French drain: how much gravel, pipe and fabric do you need?

Getting water away from where it hurts

A soggy lawn, a wet basement or water pooling against a foundation often comes from groundwater or runoff with nowhere to go. A French drain gives that water an easy path: a sloped trench filled with gravel, usually around a perforated pipe, that collects the water and carries it to a safe outlet. Despite the name, it was popularized by Henry French, a Massachusetts judge and farmer, in an 1859 book on farm drainage.

How the calculator works

1. Trench volume = length × width × depth.
2. Gravel volume = trench volume − pipe volume.
3. Gravel weight = gravel volume × bulk density, about 1.5 t/m³ (1.26 short tons per cubic yard) for crushed stone.
4. Filter fabric = (2 × depth + 2 × width + overlap) × length, enough to line the trench and fold over the top.
5. Fall = length × slope.

Example

A 50-foot drain, 12 inches wide and 24 inches deep, with a 4-inch perforated pipe and a 1% slope:

• Soil to dig out: about 3.7 cubic yards, which will bulk up by about a quarter once excavated
• Gravel: about 3.5 cubic yards, or 4.5 tons
• Filter fabric: about 350 sq ft, from a roll at least 84 inches (7 ft) wide
• Fall over the run: 6 inches

Building it

1. Call 811 a few days before digging, so buried utility lines are marked for free.
2. Dig the trench with a steady slope of at least 1%, about ⅛ inch per foot, toward the outlet.
3. Line it with non-woven filter fabric, leaving enough on each side to fold over the top.
4. Add 2 to 3 inches of clean ¾-inch gravel, lay the pipe with the holes facing down, and fill with gravel to a few inches below the surface.
5. Fold the fabric over, then finish with gravel or topsoil and sod.

Outlets and rules

Send the water to a lower part of the property, a dry well, a rain garden or a storm drain where allowed. Many towns forbid draining onto a neighbor’s land, a sidewalk or a sanitary sewer. Check local rules, and for basement and foundation problems, consult a waterproofing contractor or engineer, as the cause may also be gutters, grading or cracks.

Tips

• Use washed stone without fines; fines clog the gaps and the pipe.
• A pipe with a fabric sock helps in silty soils but can clog in fine clay; the fabric around the trench is usually enough.
• Add a cleanout at the upper end so the pipe can be flushed with a hose.