How to Build a French Drain in Your Yard
Where to run it, how deep and how much slope, the right gravel, pipe and fabric, and the step-by-step build.
A French drain is a gravel-filled trench with a perforated pipe at the bottom. Water in the soil finds its way into the gravel, enters the pipe and flows downhill to an outlet. Built well, it quietly dries out a soggy lawn or stops water pooling against a wall. Built badly (flat, or without fabric), it silts up and stops working within a few years.
Before you start: is it the right fix?
French drains handle water in the ground: saturated soil, a wet strip along a fence, seepage from a slope. If the problem is surface water running across the yard in heavy rain, regrading or a swale may work better. If water is entering a basement, talk to a foundation or waterproofing professional first.
You also need a place for the water to go that’s lower than where the drain starts: a slope, a ditch, a dry well or a storm drain where allowed. No outlet, no drain.
Example: a 40 ft yard drain
8” wide, 18” deep at the start, 1% slope, with 3” of soil and sod on top:
| Item | Quantity |
|---|---|
| Trench depth | 18” at the start, 22.8” at the outlet (4.8” of fall) |
| Washed drain stone | 1.92 tons (1.37 cu yd) |
| 4” perforated pipe | 42 ft |
| Non-woven fabric | at least 5.64 ft wide × 40 ft (6 ft roll if available) |
| Soil to dig out | about 2.1 cu yd once loose |
1. Plan the route and the fall
Run the drain through the wettest area and on to the outlet by the most direct route. Call 811 (US) before digging, since drains often cross utility lines.
A common minimum slope is 1%: about ⅛” per foot, or 1” every 8 ft. Over 40 ft, that’s 5” of fall. If the ground already slopes toward the outlet, some of the fall comes for free. Check the trench bottom with a level and a straight board, or a string line, as you dig.
2. Dig the trench
Most yard drains are 6–12” wide and 18–24” deep at the shallow end. A trenching shovel works for short runs; for long runs, renting a trencher saves a lot of effort. Pile the soil on a tarp to keep the lawn clean. Keep the bottom smooth, with no dips where water could sit.
3. Line it with fabric
Lay non-woven drainage fabric along the trench, covering the bottom and both sides, with enough left at the top to fold over the gravel and overlap by about a foot. This “burrito wrap” keeps soil out of the stone.
4. Gravel, pipe, gravel
- Spread 2–3” of clean, washed stone (¾” to 1½”, such as #57) in the bottom.
- Lay the perforated pipe on it. Most installers put the holes facing down, so water rises into the pipe and leaves the sediment behind. A pipe with a fabric sock gives extra protection.
- Check the slope again along the pipe.
- Cover with gravel to within a few inches of the surface.
Avoid crusher run or road base. The fine dust that makes them compact also blocks water.
5. Close it up
Fold the fabric over the top of the gravel. Then either finish with decorative stone right to the surface, or cover with 2–4” of soil and replace the sod.
At the outlet, finish with a pop-up emitter (which opens when water flows and stays flush with the lawn otherwise), a grate where the pipe daylights on a slope, or a connection to a dry well.
Common mistakes
- Not enough slope, or a dip in the middle where water sits.
- No fabric, so soil slowly fills the gravel.
- The wrong stone, with fines that clog.
- An outlet that’s higher than the wet area, or that dumps water onto a neighbor.
Get the trench depths and materials for your own run in the French Drain Planner.