How Much Gravel for a 20 Foot French Drain?
A typical French drain trench is about a foot wide and 18 inches deep. Over 20 feet, minus the space the 4-inch pipe takes up, that is 1.05 yards of stone. How much gravel for a 20 foot French drain by weight: about 1.5 tons.
Volume and tonnage calculator →Gravel by trench size over 20 feet
| Trench (W × D) | Cubic yards | Tons |
|---|---|---|
| 6 × 12 in | 0.31 | 0.4 |
| 12 × 18 in | 1.05 | 1.5 |
| 12 × 24 in | 1.42 | 2.0 |
| 18 × 24 in | 2.16 | 3.0 |
Figures subtract a 4-inch pipe and assume about 1.4 tons per yard of washed stone.
Related: How Much Gravel for a 100 Foot French Drain?
Which gravel
Use clean, washed stone with no fines, such as #57 or similar 3/4 to 1 inch stone. Fines clog the drain. Pea gravel also drains but shifts more. See the best gravel for a French drain.
Wrap and slope
Line the trench with non-woven filter fabric, add a couple of inches of stone, lay the perforated pipe holes down, cover with stone and fold the fabric over the top. Slope the trench about 1 percent (1 inch per 8 to 10 feet), which over 20 feet is roughly 2.2 inches of fall. Call 811 before digging.
Related: How Much Gravel for a 30 Foot French Drain?
Common mistakes
Using gravel with fines, skipping the fabric and laying the pipe with no consistent slope are the usual causes of failure. A drain with no proper outlet simply fills and holds water. Another mistake is digging without calling 811 to locate buried lines.
Cost and tools
Plan on stone, perforated pipe, non-woven fabric and fittings. For long runs, a rented trencher pays for itself in hours saved. Buying stone by the ton in bulk is far cheaper than bags.
Measuring the run
Lay out the route with paint, measure its length and note the start and end elevations. A line level or a laser level confirms you have enough fall. Plan the trench width around your shovel or trencher size.
Questions people ask
How many tons of gravel for a 20 ft French drain?
About 1.5 tons for a 12x18 inch trench.
How many yards of gravel for a 20 ft French drain?
About 1.05 cubic yards.
What size gravel for a French drain?
Clean washed stone around 3/4 inch, such as #57.
How much slope does a French drain need?
About 1 inch of fall per 8 to 10 feet.
Do I need fabric for a French drain?
Yes, non-woven filter fabric keeps soil out of the stone.
Related guides
- Best Gravel for a French Drain: Size and Type
- French Drain Along an Asphalt Driveway
- How Much Gravel for a 10 Foot French Drain?
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