French Drain
Also known as: trench drain, subsurface drain
A French drain is a gravel-filled trench, usually with a perforated pipe, that collects and redirects water away from a problem area. It is a standard fix for soggy yards, wet foundations and poor drainage.
In simple terms
A French drain is a trench filled with drain rock (often with a pipe at the bottom) that gives water an easy path to flow away from where you don’t want it — like a wet spot in the yard or against the house.
In depth
The classic design is a sloped trench lined with filter fabric, a perforated pipe at the bottom, backfilled with clean drain rock, and the fabric wrapped over the top before finishing. Water in the surrounding soil flows into the high-void rock and pipe and is carried downslope to a safe outlet. Key specs: a continuous slope (about 1% minimum), clean (washed) drain rock to preserve voids, and filter fabric to keep soil from clogging the system. It is especially valuable in clay soils that hold water.
Why it matters
Standing water damages foundations, kills plants and breeds mosquitoes — a properly built French drain is the go-to solution, and it lives or dies on using clean drain rock and filter fabric.
Common mistakes
- Backfilling with rock that contains fines, which clogs and stops draining.
- Building with no slope or omitting filter fabric, so the drain silts up and fails.
Examples & uses
- Drying out a soggy low spot in a yard.
- Carrying water away from a foundation or retaining wall.
- Intercepting runoff at the base of a slope.