A French drain is one of the most useful drainage tools and one of the most commonly botched DIY installations. The design is simple (perforated pipe in a gravel trench, sloped to a discharge), but small mistakes in slope, gravel selection, or fabric placement cause clogging within a few years. This guide covers the parts that determine whether the drain lasts 20 years or starts backing up in season two.
What a French drain solves
French drains are designed for area-wide subsurface water. The pipe collects water along its entire length through perforations, so the drain works on saturated soil rather than on a single point inlet.
Common applications:
- Wet spots in the yard where water stays after rain or where the lawn stays soggy
- Foundation perimeter drainage to collect groundwater before it reaches basement walls
- Hillside interceptors that catch groundwater flowing through a slope before it reaches the house or yard
- Retaining wall backdrains that prevent hydrostatic pressure behind the wall
- Driveway or patio edges where adjacent grade pushes water onto the hardscape
What French drains are not for: point-source water like downspouts (use solid drain pipe extensions instead) or surface water runoff during heavy storms (use swales or surface drains).
Tools and materials
For a typical 50 foot yard drain plan for:
- 50 feet of 4 inch perforated PVC drain pipe (the standard residential size)
- 50 feet of pipe sock or 100 sq ft non-woven geotextile fabric (or both)
- 4 to 5 cubic yards of 3/4 inch washed crushed stone or pea gravel
- 1 to 2 yards of topsoil or sod patches for the final cover
- Shovel, transfer shovel, and a mattock for hard ground
- Pick or trenching tool, or trencher rental (about 100 to 150 dollars per day)
- Wheelbarrow
- Mason string line and stakes for trench layout
- 4 foot or longer level with a 1/4 inch shim for slope verification
- Tape measure
- Optional: end grate for daylight discharge, pop-up emitter for unobtrusive discharge
Total materials for a 50 foot residential French drain runs 400 to 700 dollars.
Planning the route and discharge
Walk the wet area during or just after a rainstorm if possible. Note where water actually pools and which direction the natural slope falls. The French drain should run along the upslope side of the wet area, intercepting water before it reaches the saturated zone, and discharge downslope at a point that is at least 10 feet from any structure.
The discharge end must be lower than the inlet end by at least 1 inch per 8 feet of run (1 percent slope). If your site does not provide enough natural slope, the drain will not work as a passive drain and would need a sump pump. Stop planning a passive French drain in that case and consider a pumped solution.
Daylight discharges (the pipe exits to an open slope) are the simplest. Pop-up emitter discharges look better but introduce a check valve that can clog. Dry wells (gravel pits that let water dissipate) work when daylight is not feasible.
Call before you dig. Buried utilities (gas, electric, fiber, water) cross many yards in places that are not obvious from the surface. The free utility marking service catches most of them.
Excavating the trench
Mark the trench centerline with mason string and stakes. Trench width is 12 to 18 inches; deeper drains can use wider trenches for stability.
Dig the trench. Hand digging works on small projects but rents fast on long runs. A trencher rental (about 100 to 150 dollars per day at the typical big-box rental counter) cuts a 50 foot trench in under an hour. The cleanup of trencher spoil takes longer than the trenching itself.
Maintain consistent depth as you dig. Verify slope by stretching a string line between the inlet and discharge ends, then measuring down to the trench bottom with a tape every 4 to 6 feet. Adjust by digging deeper at low spots or backfilling and tamping high spots.
The trench bottom needs to be smooth, not jagged. Loose dirt and rocks at the bottom can create high spots that interrupt the pipe slope.
Laying fabric and gravel
Roll out non-woven geotextile fabric along the trench. The fabric should line the entire trench (bottom and both sides) with enough excess at the top to fold over the gravel layer later. This burrito-wrap configuration is the key to long drain life.
Pour 2 to 3 inches of 3/4 inch crushed stone or pea gravel along the trench bottom on top of the fabric. Smooth and verify slope again with the string line.
Lay the perforated pipe on the gravel layer with the perforations pointing down (toward the gravel, not up toward the sky). Pipes with two rows of perforations should have both rows at 4 and 8 o’clock positions, also pointing downward. Down-facing perforations let water enter at the bottom of the pipe where it is collected by the gravel, which is the most efficient configuration.
If using a separate pipe sock filter, the sock goes around the pipe before lowering it into the trench.
Connect pipe sections with appropriate couplings. Discharge end should extend to or beyond the daylight or dry well location.
Backfill the trench with crushed stone or pea gravel to within 4 to 6 inches of the surface. Pour gravel evenly along both sides of the pipe to avoid shifting the pipe sideways.
Fold the excess fabric over the top of the gravel layer, creating a complete fabric wrap around the gravel envelope. This prevents soil from above from migrating downward into the gravel.
Final cover
Add 4 to 6 inches of topsoil over the fabric layer. The topsoil is what supports the lawn or planting bed. Tamp lightly. Seed or sod the surface to match the surrounding lawn.
The completed drain is invisible from the surface within one growing season. The fabric, gravel, and pipe work silently underground for decades when properly installed.
Common mistakes to avoid
Perforations facing up: doubles the time water takes to enter the pipe and creates a sediment trap inside the pipe.
No fabric wrap: fine soil particles migrate into the gravel within 2 to 5 years, completely clogging the drainage capacity. The drain stops working.
Insufficient slope: water moves too slowly through the pipe, allowing sediment to settle and the system to silt up. Less than 1 percent slope is the most common failure mode.
Pea gravel only without crushed stone: pea gravel has more uniform size and slightly lower hydraulic conductivity than crushed stone. Either works but crushed 3/4 inch washed stone is the preferred material in most installations.
Discharge pointed back toward the house: a French drain that discharges its collected water onto a path that returns to the foundation defeats its own purpose. Discharge must move water permanently away.
Connecting to a sewer line: never legal and creates backflow contamination risk. Drain pipe discharge must go to surface daylight, a storm drain, or a properly sized dry well.
See the methodology page for our drainage evaluation approach. Pair this guide with the downspout extension article and the paver patio installation guide for a complete outdoor water management sequence.
Frequently asked questions
What is the difference between a French drain and a regular drain?+
A regular drain (solid pipe) carries water from a single inlet to a single outlet, like a roof gutter downspout extension. A French drain is a perforated pipe surrounded by gravel that collects groundwater along its entire length and routes it to a discharge point. The perforations let water enter the pipe anywhere along the run. The gravel around the pipe provides a high-conductivity pathway for groundwater to flow toward the pipe. French drains solve area-wide saturated soil; regular drains solve point-source water like a downspout.
How deep should a French drain be?+
The trench depth depends on what the drain is collecting. For surface water and yard saturation: 18 to 24 inches deep. For foundation drainage (against a basement wall): the bottom of the trench should be at or below the basement floor level, typically 4 to 8 feet deep. For interceptor drains on a slope: 24 to 36 inches deep, below the water table at the upslope side. The deeper the drain, the more groundwater it can collect, but also the more excavation and the higher the cost. Match depth to the actual water problem rather than digging unnecessarily deep.
What slope does a French drain need to work?+
Minimum 1 percent (1/8 inch per foot). Standard target is 1 to 2 percent. The slope is the pitch of the drain pipe along its length, draining toward the discharge end. On a 50 foot drain, the discharge end sits 6 to 12 inches lower than the inlet end. Negative slope (uphill toward discharge) does not drain at all. Zero slope drains very slowly and silts up within a few years. Slopes above 5 percent move water too fast and can cause silt washout. The 1 to 2 percent range is the engineering sweet spot for residential French drains.
Do I need a sock or fabric around the French drain pipe?+
Yes for almost all installations. Non-woven geotextile fabric wrapped around the gravel (and ideally a sock filter around the pipe itself) keeps fine soil particles out of the gravel and the pipe perforations. Without fabric, fine silt and clay particles migrate into the gravel pore space within 2 to 5 years, clogging the drainage capacity. The 30 to 50 dollars worth of fabric on a typical residential French drain extends the working life from 3 to 5 years (unfabriced) to 20 plus years (properly fabriced).
Where should a French drain discharge water?+
Three valid discharge options. Daylight: the pipe exits to a slope or low spot on your property at least 10 feet from any structure. Best option when topography allows. Storm drain: connect the discharge to a municipal storm drain if local code permits. Check requirements with the municipality. Dry well: a gravel-filled pit (typically 4 feet diameter by 4 feet deep) that lets collected water dissipate into the surrounding soil. Best for sites where daylight is not feasible and a storm drain is not nearby. Never discharge a French drain into a sewer line, septic system, neighboring property, or a drainage path that puts the water back near the house.
