How deep should you install French drain pipes?

Written by meggie hardy | 13/05/2017
How deep should you install French drain pipes?
A French drain carries water runoff underground. (Warren Price Photography/iStock/Getty Images)

Water standing in your garden is not only annoying, it’s also unsanitary; stagnant water is a breeding ground for insects. Drainage problems can be corrected by installing a French drain, an underground drainage system that uses pipes to direct water to a designated drainage point. The depth of the pipes is subjective to the topography of the property.

Evaluating the topography

Scott Lunt, owner of The Garden Center in San Antonio, Texas, says “The actual topography will dictate how deep you need to lay the pipes. Obviously, water doesn’t flow uphill, so you have to evaluate the grade of the slope before you make that determination.”

In a French drain system, pipes are perforated and placed onto gravel beds in trenches. The pipes must then be covered with another bed of gravel to help filter sediment and carry the excess water to a designated turnout. According to Lunt, covering the pipe with a weed mat to keep root systems and sediment out of the pipes is as important as digging the trench deep enough.

“The purpose of a French drain is to effectively drain water, while creating an aesthetically pleasing landscape," says Lunt. "You don’t want the pipes to show, so in a typical 30 by 30 by 10 cm (12 by 12 by 4 inch) system, you’d be digging a trench 30 cm (12 inches) wide by 30 cm (12 inches) deep and using a 10 cm (4 inch) pipe. If you place a 5 cm (2 inch) bed of gravel in the bottom of the trench, add the pipe, and cover it with another two inches of gravel, you’ll have adequate depth above the pipe and gravel bed to add topsoil and grass.”

Lunt emphasises that your drain should be at least 30 cm (12 inches) below the ground at the ground’s lowest point; depending upon the topography, you may be digging a trench of varying depth.

Caution

Check your area's legal requirements for installing a French drain. You may be in a conversation area, a national park or a listed building. If installation of any drainage system leads to the flooding of another property, you may be held financially responsible for making corrections to the system and for providing an alternative water turnout. Additionally, you may be held accountable for any damage resulting from the drainage.

At least a week before you begin digging, call all utility companies, including phone and cable, and ask for verification of underground cables. They will mark the path of the cables with spray paint to help ensure that you won’t dig into any lines. Ask for a letter stating the depth of the cables and a printout of the exact location as a backup reference.

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