How to install a French drain
A French drain fixes a soggy lawn, a wet basement wall, or water pooling against the foundation by giving that water an easy underground path away from the problem. It's a digging-heavy but mechanically simple project — a sloped, gravel-filled trench with a perforated pipe, wrapped in filter fabric. Here's how to do it, plus how much gravel and pipe to buy.
How a French drain works
Water always takes the path of least resistance. A French drain gives it one: a trench full of loose gravel is far easier to flow through than packed soil, so water sinks in, collects in a perforated pipe at the bottom, and runs downhill to a safe outlet. Two things make or break it:
- Continuous slope. The trench must fall steadily toward the outlet — at least 1%, about 1 inch of drop every 8 feet. No slope, no drainage.
- Filter fabric. Wrapping the gravel and pipe in landscape/filter fabric keeps silt from washing in and clogging it — the difference between a drain that lasts decades and one that silts up in a season.
Plan the route and slope
Work out where the water comes from and where it can safely go — downhill to daylight (an open spot on a slope), a dry well, or a storm drain where permitted. Then:
- Call 811 before you dig so the utility lines get marked — free, and the law in most places.
- Check the fall along your route with a line level; confirm the outlet is lower than the start.
- Keep clear of the septic field, and slope foundation drains away from the house, never toward it.
How much gravel, pipe, and fabric?
Enter your trench length, width, and depth and our calculator gives the gravel, perforated pipe, and filter fabric you need, plus the cost.
Open the French Drain CalculatorWhat you'll need
- 4″ perforated drain pipe (rigid PVC for the longest life, or flexible corrugated for tight routes)
- Drainage gravel — clean ¾″ crushed stone that won't pack tight
- Landscape/filter fabric (or a pre-sleeved "pipe sock") to wrap the gravel and pipe
- A trenching shovel or a rented trencher, plus a wheelbarrow for spoil and gravel
- A line level or laser level to keep the slope true
- Marking paint, stakes and string, and a tamper
Step by step
- Mark the route and call 811. Lay out the line with paint or string, and wait for utilities to be located before digging.
- Dig the trench. Aim for about 18–24″ deep and 6–12″ wide, and grade the bottom to a steady fall toward the outlet — check it as you go, since the slope is everything.
- Line it with filter fabric. Drape fabric into the trench with plenty of overhang up both sides; you'll fold it over the top at the end.
- Add a gravel base. Shovel 2–3″ of gravel along the bottom and smooth it to keep the slope consistent.
- Lay the perforated pipe. Set the pipe on the gravel, holes facing down for collecting groundwater, maintaining the slope to the outlet.
- Bury the pipe in gravel. Cover the pipe with several inches of gravel, filling to a few inches below grade (or to the surface for a drain that also takes surface water).
- Wrap it up. Fold the overhanging fabric over the top of the gravel like a burrito so soil can't migrate in.
- Cap the trench. Top with soil and sod for a hidden drain, or leave decorative stone exposed if it's collecting surface runoff. Make sure the outlet stays open and daylights cleanly.
- Test it. Run a hose at the high end and confirm water flows through and out the outlet within a few minutes.
Mistakes to avoid
| Mistake | Why it matters |
|---|---|
| Too little or reversed slope | Water sits in the trench instead of draining — keep a steady fall to the outlet |
| Skipping filter fabric | Silt washes into the gravel and pipe and clogs the drain within a season |
| Using solid pipe (or holes up) | Groundwater can't get into the pipe — use perforated pipe, holes down |
| A buried or uphill outlet | The water has nowhere to go; the outlet must daylight lower than the inlet |
| Trench too shallow | Misses the water you're trying to intercept, especially near a foundation |
| Not calling 811 first | Hitting a gas, power, or water line is dangerous and expensive |