How Much Gravel Is Needed for a French Drain?

"French drain" may sound like a kitchen gadget, but it's really a technique to protect foundations. These gravel-filled trenches capture water and route it away from a structure. They are common in wet areas and where buildings are constructed on slopes.

Trench Size

Trenches for French drains are typically 6 to 12 inches wide and 8 to 24 inches deep. A drain that includes clay tile or perforated pipe must allow a minimum of 1 to 2 inches of gravel on all sides of the pipe.

Trench Length

A French drain must be long enough that it will protect the entire length of the foundation. The drain must then be extended to reach a low spot or drainage where the water can be released safely.

Calculating Gravel Volume

Measure the average width and depth of the trench in feet (6 inches is 0.5 feet, for example). Decrease the depth slightly to allow for a layer of sand on top of the gravel. Multiply the dimensions times the total length in feet and divide the result by 27: this is the number of yards of gravel you must order delivered.

References

• Ask the Builder: A Simple Trench Drain
• Grounds Maintenance Magazine: How to Install a French Drain
• Ask Dr. Math: Adding a 6-Inch Layer of Gravel