These two get confused constantly because both involve digging a long narrow hole. The difference is where the water is when you catch it. A French drain is buried — a perforated pipe surrounded by washed stone that intercepts water already moving through the soil and gives it an easier path than your basement wall. A trench drain, also sold as a channel drain, sits flush in a hard surface with a grate on top and catches water running across concrete or asphalt before it reaches somewhere it should not.
So the question is never which is better. It is: is your problem underground or on the surface? Soggy lawn, wet basement, water seeping through a foundation wall, a hillside that stays saturated — those are subsurface problems. Sheet flow pouring down a sloped driveway into the garage, a patio that drains toward the house, a pool deck that puddles — those are surface problems.
French drain for water in the soil: wet lawns, saturated slopes, damp basements, foundation perimeters. Trench drain for water on a hard surface with nowhere to go: garage thresholds, driveway low points, patios and pool decks. If water pools on pavement, a French drain will disappoint you; if the ground itself is wet, a trench drain will not touch it.
How Each One Actually Works
A French drain works on the principle that water takes the path of least resistance. Soil, even sandy soil, resists flow. A trench packed with clean open-graded stone is 36 to 42 percent air by volume, so groundwater migrating sideways through the ground hits that void space, drops into it, finds the perforated pipe at the bottom and runs away downhill. It is passive, silent and entirely invisible once backfilled.
A trench drain does no such thing. It is a gutter set into the ground with a grate over it, and it only ever sees water that reaches the grate by running along a surface. Everything about it — the channel, the grate, the concrete haunching around it, the outlet — is sized for volume and speed, because a hard surface delivers water fast and all at once.
| French drain | Trench drain | |
|---|---|---|
| Catches | Water in the soil | Water on a surface |
| Visible? | No, or a stone strip | Yes, a grate line |
| Main material | Washed #57 or #67 stone | Channel, grate, concrete |
| Typical depth | 18–36 in | 6–12 in |
| Installed cost | $25–60 per ft | $50–150 per ft |
| DIY difficulty | Moderate — digging and grade | Hard if cutting concrete |
| Clogs from | Silt and roots | Leaves and grit on the grate |
| Maintenance | Flush every few years | Lift grate, clear debris |
Where a French Drain Belongs
Put a French drain uphill of the thing you are protecting, running across the flow rather than along it. The classic placements are a perimeter drain against a foundation, an interceptor across a slope above a soggy lawn, a line behind a retaining wall to kill hydrostatic pressure, and a curtain drain at the top of a wet yard.
- Trench: 12 in wide, 18 to 36 in deep, sloped at least 1 percent (an inch of drop per 8 feet) toward daylight or a dry well.
- Fabric: non-woven filter fabric lining the trench, wrapped over the top of the stone like a burrito. This is what keeps silt out, and skipping it is the number one reason French drains die young.
- Stone: washed open-graded #57 or #67 only. Never crusher run, never anything with fines — fines are exactly what you are trying to keep out.
- Pipe: rigid perforated PVC, holes down, bedded on 2 to 3 in of stone. Corrugated flex pipe is cheaper and clogs sooner.
- Outlet: a real one. A drain that ends in a hole with nowhere to go is a swimming pool.
Stone volume is the part people underestimate. A 12-inch-wide, 24-inch-deep, 60-foot trench holds 1 × 2 × 60 = 120 cubic feet, or 4.4 cubic yards — about 6.6 tons of washed stone, minus a little for the pipe. That is most of a tandem load before you have bought anything else. The French drain calculator handles the pipe deduction, and the full build sequence is in our French drain installation guide.
Where a Trench Drain Belongs
Anywhere a hard surface slopes toward something you do not want wet. The garage threshold at the bottom of a downhill driveway is the textbook case; so is the line where a patio meets a house wall, the low edge of a pool deck, and the entrance to a basement garage or walk-out door. In every one of those, the water is already moving fast across pavement, and the only job is to intercept it in a line and pipe it away.
- Find the true low line. Hose the surface down and watch where the water actually goes, not where you assume it goes. Set the drain across that path.
- Cut and excavate. Saw-cut existing concrete or form the gap in a new pour. The channel needs room for a concrete surround on both sides.
- Set the channel to grade. Fall of at least 1 percent to the outlet, or use a channel with built-in slope. The grate finishes a hair below the surrounding surface, never proud of it.
- Haunch it in concrete. A channel bedded in gravel alone will float, tilt or crack when a car drives over it. Match the grate load rating to the traffic.
- Pipe the outlet to daylight. Solid pipe, not perforated. A trench drain plumbed into a stone pit is just a slow flood.
Cost, and When You Need Both
French drains are cheaper per foot because the expensive component is stone and a plastic pipe, and the labour is digging. Trench drains cost more because you are buying manufactured channel, load-rated grate and concrete, and often paying someone to cut into existing pavement. But length matters more than unit cost: a trench drain is usually 10 to 20 feet across a driveway, while a French drain is often 60 to 100 feet around a foundation. The small expensive job frequently costs less than the long cheap one.
Plenty of properties need both, and they are not redundant. A downhill driveway can have a trench drain at the garage door catching surface runoff, while a French drain along the uphill side of the house handles groundwater that never touches pavement. They can even share an outlet pipe, provided it is sized for the combined flow and the French drain does not back up into the trench drain in a storm. If your problem is a soggy yard rather than a single wet spot, start with our guide to fixing yard drainage before committing to either.
Bottom Line
Watch where the water is during the next real storm and the choice makes itself. If it is standing on pavement or sheeting toward a door, you need a trench drain across its path with a concrete surround and a solid outlet pipe. If the ground is saturated, the lawn squelches, or a wall is damp, you need a French drain uphill of the problem, filled with washed open-graded stone and wrapped in filter fabric. Both live or die on the same two details: at least 1 percent fall, and an outlet that genuinely goes somewhere. Size the stone with the drainage gravel calculator or the #57 stone calculator, and check which grade to buy in our drainage stone guide.
