Best Gravel for a French Drain: Size and Type
The best gravel for a French drain is clean, washed, angular crushed stone in the 3/4 to 1 1/2 inch range, such as #57 or 3/4-inch clean stone. “Clean” is the key word: stone without fines leaves open spaces for water to flow through. Angular stone stays in place better than round stone. The gravel surrounds a perforated pipe in a trench lined with nonwoven filter fabric, which keeps soil from washing in and clogging the drain.
Volume and tonnage calculator →Gravel options compared
| Stone | Suitability |
|---|---|
| #57 or 3/4 in clean crushed stone | Excellent; most common |
| 1 to 1 1/2 in clean stone | Excellent; more void space |
| Pea gravel | Works, but rounded; can shift |
| Crusher run or stone with fines | Poor; clogs |
| River rock | Works; more expensive |
See number 57 stone and 3/4 inch clean stone.
Trench and pipe basics
- Trench width: about 6 to 12 inches.
- Trench depth: often 12 to 24 inches, deeper near foundations.
- Slope: at least about 1 percent, roughly 1/8 inch per foot, toward the outlet.
- Pipe: 4-inch perforated pipe, holes facing down in most designs.
- Gravel: 2 to 3 inches under the pipe and several inches over it.
- Fabric: wrap the gravel, overlapping at the top.
Step by step
- Dig the trench with a consistent slope.
- Line with nonwoven filter fabric, leaving extra to fold over.
- Add 2 to 3 inches of gravel.
- Lay the pipe, checking slope.
- Cover with gravel to a few inches below grade.
- Fold fabric over the top.
- Finish with more gravel, soil and sod, or decorative stone.
See landscape fabric under gravel.
How much gravel
A 50-foot trench, 12 inches wide and 18 inches deep, holds 75 cubic feet. Subtract a little for the pipe, and you need about 2.7 cubic yards or 3.8 tons. Use the asphalt base calculator for other sizes.
Outlets
The drain must end at a daylight outlet on lower ground, a dry well, a storm system where permitted, or a sump. Check local rules before connecting to storm sewers.
Foundation drains
Interior and exterior foundation drains use the same stone and fabric principles. See drainage gravel around a house foundation.
Common mistakes
- Using stone with fines.
- Skipping fabric in silty or clay soils.
- Flat or backward slope.
- No proper outlet.
Bottom line
Use clean, washed, angular stone about 3/4 to 1 1/2 inches, like #57, for a French drain. Wrap it in nonwoven filter fabric around a perforated pipe, keep at least a 1 percent slope, and give the drain a proper outlet.
Questions people ask
What size gravel is best for a French drain?
About 3/4 to 1 1/2 inch clean crushed stone.
Can I use pea gravel in a French drain?
It works but rounded stone can shift; angular stone is preferred.
Do I need fabric in a French drain?
Yes, especially in silty or clay soil, to prevent clogging.
How much slope does a French drain need?
At least about 1 percent, or 1/8 inch per foot.
Related guides
- Number 57 Stone: Size, Weight, Uses, and Coverage
- 3/4 Inch Clean Stone: Uses, Weight, and Coverage
- Landscape Fabric Under Gravel: When It Helps
- Drainage Gravel Around a House Foundation
As an Amazon Associate, AsphaltTons.com earns from qualifying purchases. Product links below are sponsored.