The short version
Lateral Rock is our drain rock: washed, screened, angular crushed stone about 3/4" to 1.5", sold by the ton. It's the rock that goes around the pipe in a French drain. A standard 12" wide × 18" deep French drain takes about 0.75 tons per 10 feet of trench (7.5 tons per 100 feet), plus about 12% for overage.
What drain rock is for
Drainage jobs, full stop:
- French drains. The rock that surrounds the perforated pipe in the trench. Water moves through the gaps in the rock into the pipe.
- Downspout drain runs. Backfill around buried downspout extensions.
- Foundation perimeter drains. Around the footing drain pipe.
- Retaining wall backfill. Behind the wall to relieve water pressure.
- Yard low spots. As fill in spots that hold water.
Size and spec
Three properties make a rock good drain rock:
- Angular. The faces lock together, so the trench holds its shape.
- Uniformly sized. Consistent gaps mean predictable water flow.
- Clean, no fines. Fines would wash into the gaps and the pipe slots and clog them.
Lateral Rock is washed and screened to meet all three. Other yards may call this kind of stone "washed rock," "drainage rock," or "57 rock." Standard 1" crushed limestone is the wrong pick here: its fines compact to a near-solid surface, which is great for driveways and bad for drains. Pea gravel is also a poor substitute, since the small rounded stones shift and pack.
How much drain rock per foot of trench
This is the same math as our French drain wizard: length (ft) × width (in) × depth (in) ÷ 144 ÷ 27 × 1.35 = tons.
| Job | Size | Tons per foot (or sq ft) | Per 100 ft |
|---|---|---|---|
| Downspout run | 8" wide × 6" deep | 0.017 per ft | 1.7 tons |
| Light yard drain | 8" wide × 12" deep | 0.033 per ft | 3.3 tons |
| Standard French drain | 12" wide × 18" deep | 0.075 per ft | 7.5 tons |
| Foundation perimeter drain | 12" wide × 24" deep | 0.10 per ft | 10 tons |
| Retaining wall backfill | 12" wide × 36" tall | 0.15 per ft | 15 tons |
| Low-spot fill | 6" deep | 0.025 per sq ft | 2.5 tons per 100 sq ft |
Add about 12% and round up to the next half ton. Example: 40 feet of standard French drain = 40 × 0.075 = 3 tons, + 12% = 3.4, so order 3.5 tons.
Not sure of your trench size? The French drain wizard on the rock & gravel page builds the rock, pipe and fabric order from your trench length, width and depth.
French drain materials list
- Drain rock: Lateral Rock, from the table above.
- Perforated pipe: 4" perforated drain pipe, one 100' coil per 100 feet of trench.
- Outlet pipe: 4" solid drain pipe from the end of the rock to the outlet, and anywhere the line crosses under a driveway or walk.
- Fabric: a full wrap needs about 2 × depth + 2 × width. The 4' × 100' roll wraps a trench up to 12" × 12" in one piece; deeper trenches need the 6' × 300' roll or two overlapping strips.
Install (French drain reference)
- Trench at a 1% slope (about 1" of drop per 8 feet) toward the outlet.
- Line the trench with fabric, leaving enough on both sides to fold over the top.
- Spread 2" of Lateral Rock in the bottom.
- Set the perforated pipe on the rock, slots down.
- Fill with Lateral Rock to within 4" of grade.
- Fold the fabric over the top. This is the seal that keeps soil out.
- Cap with topsoil and sod, or river rock for a visible decorative drain.
The full walkthrough, with rock size comparisons and North Texas clay notes, is in our guide: What gravel goes in a French drain, and how much to order.
Buying tips
- Always wrap with fabric. Without it, soil migrates into the rock and clogs the drain within a few years.
- Order the rock, pipe and fabric together so everything arrives on one delivery. See the delivery policy for same-day cutoffs.
- More rock: browse every size in Rock & Gravel, or use the rock calculator for other jobs.
Contractor notes
Bundle quotes for French drain installs (rock + pipe + fabric).
