What is A French Drain and How Does it Work?

Paul Pogue
Written by Paul Pogue
Updated July 8, 2021
hands of the worker, in blue gloves, laid a plastic pipeline in an open trench in the ground
Kurgu128/iStock / Getty Images Plus via Getty Images

French drains are ditches and pipes for diverting water from basements or yards

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Installing a French drain is an ideal solution to multiple water problems. Nothing ruins a basement quite like a flood. And outdoors, standing water both looks terrible and can quickly damage the landscape. A French drain can solve both!

At its basic level, a French drain system is a ditch lined with gravel and inset with a perforated pipe that drains water from one place to another. Usually, it diverts water to someplace like a drainage ditch, storm drain, or sewage system. French drains are commonly installed in basements to prevent flooding or in yards to prevent standing water.

Interior French drains, such as in a basement, often divert into a sump pump pit that then pumps water out of the basement.

French drains are sometimes called perimeter drains because they often run the length and width of the edges of the basement, where water seeps in.

The depth of your French drain trench is important. A deep drain trench carries more water and relieves more hydrostatic pressure from your basement walls than a shallow drain trench.

Outdoor French drains, sometimes called trench drains, simply use gravity to carry the water down the trench into a ditch or off the property. A flush grate surface is placed over the trench so it’s easy to walk on or across. These types of drains can be used for ordinary standing water or installed at the time of landscaping features to properly drain excess water.

Cost of French Drain Systems

The cost of a French drain is between $30 and $100 per linear foot on average for labor and materials. To calculate linear feet for a basement installation, simply add up the full length of the perimeter. A 36’ by 24’ foot basement would be 120 linear feet — 36 +24 +36 +24.

The harder it is to access the area and install the drain, the more you’ll pay. For instance, an outdoor French drain that merely requires digging will be on the lower end of the price range. A basement French drain that requires cutting into concrete and establishing a pipeline to the outdoors will cost more.

Installing a French Drain

An interior French drain installation is a significant project that requires specialized knowledge, and you should rely on a professional for this job. Indeed, a French drain that gets rid of too much water can also damage your foundation, so you need an expert to get the balance right.

If your French drain discharges drainage into public sewers or drain ditches, you will probably need a permit from local authorities. Ask your pro exactly what legal requirements you need to meet when installing a drain. (And as always, be skeptical of any contractor who tells you that permits are technically required but that you probably don’t need to bother with it. Skipping permits is a fast track to trouble when you try to sell.)

When you’re installing a French drain outdoors, don’t forget to call 811 before you dig. You’ll want utility markers that make sure you don’t cut straight across a gas line or other utility when your trench is being dug.

How to Maintain a French Drain

French drains, especially the pipe interior, do need to be cleaned on a regular basis, about once a year. Silt, debris, roots, and other impediments can eventually block it up, just like any other water-carrying pipe. You can clear out obstructions yourself if you have a working knowledge of how to snake lines, or you can hire a qualified drain cleaning company to do the work. The small fee will be worth preventing a major backup down the line.

Keep an eye on the French drain pipe the rest of the year. If you see debris piling up near the entrance, clear it away as soon as possible to keep it from getting into the pipe and gumming up the works.

The sump pump connected to a basement French drain installation requires its own maintenance. Follow these tips to keep a sump pump in good operating order.

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