How to tell you need a French drain under turf

You need subsurface drainage when natural infiltration and surface slope cannot keep up. Use these triggers to decide:

  • Grade is flatter than 1 percent away from structures or toward a safe discharge point.
  • Soil is clay or compacted fill that stays saturated and leaves standing water after storms.
  • Roof downspouts or uphill flow add water to the install zone.
  • Low spots near patios, curbs, mow strips, or edging trap runoff.
  • Puddles last longer than 24 hours in mild weather.
  • Shaded areas that rarely dry out, or high traffic paths that compact.
  • Putting greens or sport areas with tight surfaces that shed water quickly.

Field checks before you commit

Hose test

Run a hose at the high side for 10 to 15 minutes. If water ponds or tracks back toward structures, plan a drain or grade change.

Simple percolation check

Dig a 12 inch deep hole, fill with water twice, then time the third fill. If it takes more than 60 minutes to drop 6 inches, the soil is slow draining and a French drain or open graded base is smart.

Grade and outlet verification

  • Confirm 1 to 2 percent surface fall where possible.
  • Identify a lawful discharge: daylight at a lower slope, existing storm inlet, dry well sized for your soil, or a sump discharge where allowed.
  • Never send water onto a neighbor lot or public sidewalk.

Design basics that work

  • Trench width 6 to 12 inches. Depth to sit below the turf base, with top of stone just under the finished base.
  • 4 inch perforated pipe, rigid SDR 35 or strong corrugated with a sock. Perforations down within a stone envelope.
  • Minimum pipe slope 1 percent toward the outlet.
  • Angular, washed stone 3/4 inch for the envelope. Avoid fines and pea gravel that clog.
  • Nonwoven geotextile 4 to 8 oz to wrap the trench stone and isolate from native soil.
  • Cleanouts or an accessible basin at ends and direction changes for maintenance.
  • Outlet to daylight, catch basin to storm, or a dry well sized to soil infiltration and roof or site runoff.

Integrating the drain with the turf system

  1. Lay out the low line where water naturally collects or where downspouts discharge.
  2. Excavate the trench and a stable outlet path before building the base.
  3. Line trench with nonwoven fabric, install a few inches of stone, then set the perforated pipe sloped to outlet.
  4. Backfill with stone to within 1 to 2 inches of the planned subgrade and wrap fabric over the top.
  5. Install geotextile separation fabric over the entire area if your soil is clay or mixed fill.
  6. Build the turf base with 3 to 4 inches of compacted angular aggregate. Use open graded base in slow soils.
  7. Compact to a firm, even plane and verify final surface slope.
  8. Install turf, infill, and edge detail. Keep seams away from cleanouts where possible.

When you can skip the French drain

  • Sandy or loamy soils with 1 to 2 percent surface slope and no added roof water.
  • Open graded base or drainage mats over permeable subgrade that already drains well.
  • Downspouts rerouted to storm, cistern, or daylight away from the turf area.
  • Minor low spots resolved by regrading and adding a shallow swale.

Alternatives and complements

  • Open graded base using larger angular stone layers to increase storage and flow.
  • Drainage grid panels over concrete or dense subbase to create underlayment flow paths.
  • Catch basins with short lateral pipes to a main drain.
  • Swales or micro grading to move sheet flow to a basin or safe lawn area.

Cost, timeline, and planning

  • Typical French drain under turf installs run 30 to 75 dollars per linear foot depending on depth, outlet, and access.
  • Small yards often need 30 to 80 linear feet. Expect 1 day for 50 to 100 feet with a trained crew.
  • Add budget for dry wells, saw cuts, or long daylight runs.

Code, safety, and maintenance

  • Call 811 before you dig and verify local stormwater rules.
  • Discharge to an approved location and protect slopes from erosion.
  • Flush cleanouts annually and after leaf season. Keep downspout filters clear.
  • Protect fabric and pipe from fines. Keep concrete washout and soil spoils away from open trenches.

Ready checklist

  • Confirmed slow soils, flat grade, or added roof water.
  • Defined outlet and pipe route with 1 percent fall.
  • Specified pipe, stone, and fabric that will not clog the system.
  • Integrated trench elevation with base thickness and final turf grade.
  • Maintenance access in place via cleanouts or basins.