A French drain is a subsurface tool for saturated soil. It is not a general-purpose fix for water running across a surface, which is what most people install it for.
The correct cross-section
Every layer has a job. Skip one and the system silts closed within a few seasons.
- Trench 12–18 inches wide, 18–24 inches deep, with continuous fall of at least 1%
- Non-woven filter fabric lining the trench, wrapped over the top
- 3–4 inches of clean 3/4-inch drain rock as bedding
- Perforated pipe, holes down, rigid not corrugated
- Backfill with clean rock to within a few inches of grade — not soil, not fines
- Cleanouts at ends and turns, and a legal daylight outlet
Soil changes the answer
In sandy coastal soils around Pacific Beach or Cardiff, French drains and infiltration work extremely well. In the expansive clay common inland, water moves through soil slowly, so a French drain intercepts far less than people expect — surface grading, catch basins and channel drains usually do more.
Cost and lifespan
Expect $45–$95 per linear foot installed, more through hardscape or across mature roots. Built properly with fabric and clean rock, service life is 20–30 years; built with soil backfill, closer to three.
Frequently asked
- Can a French drain go against the house?
- A footing drain against a foundation is different work with waterproofing implications — get it designed, not improvised.
- Gravel with no pipe?
- A rock-only trench holds a fraction of the flow. Pipe costs little and multiplies capacity.
- Will roots invade it?
- Filter fabric and clean rock slow it. Keep runs away from ficus and willows entirely.