How to Install a French Drain (Step by Step)

A French drain is simple technology: a gravel path of least resistance with a pipe at the bottom. The installation is 80% digging and 20% discipline about slope, fabric, and stone. Here is the full sequence, with the details that decide whether the drain works for two years or twenty.

Before you dig: the two go/no-go checks

1. The outlet exists. Walk the route with a string line or laser level and confirm somewhere lower than the entire pipe run: daylight on a slope, a dry well, or a legal storm connection. The required drop is length × 1%, so a 50-foot drain needs 6 inches from inlet to outlet. The French drain calculator computes this with your materials list. No outlet, no drain: this is the single most common fatal error.

2. 811 has marked utilities. Free, required, and takes a few business days. Plan the call a week before the dig.

The 9 steps

1. Mark the route. Paint or string the line, avoiding tree root zones and marked utilities. Straighter is better; every bend costs flow capacity.
2. Set the slope line. Stake both ends, run a string at a constant height above the finished trench bottom, and set it to a 1% fall (1/8 inch per foot). Details and a drop chart are in the slope guide.
3. Dig. 12 inches wide, 18-24 inches deep for a standard yard drain (see how deep to go for other cases). Measure from the string as you go so the bottom falls continuously, with no bellies where water will sit.
4. Compact and smooth the bottom. Remove loose spoil and rocks that would puncture fabric.
5. Line the trench with non-woven geotextile. Drape it across the full trench with 12+ inches of excess on each side. Overlap sections by at least 12 inches, upstream piece on top.
6. Bed the pipe. 2-3 inches of washed, angular 3/4-1.5 inch stone on the fabric, then the perforated pipe, holes facing down, bell ends pointing downstream. Check slope again on top of the pipe.
7. Backfill with stone. Washed stone only, no fines, to within 3-4 inches of grade. Shovel gently around the pipe first so it does not shift off slope.
8. Fold the fabric over the stone. This burrito wrap is what keeps soil out of the system. Leave no soil-to-stone contact anywhere.
9. Cap and finish the outlet. Topsoil and sod for an invisible drain, or decorative stone for a visible one. At the outlet: a pop-up emitter, daylight grate with rodent screen, or dry well connection. The outlet must stay open forever; put it where mowing and mulch will not bury it.

Materials and tools checklist

Exact quantities for your length and trench size come from the calculator; full pricing is in the cost guide. Plan spoil disposal too: a 50-foot trench produces about 5-6 loose yards of soil.

The mistakes that kill drains

No fabric, or fabric around the pipe only. Soil migrates into unprotected stone and turns the trench into a buried dam within a few years. Line the whole trench. The failure modes are cataloged in Why French Drains Fail.

Wrong stone. Crusher run, screenings, or anything with fines will cement solid. Rounded pea gravel migrates. Washed angular stone only.

Bellies in the pipe. A low spot collects silt at exactly the point you cannot reach. Check the slope at every step that touches the pipe.

Burying the outlet. An emitter under mulch or a daylight pipe crushed by a mower ends the drain. Mark it, screen it, maintain it.

When to hire instead

Foundation-depth perimeter drains, basement interior systems, slopes showing movement, or any trench past 4 feet deep belong with a contractor; costs for those systems are in the cost guide. And if the water problem is surface runoff rather than saturated soil, regrading may fix it for less: start with the standing water diagnosis.

Sources & standards: quantities and factors in this guide follow the references documented in our methodology, including ACI 302/318 (concrete practice), OSHA 29 CFR 1926 Subpart P (excavation safety), ASTM aggregate designations, and the International Residential Code. Local codes, climate, and site conditions govern.