French Drain Installation:
How It Works & What to Get Right

Before anything is designed or dug, we spend time understanding exactly where the water is coming from and where it's going. This sounds obvious, but it's the step that most drainage problems skip — and why so many French drains fail to solve the underlying issue.

Standing water in your yard isn't always caused by one thing. It could be surface runoff from uphill areas flowing onto your property. It could be groundwater rising to the surface during or after heavy rain. It could be water sheeting off a structure — a roof, a barn, a driveway — and concentrating in one spot. Or it could be a combination of all three. The solution for each is different.

A French drain with perforated pipe is excellent at intercepting groundwater and shallow subsurface flow. It's less effective at capturing large volumes of surface sheet flow, which may be better served by a swale, a catch basin, or a combination approach. Understanding the source tells you what type of system to build — and where to put it.

The outlet is the most important decision in a French drain design, and it's the one that constrains everything else. Water has to go somewhere — you can't just move it underground without a plan for where it exits the system.

On most residential properties in the West Houston area, a daylight outlet is the standard approach — the pipe runs until it exits above grade at a low point on the property or along a fence line, ditch, or drainage easement. This is the simplest, most reliable option because there are no mechanical parts and no potential for back-pressure buildup. The outlet should be located somewhere that the discharged water won't pool or cause erosion, and ideally it should connect to or discharge near an existing drainage feature like a roadside ditch or drainage channel.

In the Houston area, many properties border drainage districts or municipal drainage easements. You cannot discharge into those features without the appropriate permissions. We'll flag anything that needs coordination when we assess your site.

Any time you're planning to dig, you call 811 first. Texas law requires it, and it's the right thing to do regardless. Utility locates are free, and the service marks underground gas, electric, water, and telecom lines so you know what's there before a machine goes in the ground.

On rural and semi-rural properties common in the West Houston area — Katy, Richmond, Fulshear, Needville — private utility lines like septic systems, water wells, and irrigation systems are not included in the 811 locate. Those are the property owner's responsibility to locate and identify. If you have any private underground systems, know their location before we start, because they're not always where you expect.

French drain trenches typically run 18 to 36 inches deep. That's deep enough to hit things. A few days waiting for a locate is always the right trade-off.

With the outlet located and utilities marked, we shoot grades with a laser level across the planned pipe route. This gives us the existing ground elevation at regular intervals, which we use to design the pipe run: where it goes, how deep it needs to be at each point, and whether a straight run is feasible or whether the route needs to be adjusted to maintain adequate slope.

Slope is non-negotiable. A French drain that doesn't move water by gravity will silt up and fail. On Houston's flat terrain, achieving minimum slope often means digging deeper than you might expect — particularly at the collection end of the run. We design to a minimum of 0.5% grade (about ⅝ inch of drop per 10 feet), and prefer 1% or better where the site allows. That number sounds small, but on a property where you only have 6 inches of total fall over 100 feet, you have to account for every inch.

Trench excavation is where the design becomes physical. We cut a 12-inch-wide trench along the planned route, maintaining the designed slope from collection end to outlet. The bottom of the trench is graded continuously — no low spots or flat sections that could hold water and allow sediment to accumulate.

Typical trench depth for a residential French drain in our area runs 18 to 30 inches, though flat-site installs sometimes go deeper at the collection end to get adequate slope. On larger properties or jobs with significant elevation changes, we'll have nodes at different depths with calculated invert elevations at each point.

All excavated material — the spoils — come out and have to go somewhere. On residential jobs, we haul them off. On larger rural properties, it may make sense to grade them into a low spot on the property rather than pay trucking costs. Either way, they can't stay piled next to the trench long-term.

Before any aggregate goes in, the trench is lined with geotextile filter fabric. The fabric runs up both sidewalls and extends over the top of the trench — it wraps the entire gravel column like a sock. This is one of the most important longevity decisions in a French drain system, and it's often where cheaper installs cut corners.

The fabric's job is to let water pass through while blocking fine soil particles from migrating into the gravel bed. In Houston's clay soil, those fines are constant — every rain event pushes clay particles toward any permeable opening. Without the fabric, the gravel column slowly fills with silt, permeability drops, and the drain stops working. With properly selected and installed fabric, the system can function effectively for 20 years or more.

With the fabric in place, we place a bed of clean drainage stone — typically #57 washed limestone or gravel — in the bottom of the trench. This bed is the foundation the pipe will rest on, and it's graded to match the designed pipe slope.

The stone has to be clean — no fines, no crusher dust. Angular stone locks together and resists compaction; round pea gravel works but doesn't perform as well as an angular-crushed aggregate. The goal is a highly permeable void space that water can move through quickly and that won't compact down under the weight of backfill over time.

Bed depth is typically 4 to 6 inches beneath the pipe, which is enough to establish the correct grade and provide stable, even support along the full length.

The pipe goes in on the prepared gravel bed. We use 4-inch Schedule 40 perforated PVC as our standard — it's rigid enough to maintain its shape and slope under backfill, durable enough to last for decades, and widely available for any future maintenance or extensions.

Perforations go down, not up. This is a common installation error. Water rises up through the gravel bed and enters the pipe through the bottom perforations — if the holes face up, they'll fill with sediment and stop functioning. With the holes facing down into clean gravel, water enters freely and sediment has nowhere to accumulate inside the pipe.

We verify slope one more time with the laser level before the stone goes on top — it's the last chance to catch and correct any deviations before the system is buried. On branching systems where multiple runs meet, we install a clean-out access point at each junction, which allows the system to be flushed and inspected years down the road without excavation.

With the pipe in and slope verified, we backfill the remainder of the trench above the pipe with the same clean drainage stone, typically to within 6 to 8 inches of the surface. The gravel column — both below and above the pipe — is the heart of the system. More stone means more storage capacity for water to collect during heavy rain events while the pipe catches up and conveys it to the outlet.

Once the stone is in, the filter fabric is folded over the top, creating a complete envelope around the entire gravel column. The overlap at the top should be generous — we typically overlap 12 inches or more — so that the fabric seal holds even if the surface settles slightly over time.

The final few inches of trench above the fabric are filled with topsoil and graded to match surrounding ground. This layer is thin enough that any grass or vegetation planted on top won't send roots into the gravel column. If the drain runs under a turf area, sod goes back on top. If it runs through a bed or bare area, a light layer of topsoil and reseeding takes care of the restoration.

The outlet end of the system gets specific attention. We install a critter guard or rodent screen over the pipe end to prevent animals from nesting inside the pipe — this is more common than you'd think, and a blocked outlet can back up the entire system. The outlet is set above the expected water level in whatever it discharges into, so back-pressure can't push water up into the pipe.

If the outlet discharges onto bare ground or a slope, we protect against erosion at the outlet point with riprap (rough stone) or a splash pad to disperse the flow. A small erosion channel at an unprotected outlet will widen quickly during heavy rain events.

On the maintenance side, a well-installed French drain with proper filter fabric needs very little attention. The main thing to watch for over time is reduced performance — slower drainage than it used to have, or ponding in areas that previously drained quickly. That's usually a sign the fabric needs flushing or that sediment has accumulated at a joint. Clean-out access points make this process quick without any excavation.