French drains
Summary: A French drain is a fabric-lined trench of washed stone around a perforated pipe that collects groundwater and carries it to an outlet. Celtic Rainwater Systems builds exterior drains topped with rock or grass and interior drains feeding sump pumps[1][2].
What is a French drain?
A French drain is a trench filled with gravel or rock, usually containing a perforated pipe, that collects surface water and groundwater and carries it away from an area[1]. The name is usually traced to Henry Flagg French, whose 1859 book Farm Drainage promoted the method in the United States. Celtic Rainwater Systems describes its version as “the digging of a large ditch, lined with geotextile fabric and a wrapped perforated pipe designed to allow water to freely enter through the washed stone base”[2].
What are the parts?
| Layer | Purpose |
|---|---|
| Trench | Sloped continuously toward an outlet |
| Geotextile fabric | Lines the trench so soil does not wash into the stone |
| Washed stone | Gives water an easy path to the pipe |
| Perforated pipe (often wrapped in a sock) | Carries collected water to the outlet |
| Top finish | Decorative rock, or soil and grass |
The company says its drains can be topped “with decorative rock, creating an attractive and functional landscape piece, or can have grass grown back over top”[2]. The outlet matters as much as the trench: water must reach daylight, a dry well, an emitter or a sump basin.
Interior versus exterior
An exterior French drain intercepts water in the yard or along the outside of a foundation. An interior drain, sometimes called a perimeter drain, sits under the edge of the basement floor, collects water that comes through the wall or floor joint and leads it to a sump pump. The company offers both, and describes interior work ranging from one leaking wall to an entire perimeter system[2][3].
When is a French drain the right fix?
A French drain suits persistent groundwater and slow-draining areas where grading alone cannot move water, such as a flat yard, the uphill side of a house on a slope, or a low spot along a fence. It is the wrong first fix when the water is coming from the roof. DOE building science guidance advises against connecting downspouts to the foundation drain system[4], because a drain sized for seepage can be overwhelmed by a roof’s flow. Fix gutters, downspout discharge and grading first; see grading and foundation drainage.
Do French drains need maintenance?
They can clog over time if fine soil gets past the fabric, or if roots get into the pipe. Cleanouts at the ends make it possible to flush or snake the pipe. Outlets should be checked so they stay clear of mulch, leaves and ice. The company lists drainage repair and snaking among its services[3].
See also
Common questions
What is a French drain made of?
A trench lined with geotextile fabric, filled with washed stone and a perforated pipe, sloped to an outlet.
Can a French drain go inside a basement?
Yes. An interior perimeter drain under the floor edge carries water to a sump pump.
Should downspouts connect to a French drain?
DOE guidance advises keeping roof water out of the foundation drain system; use a separate solid pipe instead.
References
- ^ a b French drain. Wikipedia. Accessed 9 November 2026.
- ^ a b c d Drainage (PVC downspout drainlines, French drains, basement drainage and sump pumps). Celtic Rainwater Systems (celticrainwater.com). Accessed 9 November 2026.
- ^ a b Celtic Rainwater Systems (home page: mission, about us, areas of expertise). Celtic Rainwater Systems (celticrainwater.com). Accessed 9 November 2026.
- ^ Foundation Drain, Gutters and Downspouts. U.S. Department of Energy, Building Science Education Solution Center. Accessed 9 November 2026.