Recommended Residential Drainage Slope for Pipes

Subsurface Drainage

• Underground pipes are used on residential properties for foundation drainage and to drain runoff where the surface is too flat, or soils are saturated. Drainage pipes are usually flexible or rigid plastic, and may be perforated or solid. Perforated pipes are punctured with small holes, letting water enter and exit the pipe, moving runoff away from the structure, yet still allowing some to filter down into the water table. Solid pipes are used for situations where it's desirable to direct water away quickly.

Flow Considerations

• Water has to flow through the pipe with enough velocity to self-clean -- prevent silting. Smaller pipes need steeper grades to maintain self-cleaning velocity; the minimum pipe size for subsurface drainage is 4 inches in diameter. Rigid pipe maintains a more consistent slope than flexible pipe and it's easier to clean with a sewer snake when it's blocked. Corrugated pipes are available with a smooth interior wall, which allows better flow than where the inside pipe is corrugated.

Sizing the Pipe

• Pipes are sized according to the site and climatic conditions. Site assessment for drainage includes the available slope gradient, the infiltration rate of the soil, the size of the area to be drained, and the amount of precipitation that's expected. Adjustments with installation methods and the type of pipe can help meet design criteria. Residential drainage usually follows guidelines and regulations established by local building officials and experts familiar with the conditions for the area.

Installation

• Subsurface pipes are laid on a gravel bed, or channel. The gravel channel doesn't have to slope to the minimum grade that the pipe has to achieve -- the gravel slows the runoff and lets it infiltrate into the ground. The pipe is collecting water and discharging it to a safe point, and the minimum slope is critical for the pipe to maintain the flow. The recommended minimum slope for subsurface pipes is 1/8 inch per foot, or 1 percent.