Why should I use #57 washed stone instead of pea gravel or bank-run gravel?

#57 stone consists of 1/2" to 1" angular crushed stone that has been washed free of rock dust and silt. Its uniform shape provides approximately 40% open void space for rapid water movement and temporary retention, whereas unwashed gravel contains fines that immediately clog geotextile fabric.

How much slope does a French drain need to function?

The minimum recommended slope is 1% (1/8 inch drop per linear foot, or 1 foot drop per 100 feet). While a 0.5% slope can move water, a 1% to 2% slope ensures self-cleaning velocity (≥ 2.0 ft/sec) to keep silt from settling inside the pipe.

How deep and wide should my French drain trench be?

For surface yard drainage, a trench 10 to 12 inches wide and 18 to 24 inches deep is standard. For footer drains protecting basement foundations, the trench must sit adjacent to or below the foundation footing, often 4 to 8 feet below grade.

What is the difference between a French drain and a catch basin dry well?

A French drain captures diffuse ground water over a long linear stretch via a gravel trench and perforated pipe. A catch basin collects concentrated point-source surface water (like downspouts or patio drains) using solid non-perforated pipe to quickly convey water to an outlet or dry well.

Can I connect roof downspouts directly into my perforated French drain?

No! Direct downspout connection into a perforated French drain introduces roofing granules, leaves, and massive surge volumes directly into your gravel trench, leading to premature failure. Run gutter downspouts in a separate, solid non-perforated pipe.

Free French Drain Perforated Pipe Capacity Calculator

🌧️ Trench & Pipe Specifications

Pipe Type & Diameter

Trench Length (ft) Total linear run

Trench Slope Gradient (%)

Trench Width (inches) Standard 10" - 16" width

Trench Depth (inches) Standard 18" - 24" depth

Backfill Aggregate Type Washed stone prevents silt clogging

⚡ Drainage Flow & Material Estimates

Full Pipe Flow Rate -- Gallons Per Minute

Washed Gravel Required -- Tons of #57 Stone

Hourly Discharge Volume --

Water Flow Velocity --

Total Trench Excavation --

Geotextile Fabric Needed --

Gravel Trench Water Storage --

Recommended Tools & Equipment

Tested hardware and components for high reliability

100% Free Tool Zero Sign-Up

The Engineering Principles of Subsurface French Drains

A French drain is a subsurface gravel trench designed to redirect groundwater and surface runoff away from building foundations, retaining walls, and saturated lawns. Rather than relying solely on the perforated pipe, a true French drain functions as a combined high-void underground gravel reservoir and gravity conduit.

Smooth PVC vs. Corrugated HDPE: The Manning's n Difference

Contractors and homeowners frequently choose flexible corrugated black pipe for convenience, but smooth interior PVC (such as SDR-35 or Schedule 40 perforated sewer and drain pipe) offers massive hydraulic advantages:

Roughness Factor (Manning's n): Corrugated pipe has severe internal ribs with a high roughness coefficient of n = 0.024. Smooth PVC has an extraordinarily slick interior of n = 0.009.
Discharge Capacity: At the same 1% slope, a 4-inch smooth PVC pipe discharges over 2.6 times more water per minute than a 4-inch corrugated pipe!
Sediment Resistance: Corrugated valleys trap mud and silt when water velocity falls below 2.0 ft/s, whereas smooth PVC maintains continuous laminar self-scouring velocity.

Perforations Down: The Proper Pipe Orientation

One of the most persistent installation mistakes is placing perforated holes facing upward. French drain pipes should almost always be installed with the holes facing down (at 4 and 8 o'clock) resting on a 2-inch bed of washed gravel. As the water table rises through the gravel trench from below, water enters the holes immediately and flows away by gravity before ever reaching the surface of the soil.