Gravel is the trench minus the pipe
Work out the trench box, then take out the space the perforated pipe fills. Gravel depth is the stone below the pipe, plus the pipe diameter, plus the stone above it. A 4-inch pipe takes up about 0.087 ft³ per foot of run, so on a 50-foot drain that is roughly 4 ft³ removed from the gravel total.
Standard dimensions
| Part | Minimum | Recommended |
|---|---|---|
| Trench width | 8 in | 12 in |
| Trench depth | 18 in | 24 in |
| Pipe diameter | 3 in | 4 in |
| Slope | 1% (1:100) | 2–3% |
| Gravel below pipe | 6 in | 12 in |
| Gravel above pipe | 2 in | 4 in |
A 4-inch pipe suits most single homes up to about 2,000 ft². Step up to 6 inches for larger homes or clay soil — a 6-inch pipe carries roughly 225% more water than a 4-inch.
The details that decide whether it lasts
- Perforations face down. Water enters the pipe from beneath, through the gravel.
- Use clean 3/4-inch angular gravel with no fines. Dirty gravel or sand clogs and holds water instead of draining it.
- Wrap it in filter fabric with a 12-inch overlap at seams, so fine soil cannot migrate into the stone.
- Keep the fall consistent. One clean slope of 1–2% beats a steeper drain with flat spots where water pools.
Common questions
How much gravel do I need for a French drain?
Multiply trench length by width by gravel depth, then subtract the volume the pipe takes up. A 50-foot trench, 12 inches wide and 18 inches deep of gravel is about 75 cubic feet, or roughly 2.8 cubic yards, before subtracting the pipe.
What slope does a French drain need?
At least 1%, meaning a 1-foot drop over 100 feet. Aim for 2 to 3%. Below 1% the water sits in the pipe instead of draining away.
Which way do the holes in the pipe face?
Down. Water rises up through the gravel and enters the pipe from below. Holes facing up let water pass through the gravel without collecting it properly.
Do I need filter fabric?
Yes. Non-woven geotextile wrapped around the gravel stops fine soil from washing in and clogging the pipe. Without it a drain can silt up in two or three years instead of lasting decades.
