A French drain solves yard drainage problems by collecting groundwater and surface runoff in a gravel trench and directing it away through a perforated pipe. When installed correctly, it lasts 20+ years. When installed wrong, it fails in 2–3.
The most important thing to remember: The holes in the perforated pipe face DOWN. Water rises from the saturated gravel into the pipe from below — it does not rain into the pipe from above. Getting this wrong is the #1 cause of French drain failure.
| Material | Quantity |
|---|---|
| 4" perforated drain pipe | Trench length + 10% |
| 3/4" clean crushed stone | ~0.8 cf per linear foot |
| Non-woven geotextile fabric | Trench length × 4 ft |
| Pipe outlet end cap (perforated) | 1 |
| Topsoil or decorative stone | As needed |
| Stakes and string line | Per trench length |
Use the Gravel Calculator to estimate how much crushed stone you need for your trench dimensions.
Walk your yard during or right after a heavy rain to see where water collects and which direction it naturally flows. The drain trench needs to intercept water before it reaches the problem area. Plan a route that follows the natural low spots and leads to a lower elevation where water can exit (daylight). A French drain only works by gravity — if there's no lower place to drain to, you need a sump pump system instead.
Call 811 (in the US) or your country's equivalent to mark buried utilities. This is free, required by law in most states, and takes 2–3 business days. Striking a buried gas, water, or electric line is dangerous and expensive. Never skip this step regardless of how well you think you know your yard.
The drain needs at minimum 1% slope — 1 inch of drop per 8 feet of horizontal run. Use stakes and a string line with a line level to establish your grade line before digging. Mark the high end and low end, calculate the total drop needed (run length ÷ 8 for 1% slope), and adjust your entry and exit points accordingly. More slope is always better — 2% is ideal.
Dig the trench 18 inches deep and 12 inches wide as a starting point for yard drainage. For foundation drainage, go deeper — below the footing level, typically 24–36 inches. Use a trenching shovel for clean edges. A rented trenching machine is worthwhile for runs over 30 feet. Keep the trench bottom at consistent slope — check with your line level every 8–10 feet as you dig.
Line the trench with geotextile landscape fabric (non-woven filter fabric — not the weed barrier kind). Leave enough fabric draping over the sides to fold over the top later — you need about 2× the trench width plus 6 inches on each side. The fabric keeps soil from migrating into the gravel and clogging the system over time. This is what makes the difference between a drain that works for 20 years vs one that fails in 5.
Add 2–3 inches of clean washed gravel (3/4-inch crushed stone — not pea gravel) to the bottom of the fabric-lined trench. Then lay the 4-inch perforated pipe on top of the gravel with the HOLES FACING DOWN. This is the single most important detail. Water in the gravel rises up into the pipe through the holes — the pipe doesn't collect rain from above. If holes face up, the pipe fills with dirt within months. The pipe should slope toward the exit point throughout its entire length.
Fill the trench with clean washed gravel to within 3–4 inches of the surface. Use 3/4-inch or 1-inch crushed stone — clean, angular stone without fine particles. Do not use pea gravel (it migrates) or crusher run (too many fines that clog). Pack the gravel gently to settle it without disturbing the pipe position. Check that the pipe hasn't shifted and the holes are still facing down.
Fold the landscape fabric over the top of the gravel, overlapping the edges in the center of the trench. This keeps topsoil from eventually mixing with the drainage gravel. Add topsoil over the fabric to fill within an inch of grade, then seed or sod over it. Alternatively, finish with 3–4 inches of decorative stone visible at grade — some homeowners prefer this for a clean look in landscape beds.
| Trench Length | Drop at 1% Slope | Drop at 2% Slope |
|---|---|---|
| 20 ft | 2.4" | 4.8" |
| 30 ft | 3.6" | 7.2" |
| 50 ft | 6" | 12" |
| 75 ft | 9" | 18" |
| 100 ft | 12" | 24" |
The most common DIY mistake. Water rises from below into the pipe — it doesn't rain into it. Holes face down, always.
Less than 1% slope and water moves slowly or pools in low sections of the pipe. Establish grade with a line level before digging.
Without fabric, soil eventually migrates into the gravel and clogs the drain. The system becomes ineffective within 5–10 years.
Pea gravel migrates and shifts. Crusher run has too many fines that clog the system. Use clean 3/4-inch crushed stone.
If the pipe has nowhere to go, you're just storing water underground temporarily. The pipe must daylight at a lower elevation or connect to a dry well or storm drain.
Gas line, electrical conduit, and water lines run through yards constantly. One shovel strike can be fatal. Call 811 every time, no exceptions.
A French drain is a gravel-filled trench with a perforated pipe that collects and redirects groundwater and surface runoff. You need one when water pools in your yard after rain, when water flows toward your foundation, or when a low area stays wet for days after precipitation.
The holes face DOWN. This is the most common DIY mistake. Water rises from the saturated gravel into the pipe through the holes — the gravel acts as a filter and collection medium. If holes face up, dirt washes in and clogs the pipe within a season.
Minimum 1% slope — that's 1 inch of drop for every 8 feet of run. More slope is better. Use a string line and line level to establish grade before you dig. If you can't get enough slope to daylight the pipe at a lower elevation, you may need a dry well or sump pump.
For yard drainage and surface water: 18–24 inches deep is typical. For foundation/perimeter drainage catching groundwater: 24–36 inches, going below the footing level. For agricultural or field drainage: depths vary widely by soil and application.
The drain must terminate (daylight) somewhere: a lower area of your yard where water can spread, a storm drain connection (check local code), a dry well, or a gravel pit/infiltration basin. Never dead-end a French drain — you're just moving the water problem, not solving it.
DIY materials for a basic 50-foot French drain run $300–$600 (pipe, gravel, fabric, rental equipment). Professional installation runs $20–$50 per linear foot, so a 50-foot drain costs $1,000–$2,500 installed. Longer or deeper drains, or those near structures, cost more.