Whether you hire it out or are just trying to judge a quote, it helps to know what a properly built French drain looks like. These are the typical steps and the choices inside each one. Exact depths and specs depend on the yard, so treat the numbers here as typical ranges, not rules.
- 1Surface and topsoil. Sod, rock, or mulch over the trench. Water soaks down to the gravel faster than through the clay around it.
- 2Filter fabric. Wraps the gravel on all sides and keeps clay fines out. Skipping it is the usual reason drains clog.
- 3Washed gravel. Clean, uniform stone gives water an easy path to the pipe.
- 4Perforated pipe on a slope. Holes facing down, laid with a steady fall so water flows the whole way.
- 5The outlet. Daylight, a curb outlet where allowed, or a dry well. Without one, a drain just fills up.
Illustration. Not to scale; depth, width, and pipe size vary by yard.
1. Find the source and the outlet
A good installer starts by working out where water comes from and where it can go. Downspouts, slope, soil, and water arriving from uphill all point to different fixes, covered in the standing water guide. Then comes the outlet: lower ground where the pipe can daylight, a curb outlet where the city allows it, or a dry well. The route is planned backward from the outlet so the pipe falls the whole way.
2. Mark utilities before digging
Irrigation lines, gas, electric, and cable run through residential yards. In California the free 811 service (USA North 811 in Northern California) arranges marking of public utility lines, and its operator says notice is required at least two working days before excavation, including for homeowners. Private lines such as irrigation and landscape lighting aren't covered by 811 and need to be located separately.
3. Dig the trench
- Depth. Shallow lawn and side-yard drains are often 18 to 24 inches deep; foundation-edge and interceptor drains typically go deeper, and hillside drains deeper still. A Bay Area contractor's guide groups them the same way: shallow at 18–24 inches, foundation-grade at 24–36 inches or more.
- Width. Often around 6 to 12 inches, wide enough for the pipe with gravel around it.
- Slope. The trench bottom falls steadily toward the outlet. A common rule of thumb is at least about 1% (roughly 1 inch per 8 to 10 feet), with more being better. Low spots hold water and collect silt.
4. Fabric, gravel, and pipe
- Filter fabric (a non-woven geotextile) lines the trench, with extra left to fold over the top. It keeps soil out of the gravel. In clay soil it's the single most important material.
- Washed gravel, commonly around 3/4 inch, goes in as a bed a couple of inches deep. "Washed" matters: dirty gravel brings its own fines.
- Perforated pipe is set on the gravel with holes facing down so water rises into it from below. Rigid PVC and corrugated pipe are both used; rigid pipe holds its slope and can be flushed more easily.
- More gravel covers the pipe, then the fabric is folded over the top so the gravel is wrapped on all sides.
- Solid pipe takes over where the line passes the house or runs to the outlet, so water isn't leaked back into the soil.
5. Outlet and cleanouts
The pipe ends at a pop-up emitter, a daylight point protected by a grate or rock, a curb outlet, or a dry well. A cleanout (a capped riser at the high end or at bends) lets the line be flushed later. It's an inexpensive add-on that makes maintenance possible.
6. Backfill and restore
The top of the trench is finished with soil and sod, decorative rock, or mulch. A strip of gravel at the surface collects surface water too, but in a lawn it also collects debris. Ask how the contractor will repair irrigation and sod, and whether that's in the price.
How long it takes
A straightforward residential yard drain is often a one- to three-day job, longer with hand digging in narrow side yards, concrete cutting, or a dry well. Clay soil is slower to dig than sandy soil.
Signs a corner is being cut
- No filter fabric, or fabric only on the bottom
- Dirty fill or native soil instead of washed gravel around the pipe
- Perforated pipe all the way to the outlet, including past the foundation
- No measurement of slope, or a trench that visibly dips
- An outlet that's just "the end of the trench"
- No cleanout
Should you do it yourself? A short, shallow drain in an open lawn is within reach for some homeowners. Long runs, foundation-edge or hillside drains, anything under concrete, and any yard where the outlet is uncertain are worth handing to a contractor. Prices for both are on the cost guide, and the hiring checklist is on choosing a yard drainage contractor.
Frequently asked questions
How deep does a French drain need to be buried?
Shallow lawn and side-yard drains are often 18 to 24 inches deep; drains protecting a foundation or intercepting water on a slope usually go deeper. The right depth depends on where the water is moving and the fall available to the outlet.
How much slope does a French drain need?
A common rule of thumb is at least about 1%, roughly an inch of drop for every 8 to 10 feet, and more where possible. The key is a steady fall with no low spots.
Should French drain holes face up or down?
Down is the common practice: water rises from the gravel bed into the pipe from below, and less sediment settles into the holes.
Do I need to call 811 before installing a French drain?
USA North 811, which handles Northern California, says calling before digging is a legal requirement in California, applies to homeowners, is free, and needs at least two working days' notice.
Is it better to install a French drain inside or outside a crawl space?
It depends on where the water enters. On a slab home the question is exterior drainage: grading, downspouts, and a yard or foundation-edge drain. For a crawl space, a contractor should inspect it first; exterior drainage often comes first, with an interior system only if water still gets in.
Cost ranges and technical notes on this page are general information from the sources named, not a quote or professional advice. Prices vary by yard, soil and drain design; get a written quote for your property. See the disclaimer and cost sources.