French drain
A French drain is a trench of clean rock with a perforated pipe at the bottom, laid at a deliberate grade toward an outlet. It collects groundwater and shallow storm water and carries it somewhere the ground can take it — or to daylight. It is the most-installed and most-misinstalled drainage object in residential yards.
Checked 2026-09-10 · figures re-verified against sources below
The four failure modes
Why French drains fail
| Mode | Cause | The fix that prevents it |
|---|---|---|
| Flat pipe | No survey; “downhill-looking” is not a grade | 1% minimum — measure it, don't eyeball it |
| No outlet | Pipe ends in the ground and hopes | Daylight, dry well, or catch basin — pick one, on purpose |
| Clogged sock | Fabric wrap silts shut over years | Clean rock, filter separation, cleanouts — see the sock note |
| Wrong fix | The real problem was surface water or a broken gutter | The three-number check before any trench |
When it is the wrong fix
- Surface puddle, high clay, no slope. A pipe can't leave a trench that has nowhere lower to go. On dead-flat clay lots the honest answer is usually a swale to a pumped catch basin — or accepting a ponded area as designed detention.
- One soggy spot fed by a downspout. Fix the downspout first: extend it, or carry it to the swale with the arithmetic note. A French drain for a downspout problem is a trench digging its own alibi.
- Water against the foundation from a high water table. A drain beside the footing can lower local levels, but the desk stops there: anything touching a foundation belongs to a local engineer — see the refuse list.
Numbers that survive contact with a shovel
| Number | Value | Why |
|---|---|---|
| Minimum pipe grade | 1% | Below 1% silt wins; 1–2% is the working range for 4-in pipe |
| Pipe size, residential | 4 in | 6 in for long runs or many inlets |
| Rock | 3/4–1.5 in clean | Washed, angular. “Clean” is the word that matters |
| Trench width | ≥ pipe + 8 in | Narrower and the rock envelope can't filter |
| Cleanouts | every 50–75 ft | And at every bend sharper than 45° |
| Slope to house | ≥10 ft away | Setback from foundation; check your code |
Holes up or down?
The desk refuses to repeat the one-line answer vendors give, because the manuals split on it. The full dispute — and what each camp is really optimizing — is in the holes note.
Where to go next
- Diagnose standing water — before any trench, always.
- Dry well — the outlet when there is no daylight.
- Holes up or down — the dispute the manuals won't settle.
Sources
- Minnesota Stormwater Manual — stormwater.pca.state.mn.us
- ICC International Plumbing Code, storm drainage provisions — iccsafe.org
- US EPA NPDES — epa.gov/npdes