Fixing a Reverse French Drain Near a Slab Foundation

Fixing a Reverse French Drain Near a Slab Foundation

Construction debris created a rock-filled trench against a slab, causing indoor moisture. Solution: remove rock, install proper drainage away from foundation.

Recommended Approach

1. Excavate rock against slab.
2. Install perforated pipe at foundation footing level, sloping away (1% grade).
3. Backfill with gravel, then filter fabric, then native soil.
4. Compact soil in 6-inch lifts.
5. Ensure surface grade slopes away from house (6 inches in 10 feet).

Soil for Backfill

Use native clayey soil (not sand) for the top 12–18 inches to shed surface water. Gravel layer below handles subsurface flow.

Key Principle

Water must move away from foundation both at depth (pipe) and surface (grade).