation. Water can leak into basements and damage the foundation; some regions may have special requirements for foundation drainage. Much of the stormwater runoff for the site is generated from the roof. The downpipe should be placed away from the foundation and discharged where the ground won't erode. Ensure all paved areas are sloped adequately to shed water, and that site preparation for them includes a well-drained and compacted subgrade.

Subsurface Drainage

• If the site conditions don't allow the minimum slope, it may be necessary to install subsurface drainage. Perforated pipe is suitable for most situations; it can be placed as deep as 4 feet below the surface. For locations where a heavy load is expected, rigid pipes may be required. Foundation drainage is a particular concern. In some locations, the building code may call for special measures for foundation drainage that includes the installation of pipes.