Drain Rock
Also known as: drainage rock, drainage gravel
Drain rock is clean, rounded or angular gravel — commonly 3/4 inch — with no fine particles, so water flows freely through it. It is used in French drains, behind retaining walls and under foundations to move water away.
In simple terms
Drain rock is gravel that has been washed so there is no sand or dust clogging the gaps between stones. Those open gaps let water drain through quickly, which is exactly what you want around drains and wet spots.
In depth
Drain rock is specified as "clean" or "washed" because the absence of fines preserves void space (porosity), typically 30–40% by volume, allowing high flow rates. Rounded river-type drain rock packs loosely and drains fastest; angular crushed drain rock interlocks slightly and is used where some stability is needed. Common sizes are 3/8 inch, 3/4 inch and 1–1.5 inch. It is usually wrapped with filter fabric to keep surrounding soil from migrating in and clogging the voids.
Why it matters
Proper drainage protects foundations, retaining walls and plant roots from standing water — a real concern in Placer County’s heavy clay soils that hold water after winter rains.
Common mistakes
- Using unwashed rock with fines, which clogs and defeats drainage.
- Skipping filter fabric, so soil silts into the rock and the drain fails over time.
Examples & uses
- Backfill for a French drain or trench drain.
- Gravel layer behind a retaining wall’s weep holes.
- Dry wells and downspout splash zones.