How to Design Gravity Drain Lines

Gravity Drainage

• It's essential to drain water away from structures to keep foundations and building materials dry. The preferred method is to slope the surface of the land away from the structure and direct runoff toward an approved outlet, such as a municipal drain or ditch. A gravity subsurface drainage system collects water in underground pipes and carries it downhill to the proper outlet. This type of drainage is used for developments where the land is too flat to achieve an adequate surface slope.

Designing a Gravity Drain Line System

• Determine the starting point for the drain line, or inlet, and the end point -- the outlet or discharge point. The outlet has to be downhill from the start of the line. Water is collected from critical points such as downspouts, where runoff from the roof is discharged, and at the base of footings. Usually the drain line is directed to a swale -- shallow depression -- where water is taken to a ditch in the right-of-way that drains into the municipal system.

Layout

• Determine how many lines -- pipes -- are needed for the system. Typically there is a line for the footing and one for each downspout, where surface drainage isn't sufficient. Window wells may have vertical lines that connect to the footing drain. Measure the horizontal distance for each line from the inlet to the outlet, or to where one pipe connects to another. Check with local building officials and suppliers for recommended pipe sizes and depth of the trench for the pipe.

Selecting Pipes

• Generally, either solid or perforated pipes are used for gravity drain lines. Perforated pipes have small openings that collect water from the soil -- footing drains are often the perforated type. Solid pipes are used where water needs to be carried away quickly, such as the discharge from downspouts. Rigid material is stronger than corrugated and withstands heavy loads, making it suitable under a driveway or deep trenches, where the weight of vehicles or backfill may crush corrugated pipe.