What is a French Drain?
A French drain is a gravel-filled trench containing a perforated pipe that redirects surface water and groundwater away from a specific area. Named after Henry French, who popularized the concept in the 1850s, French drains are one of the most effective and widely used methods for managing water drainage around foundations, yards, and retaining walls.
The system works by gravity: water enters the gravel-filled trench, seeps down to the perforated pipe, and is channeled away to a suitable discharge point such as a storm drain, dry well, or lower area of the property. A layer of landscape fabric prevents soil from clogging the gravel and pipe over time.
French Drain Formulas
The gravel weight conversion assumes crushed stone at approximately 2,800 lb per cubic yard (1.4 tons).
Materials Needed
| Material | Purpose | Recommended Type |
|---|---|---|
| Perforated Pipe | Collects and channels water | 4" corrugated or rigid PVC |
| Gravel | Allows water to flow to pipe | 3/4" washed crushed stone |
| Landscape Fabric | Prevents soil from clogging gravel | Non-woven geotextile |
| End Cap / Outlet | Terminates or directs pipe outflow | PVC fitting or pop-up emitter |
Installation Guidelines
- Slope: The trench should have a minimum slope of 1% (1 inch per 8 feet) for proper drainage. Steeper slopes improve flow rate.
- Depth: The trench should be 18 to 24 inches deep for yard drainage, or deeper for foundation drainage (below the footing level).
- Width: A minimum of 12 inches wide is recommended. Wider trenches provide greater water capacity.
- Fabric overlap: Leave 6 to 12 inches of extra fabric on each side to fold over the top of the gravel before backfilling.
Worked Example
A French drain 50 feet long, 12 inches wide, and 18 inches deep:
Frequently Asked Questions
How deep should a French drain be?
For yard drainage, 18 to 24 inches deep is standard. For foundation drainage, the drain must be at or below the footer level, which may be 3 to 6 feet deep depending on your foundation type and frost line depth.
What type of gravel is best for French drains?
Washed crushed stone in 3/4-inch to 1-inch size is ideal. Avoid using pea gravel (too small), river rock (too round, packs poorly), or unwashed stone (fines will clog the system). The gravel must be angular and clean to allow water flow.
Do French drains need maintenance?
Properly installed French drains require minimal maintenance. However, over time, sediment can accumulate. Flushing the pipe with a garden hose every 1-2 years helps maintain flow. Signs of clogging include standing water or slow drainage.
Can I install a French drain myself?
Yes, French drain installation is a common DIY project. The main challenges are digging the trench and ensuring proper slope. Call 811 before digging to locate underground utilities. Renting a trenching machine can make the excavation much easier.