How to build a French drain
A French drain is a trench, a wrapped pipe and a lot of clean rock, built at a consistent fall so water takes the easy route instead of the one through your basement wall. It is not complicated. It is, however, extremely unforgiving about two things — fall and fines.
The short version
- Washed rock only. Anything with fines in it will silt up and the drain will stop working.
- 20mm (3/4″) washed rock is the standard size — big enough to hold voids, small enough to place.
- Fall of at least 1% — 1″ every 8 ft, consistently, in one direction.
- Wrap the whole trench in fabric, not just under the rock. Silt is what kills French drains.
- A cubic yard fills roughly 5 ft of a 12″ × 18″ trench, once pipe volume is allowed for.
The rock and the fabric
Devonian carries the washed rock and the landscaping fabric — the two components that determine whether a drain keeps working in five years. Perforated pipe, fittings and catch basins come from a plumbing or drainage supplier, not from us. We’d rather tell you that than send you home short.
Materials
Tools: spade or trenching shovel · wheelbarrow · line level or laser · tamper. From a drainage supplier: perforated pipe (typically 4″) · end fittings · catch basin if you’re collecting at a low point
Depth & coverage
A 50 ft standard trench takes roughly 10 yards of washed rock. Round up — trenches are always bigger than they measure.
Steps
1 · Find out where the water actually goes
Watch the yard during a real downpour, or run a hose for twenty minutes. Water rarely comes from where people assume. Mark the wet line.
2 · Pick the outlet before you dig
A French drain moves water; it doesn’t make it vanish. You need a legitimate low point — a swale, a drainage easement, a dry well. Never discharge onto a neighbour’s property. Rules on where you’re allowed to send water vary between municipalities in the Edmonton region, so check with yours before you commit to an outlet.
3 · Call before you dig
Alberta One-Call, every time.
4 · Dig the trench
12″ wide, 18″ deep for a standard yard drain, running from the wet area to the outlet.
5 · Set the fall and check it twice
Minimum 1% — an inch of drop every 8 ft — held consistently the whole way. This is the step that decides whether the drain works. A trench that dips and rises holds standing water in the low spots and silts up there first. Line level or laser, not eyeballing.
6 · Line the trench with fabric
Fabric goes in first, up both walls, with enough overhang to fold over the top later. You are building a sock. Staple the excess back so it stays out of the way.
7 · Base layer of washed rock
2–3″ of 20mm washed rock along the bottom, levelled to preserve the fall you just set.
8 · Lay the pipe, holes down
Counter-intuitive, and it’s right: water enters the pipe from below as the water table rises through the rock. Holes down also keeps them clearer for longer.
9 · Backfill with washed rock
Bury the pipe and fill to within about 4″ of the surface. Use washed rock the whole way — not road crush, not pit run. Anything with fines in it turns to a plug.
10 · Close the fabric
Fold the overhang across the top of the rock and overlap it. The trench is now fully wrapped, which is what stops silt migrating in from the surrounding soil.
11 · Finish the surface
Either topsoil and sod over the top, or leave the rock exposed as a dry creek bed using round creekstone for a natural look.
12 · Test it with a hose
Run water in at the top and watch the outlet. Better to find a flat spot now than in April.
Drainage in the Edmonton area
Much of the Edmonton area sits on heavy clay, which drains very slowly. Water that would soak away elsewhere sits and finds the path of least resistance — often a foundation wall.
- Freeze-thaw and spring melt are the real load. The heaviest hydraulic event of the year isn’t a summer storm, it’s break-up: frozen ground beneath, a winter’s worth of snow melting on top, and nowhere for it to go. A drain sized for a rainstorm can be overwhelmed in April.
- Drains freeze, and that’s usually fine. A properly built French drain is below the active surface and carries no standing water, so it comes back into service as the ground thaws. A drain with dips in it holds water, freezes solid and stays blocked.
- Discharge sensibly. Meltwater directed at a driveway or a walk turns into an ice hazard by morning. Think about where the water ends up in February, not just in June.
Common mistakes
- Using road crush or pit run instead of washed rock. The fines fill the voids and the drain becomes a buried trench of wet gravel. This is the number one failure.
- Inconsistent fall. Any low spot becomes a silt trap.
- No fabric, or fabric only underneath. Silt enters from the sides. Wrap the whole trench.
- Pipe holes up. Halves the drain’s effectiveness.
- No planned outlet. The water arrives somewhere. Decide where.
- Trench too shallow. A drain in the top 6″ intercepts almost nothing.
- Compacting the rock. It needs its voids. That’s where the water goes.
What it costs
Priced by the yard plus fabric — calculate your trench volume first.Rock is the bulk of it, and length drives the number. Work out your trench volume with the material calculator, or call the yard — tell us the trench dimensions and we’ll tell you the yardage.
FAQ
20mm (3/4″) washed rock. Washed matters more than size — any material with fines in it will silt up and stop draining.
No. Road crush is 40% fractured rock and 60% sand, which is exactly right for a compacted driveway base and exactly wrong for drainage. The sand fills the voids.
18″ for a standard yard drain, deeper along a foundation. Too shallow and it intercepts almost nothing.
At least 1% — 1″ of fall for every 8 ft — held consistently the whole run.
Down. Water rises into the pipe through the rock from below.
Yes, and around the whole trench rather than just underneath. Silt entering from the sides is what ends a French drain's life.
A properly built drain holds no standing water, so freezing does no harm and it works again as the ground thaws. Drains that freeze solid are usually drains with dips in them.
Roughly 0.2 yd³ per running foot of a 12″ × 18″ trench — about 10 yards for a 50 ft run.
Order the materials
Tell us the trench length, width and depth and we’ll work out the washed rock with you. Delivered anywhere in Edmonton metro inside 24 hours.
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