Why clay soil changes the drainage plan

Clay has tiny pores and very low infiltration. Water that passes through artificial turf drains fast into the base, but in clay it cannot soak away quickly. Without a path out, water can perch under the turf and keep the surface soft or soggy after storms. That is why a French drain paired with an open graded base is often the smartest move.

When a French drain is required vs optional

Use a French drain when any of these are true

  • Surface slope is under 2 percent and there is no natural path to daylight.
  • Ponding lasts more than 24 hours after a typical storm.
  • The area is large, generally over 400 square feet, or collects runoff from roofs or hardscape.
  • High rainfall region or frequent hose washdowns, especially pet runs and play spaces.
  • Adjacent grades or edging create a bowl that traps water.

You may not need one when

  • You have 2 to 3 percent uniform surface slope to a safe outfall, and no upstream runoff loads the area.
  • Underlying soils are sandy or loamy with good infiltration.
  • The area is small and elevated on a thick open graded base with an open edge that weeps to landscape.

Drainage design that works in clay

Recommended cross section from top to bottom

  1. Artificial turf with stitched drainage holes and appropriate infill.
  2. Optional shock pad or drainage pad for play or sport.
  3. Leveling layer 0.5 to 1 inch of clean screenings or 1/4 inch chips with minimal fines.
  4. Open graded base 4 to 6 inches of clean, washed stone such as 3/4 inch clear aggregate. Compact in lifts.
  5. Non woven geotextile separator to keep clay fines out of the base.
  6. Compacted subgrade shaped to shed toward the drain.

This open graded system stores and moves water quickly to the underdrain. Dense graded bases that use fines hold water in clay and should be avoided.

French drain layout and sizing basics

  • Pipe: 4 inch perforated SDR 35 or corrugated with a filter sock.
  • Trench: 8 to 12 inches wide, depth so the pipe invert sits at or just below the base bottom. Surround with clean stone and wrap in geotextile.
  • Slope: Target 1 percent to an outlet. Minimum 0.5 percent if space is tight.
  • Placement: Along the low side of the turf zone. For large fields, add laterals every 10 to 15 feet to a collector line.
  • Cleanouts: At ends and direction changes, or every 50 feet.
  • Outlet: Daylight to grade with a rodent guard, discharge to a dry well sized for your storm, or connect to an approved storm line where permitted.

Outfall options and quick sizing

  • Daylight: Best option. Keep the outlet above finished grade and protected.
  • Dry well: Size to the contributing area. A rule of thumb is 0.623 gallons per square foot per inch of rain. Account for stone voids around 40 percent when calculating volume.
  • Storm connection: Only with local approval. Install a backwater device if required.

Step by step install summary

  1. Map slopes, runoff sources, and your intended outfall.
  2. Excavate turf area and drain trench. Shape subgrade to fall toward the drain.
  3. Install non woven geotextile over subgrade and in the drain trench.
  4. Set perforated pipe on grade with a 1 percent fall. Add cleanouts.
  5. Backfill trench with clean stone and wrap the fabric.
  6. Place and compact open graded base in 2 to 3 inch lifts to reach design thickness.
  7. Add leveling layer, screed true to plane and maintain surface fall.
  8. Install turf, seam, secure edges, and infill to spec.
  9. Test with a hose. Confirm water moves to the outlet without pooling.

Costs and timeline

  • French drain install: typically 12 to 25 dollars per linear foot depending on depth, access, and outlet type.
  • Open graded base upgrade: typically 2 to 4 dollars per square foot compared to dense graded blends.
  • Time: usually adds 1 to 3 workdays to a standard residential install.

Actual numbers vary by region, soil, and access. Get a site specific bid.

Common mistakes to avoid

  • Using dense graded base with lots of fines in clay.
  • No defined outlet or too little pipe slope.
  • Skipping geotextile, which lets clay contaminate the base.
  • Burying downspouts under turf without routing to the drain network.
  • Edge details that trap water instead of letting it move to the drain.

Maintenance and performance

  • Inspect outlets after big storms and keep them clear.
  • Flush the drain through cleanouts annually or as needed.
  • Top up infill and groom turf to keep drainage holes clear.

Regional notes

  • High rainfall zones like the Pacific Northwest, Gulf Coast, and parts of the Midwest with Group D clays almost always benefit from an underdrain.
  • Arid regions with caliche or hardpan can still drain poorly. Verify infiltration and plan an outlet.

Need backup from a pro?

We are here to help. FusionTurf connects you with certified installers who design drainage right the first time so your Artificial surface stays game ready.