What a French drain actually does
A French drain takes water out of the ground, not off the surface. Rain that has soaked in, irrigation that keeps a bed wet and water perched on a clay layer all move sideways underground until something intercepts them. A gravel trench with a perforated pipe at the bottom is that interception: water enters the stone from every side, drops to the pipe and runs to the outlet instead of standing in the soil against a wall.
The code's version of it is a foundation drain, and it is not asked for everywhere. The California Building Code describes the detail: clean gravel or crushed stone around and over the pipe, the trench topped with an approved filter membrane, the pipe invert kept below the floor it protects, and discharge by gravity or mechanical means into an approved drainage system. It is required at basements and below-grade floors rather than around every building, so on a slab-on-grade property this is a chosen remedy, not a code duty.
Crawl spaces are where the code and the wet ground meet. Where ground water rises close to the outside ground level, or surface water will not readily leave the site, the under-floor ground has to sit as high as the finished ground outside unless an approved drainage system is provided. A perimeter drain is one way that is done, and it is worth doing early: a crawl space that stays wet through winter costs more in timber than the trench ever did.
Almost every failure is at the two ends. A drain with no fall, or with an outlet higher than the pipe, holds water instead of moving it. One backfilled with the spoil that came out, or built without the membrane over the stone, silts up from above until the voids close. No source opened for this trade gives a service life for one, so nothing here claims a number; what is claimed is a fall, a wrap and an outlet that is genuinely lower.
What french drains includes
The wet ground is checked against the alternatives before anything is dug, because surface water lying on a flat lot is a grading problem and a drain in the wrong place collects nothing. Where the water is underground, the line, the depth and the fall are set from the outlet back.
Services are delineated and the notification goes in before the trench is opened, and irrigation, lighting and downspout lines near a building are exposed by hand.
The trench is cut to a continuous fall towards the outlet, deep enough to sit below what is being protected, with the sides protected as required by Title 8 section 1541 and inspected after rain.
Clean stone is placed, the perforated pipe is bedded on it and covered with more of the same, and the stone is wrapped so that soil cannot migrate into the voids.
Backfill goes in clean and compacted, and the beds, lawn or paving over the trench are put back.
Water is run into the drain to prove it moves, the outlet is checked at the approved point, and the line, depth and photographs go into the record.
A French drain is only as good as the place it lets go. Without an outlet lower than the pipe, and a discharge point the city or district accepts, the trench is a reservoir with gravel in it.
Where French drains are used
Foundation lines
Along walls where the ground beside the building has stayed wet after every storm and the fall on its own cannot be improved further.
Crawl space perimeters
Around under-floor spaces that hold damp through the winter, where an approved drainage system is the alternative to raising the ground inside.
Courtyards and light wells
Enclosed ground with no route out at the surface, where water can only leave in a pipe.
Landscaped commons
HOA lawns and planted beds kept wet by irrigation and roof water, sitting against ground-floor walls.
Wet yards at homes
Back gardens on heavy soil that stay soft for weeks, where the water is in the ground rather than running over it.
Planter strips at retail
Strips between the building face and the paving that hold water, feeding it back under the slab edge.
Questions about french drains
Not on its own. Surface water needs somewhere to run before it needs somewhere to soak, so a pool on flat paving or lawn is usually a grading fix with an inlet. A French drain earns its place where the water is already in the ground.
It has to reach an approved drainage system, by gravity or by pump. What is approved is the city's call, or the district's where the receiving system is a canal or lateral, so the outlet is settled before the trench is set out.
No. A French drain is buried and collects water from the ground through stone; a trench drain is a grated channel at the surface that catches water running across it.
Yes, with care. Excavation next to a foundation must not take away its support, so depth, distance and sequence are set with that in mind, and the wall is protected before the trench is opened.
Clean stone with very little fine material in it, the filter membrane over the top, and backfill that is not the muddy spoil that came out of the trench.
French drains by city
French drains are requested through the grading and drainage page for your city, which covers its soils, its water table and where drains discharge.
Related grading and drainage services
