Stone & aggregate · Guide
Drainage stone and how to build a French drain
A French drain is a trench full of void space with somewhere to go. Get the stone, the fabric and the fall right and it works for decades.
5 min read · Updated August 2026
Sizing the trench
A standard residential French drain is 12 inches wide and 18-24 inches deep, running to a daylight outlet or a dry well. Deeper is only needed where the drain has to intercept groundwater rather than surface water.
Stone volume is simply width × depth × length. A 12 in × 24 in trench 40 feet long is 80 cubic feet, about 3 cubic yards, or roughly 4 tons of #57 stone before deducting the pipe volume.
Fall and outlet
Minimum slope is 1% — 1/8 inch per foot, or one foot of fall per hundred. Less than that and silt settles in the pipe; much more is fine and self-cleaning.
Every drain needs an outlet: daylight on a slope, a dry well sized for the catchment, or a storm connection where local code allows it. A trench with no outlet is a bathtub that fills up and holds water against whatever it was meant to protect.
Stone, pipe and fabric
Use clean, washed #57 (3/4 inch) stone. Anything with fines silts the voids and the drain fails. Rigid perforated PVC outperforms corrugated flexible pipe — it holds its slope, resists crushing and can be rodded out later.
Lay perforations facing down. Water enters from below through the stone column, and downward perforations let the pipe drain completely between events.
Line the trench with non-woven geotextile before the stone, and fold it over the top before backfilling. This 'burrito wrap' is what stops soil fines migrating in — a drain built without it typically loses most of its capacity in 3-7 years.
- #57 clean stone only — never crusher run
- 1% minimum fall to a real outlet
- Non-woven geotextile wrapped over the top of the stone
Surface finish
A drain that intercepts surface water should stay open to the surface — stone all the way up, with fabric just below the top layer. Capping a surface drain with topsoil and turf turns it into a subsurface drain and cuts its intake dramatically.
Where turf is required over the top, add a channel drain or catch basins at the surface so water still has an entry path.
Frequently asked questions
+How much gravel do I need for a French drain?
Trench width × depth × length gives cubic feet; divide by 27 for cubic yards and multiply by about 1.35 for tons of #57 stone. A 12 in × 24 in trench uses roughly 0.074 cubic yards per foot of length.
+Should French drain pipe holes face up or down?
Down. Water enters the pipe from the stone column below, and downward perforations let the pipe empty completely between rain events.
Part of a series
The complete stone and gravel guide
Sizing, choosing and ordering aggregate — coverage maths, product names decoded, and the depth each application actually needs.