A French drain is a gravel-filled trench containing a perforated pipe that redirects groundwater and surface water away from a problem area. Despite the name, the concept has nothing to do with France — it's named after Henry French, a Massachusetts farmer who popularized the technique in the 1850s.

When You Need a French Drain

Not every wet yard or damp basement calls for a French drain, but they're the right solution when you're dealing with:

Standing water in the yard after rainfall, especially in low spots
Hydrostatic pressure against basement or retaining walls
Soggy ground that stays saturated long after rain stops
Downhill water flow from neighboring properties toward your structure
Perimeter foundation drainage to protect footings from water accumulation

If the problem is purely surface runoff, a swale or surface drain may be simpler. French drains solve subsurface water problems.

How a French Drain Works

The principle is simple: water follows the path of least resistance. A French drain creates an easy path.

Water percolates through the soil and enters the gravel-filled trench
Gravel allows water to flow freely toward the perforated pipe
The pipe collects the water and carries it by gravity to a discharge point
The discharge point releases water to daylight, a dry well, or a storm sewer

The system works passively — no pumps, no power, no moving parts. Gravity does all the work, which means getting the slope right is critical.

Design Considerations

Sizing

Trench width: 12 inches minimum, 18–24 inches for heavy water loads
Trench depth: 18–24 inches for yard drainage, deeper for foundation perimeter drains (typically to the bottom of the footing)
Pipe diameter: 4-inch perforated pipe is standard for residential applications

Slope

The pipe must maintain a consistent downhill grade to the discharge point. The minimum slope is 1% (1 inch of drop per 8 feet of run). Steeper is better — aim for 1–2% where the terrain allows it. A laser level or transit is essential for getting this right over long runs.

Discharge

Water has to go somewhere. Options include:

Daylight outlet — Pipe exits on a downhill slope and discharges to the surface. This is the most reliable option.
Dry well — A buried pit filled with gravel that allows water to percolate into surrounding soil. Suitable when daylight discharge isn't possible and soil percolation is adequate.
Storm sewer connection — Tying into the municipal system. Check local codes; many jurisdictions require a permit.

Never discharge a French drain onto a neighbor's property or into the sanitary sewer.

Installation Steps

1. Plan the Route

Mark the trench path with stakes and string. Identify the high point (where water enters) and the low point (discharge). Call 811 to locate underground utilities before digging.

2. Excavate the Trench

Dig to the required depth and width, maintaining consistent slope. Check grade frequently with a level. The trench bottom should be smooth and free of rocks that could create low spots.

3. Line with Filter Fabric

Lay non-woven geotextile filter fabric along the entire trench — bottom and sides — with enough excess to wrap over the top later. The fabric prevents fine soil particles from migrating into the gravel and clogging the system.

4. Add Base Gravel

Place 2–3 inches of washed, crushed stone (3/4-inch to 1-1/2-inch) on the bottom of the trench. Avoid round river rock — angular crushed stone interlocks better and creates more void space.

5. Lay the Pipe

Place 4-inch rigid or flexible perforated pipe on the gravel bed. Holes face down. This is counterintuitive but correct — water rises into the pipe from below. If holes face up, soil and debris enter the pipe directly.

Orient the pipe so water flows toward the discharge point. Connect sections with appropriate couplings and use sweep fittings (not sharp 90-degree elbows) at turns.

6. Backfill with Gravel

Cover the pipe with gravel to within 2–4 inches of the surface (for buried drains) or to the desired finish grade. Use the same clean, washed crushed stone.

7. Wrap the Fabric

Fold the filter fabric over the top of the gravel, overlapping by at least 6 inches. This creates a complete envelope that keeps soil out of the drainage aggregate.

8. Finish the Surface

Top with soil and sod, or leave gravel exposed as a decorative swale. For foundation perimeter drains, backfill per the project's waterproofing detail.

Common Mistakes

Using the Wrong Gravel

Pea gravel and river rock are popular but not ideal. They lack the angular edges that create stable void space. Worse, some contractors use fill gravel containing fines that clog the system within a few years. Always specify washed, crushed stone.

Skipping the Filter Fabric

Without filter fabric, soil particles gradually fill the voids between gravel and eventually clog the pipe. This is the number one reason French drains fail within 5–10 years. The fabric is cheap; replacing a failed drain is not.

Insufficient Slope

A French drain with flat or inconsistent slope will hold standing water instead of moving it. This turns your drain into an underground puddle. Verify slope during installation, not after backfilling.

Holes Facing Up

Placing the pipe with perforations facing upward allows sediment and debris to fall directly into the pipe. Holes should face down so water rises into the pipe through hydrostatic pressure.

No Cleanout Access

On long runs, install cleanout risers (vertical pipe sections capped at the surface) every 50–100 feet. These allow you to flush the system with a garden hose if it slows down over time.

French Drains and Foundation Waterproofing

A perimeter French drain is a key component of any foundation waterproofing system. When installed alongside a waterproofing membrane and drainage board, it forms a complete water management assembly:

Membrane keeps water from penetrating the wall
Drainage board channels water downward along the wall face
French drain collects the water at the footing and carries it away

For interior applications, an interior French drain (also called a curtain drain) runs along the inside perimeter of a basement slab. Water entering through the wall-floor joint is intercepted and directed to a sump pit with a pump. This is a common retrofit solution for existing basements with water intrusion.

Maintenance

French drains are low maintenance but not zero maintenance:

Inspect discharge outlets seasonally to ensure they're clear of debris and sediment
Flush through cleanouts with a garden hose annually
Keep surface inlets clear of leaves and soil
Monitor performance during heavy rain — if water is surfacing along the trench line, the system may be clogging

A properly installed French drain with filter fabric and clean stone should last 20–30 years or more. Cut corners on materials or installation, and you'll be digging it up in under a decade.