French drains need
clean rock.

How a French drain works

Water in the soil moves toward the open, rock-filled trench, flows through the gaps between stones into the perforated pipe, and runs out at a discharge point. The gaps are the whole point. Anything that fills them stops the drain.

Why the rock has to be clean

Clean drain rock (for example 3/4 to 1-1/2”) has the fines screened out, so there are large voids for water to travel through. A “minus” rock has dust that packs into those voids and slows or blocks the flow. That’s great for a driveway and fatal for a drain. See 3/4 minus vs. 3/4 clean.

Why the fabric matters too

Valley clay and silt are fine enough to wash into the rock. A wrap of non-woven filter fabric around the rock keeps soil out while letting water in. Without it, even clean rock slowly clogs.

The parts of a good drain

1. A trench with a steady fall toward the outlet.
2. Non-woven fabric lining the trench.
3. Clean drain rock around perforated pipe.
4. A discharge point where water can leave without causing a new problem. We call it daylighting.
5. A cap of drain rock or soil and sod, never fill that blocks the top.

Mix-ups that clog drains

• “Minus” rock ordered because it was cheaper or matched the driveway.
• No fabric, or fabric laid under the rock but not wrapped over it.
• A drain that goes nowhere, with no outlet and nowhere for the water to leave.
• Fill dirt dumped on top, which seals the surface.

Is a French drain even the right fix?

Sometimes the answer is a swale, regrading or a culvert. See which drainage fix fits your problem, and our drainage service page for what we build.

How much rock?

A trench takes more rock than most people expect. Enter the length, width and depth into our rock and gravel calculator to see cubic yards and tons.