A French drain is a gravel-filled trench with a perforated pipe at the bottom that intercepts groundwater and carries it somewhere harmless. It is honest hard labor — expect a full weekend for a 50-foot run, most of it digging — but the materials are cheap: pipe, fabric, and washed stone typically land between $300 and $700 for that length. The build standard is not negotiable: a trench at least 12 inches deep, washed #57 stone, 4-inch perforated pipe, the whole assembly wrapped in geotextile fabric like a burrito, falling at a slope of at least 1% toward an outlet.
Tools & Materials
- Trenching shovel and mattock, or a rented trencher for long runs
- 4-inch perforated pipe — rigid PVC with two rows of holes preferred over flexible corrugated
- Non-woven geotextile fabric, 4 ft wide roll
- Washed #57 stone (clean stone, no fines — this matters)
- Line level and string, or a laser level; stakes; marking paint
- Pop-up emitter or rodent grate for the outlet
Step 1: Plan the Route and Find Your Outlet
Start at the outlet and work backwards. A French drain with nowhere to discharge is just a buried bathtub. You need a spot lower than the trench bottom at the far end: a slope that daylights, a curb, a dry well, or a drainage swale. Call 811 before digging — utility strikes are free to avoid and expensive to cause. Route the drain along the uphill side of whatever you are protecting, so it intercepts water before it arrives. Check the total fall available: at a minimum 1% slope, a 50-foot run needs at least 6 inches of elevation drop from start to outlet. If the math does not work, shorten the run or find a lower outlet.
Step 2: Dig the Trench
Dig at least 12 inches deep and 6–12 inches wide — deeper if you are protecting a basement footing, where the pipe should sit near footing level. Set the trench bottom to a consistent fall of 1% or better (that is 1 foot of drop per 100 feet, or about 1/8 inch per running foot). Check it with a string line and line level as you go, not at the end. Bellies in the trench bottom become sediment traps that slowly choke the pipe.
Step 3: Line the Trench with Fabric
Drape non-woven geotextile fabric across the trench so it lines the bottom and both walls, with enough excess on each side to fold over the top later. This is the burrito wrap, and it is the difference between a drain that lasts 30 years and one that silts solid in five. The fabric lets water through while keeping soil fines out of the stone. Use non-woven here — it is the filtration fabric. Woven stabilization fabric belongs under driveways, not around drains; the fabric guide covers the distinction in detail.
Step 4: Bed the Pipe — Holes Down
Place 2–3 inches of washed #57 stone on the fabric, then lay the pipe on top. Now the eternal argument: holes up or holes down? Holes down. Water in the surrounding stone rises from the bottom of the trench; with the perforations at the bottom, the pipe starts collecting as soon as water reaches trench bottom, keeping the water table in the trench as low as possible. With holes up, water must fill the trench past the top of the pipe before any of it enters — which defeats the purpose of a drain meant to keep water levels down. Manufacturer installation guides for dual-wall perforated pipe say the same thing. Confirm the slope again with the pipe in place before burying it.
Step 5: Fill with Washed Stone and Close the Burrito
Fill around and over the pipe with washed #57 stone to within a few inches of grade. Washed stone only — crusher run or anything with fines will clog the voids that make the drain work. Fold the fabric flaps over the top of the stone, overlapping them, so the stone column is completely enclosed. Size the stone order with the French drain calculator or the #57 stone calculator; a 50-foot trench 12 inches wide and 12 inches deep holds about 1.9 yd³, and at roughly 1.4 tons per loose cubic yard you are ordering about 2.5–3 tons.
Step 6: Cap and Finish the Surface
What goes on top depends on the job. For a curtain drain catching surface water too, bring the washed stone all the way to grade and leave it exposed. To hide the drain, cap the burrito with 3–4 inches of topsoil and sod — understanding that surface water will reach the drain more slowly. Never cap with heavy clay; you would be roofing over your own drain.
Step 7: Daylight the Outlet
Transition to solid (non-perforated) pipe for the final run to the outlet so collected water cannot leak back out. Daylight it on a slope with a rodent grate, or use a pop-up emitter in a lawn. Keep the outlet visible and mowable — buried, forgotten outlets get crushed, and a blocked outlet backs the entire system up. If the discharge would cut a channel across the lawn, spread it through a dry creek bed, which turns the outfall into landscaping instead of erosion.
Troubleshooting an Existing (or Failing) Drain
Water backs up at the inlet end or pools on the surface above the trench
This almost always means the pipe or the stone around it has silted in somewhere downstream, usually at a low spot or belly where the slope was not held to a consistent 1%. Find the nearest cleanout or the outlet and run a garden hose backwards into the pipe; if water backs up quickly, the blockage is close to that end. On drains without a cleanout, this is the point where you excavate a short section at the suspect spot, cut the pipe, flush it with a hose or a plumber’s snake, and splice in a repair coupling. Going forward, add an inline cleanout (a vertical pipe stub capped at grade) at the high end of any new drain — it turns a future dig into a five-minute flush.
The outlet freezes shut every winter
A daylight outlet that sits in a shaded, low-airflow spot can ice over in freezing weather, and once the outlet is blocked the whole system backs up behind it — water in the trench then has nowhere to go and can heave under freeze-thaw pressure. Two practical fixes: bury the outlet pipe below the local frost line for as much of its run as possible so freezing happens only in the last exposed foot or two, and slope the final run steeper than the 1% minimum so any water in the pipe drains out completely between rain events instead of sitting and freezing. A pop-up emitter helps here too, since it closes and reduces standing water right at the discharge point.
Roots have grown into the pipe
Perforated drain pipe is exactly the kind of moist, oxygenated environment tree roots seek out, and thin feeder roots can work through the perforations over a few seasons, eventually filling the pipe. If a drain that worked fine for years suddenly slows down, suspect roots before anything else, especially if the route passes within 10–15 feet of a mature tree. A plumber’s auger or a rented root-cutting snake through the cleanout or outlet will clear it short-term. For a permanent fix, either reroute the drain further from the tree line or switch to solid pipe with occasional catch basins in the sections nearest known root zones, since roots cannot enter pipe that has no holes.
Mistakes That Clog French Drains
- Skipping the fabric wrap. Soil fines migrate into the stone and the drain silts shut. The burrito is mandatory.
- Stone with fines. Crusher run compacts into a nearly waterproof mass. Washed stone only.
- Flat or back-sloped sections. Water sits, sediment settles, capacity dies. Hold 1% minimum the whole way.
- Sock-wrapped corrugated pipe as a substitute for the burrito. A pipe sock protects the pipe, not the stone around it.
- No outlet plan. Discharging at a property line or back toward the house just relocates the problem.
Bottom Line
A French drain is five decisions done right: a real outlet, a trench at least 12 inches deep falling at 1% or better, 4-inch perforated pipe laid holes down, washed #57 stone, and a complete non-woven fabric burrito. Get those five right and the drain will quietly move water for decades. Cheat any one of them and you will be digging the whole thing up again — through mud this time.
