French Drain Installation Guide: Depth, Slope, Cost and Steps
A French drain is a gravel or aggregate-filled trench with a slotted pipe that collects water from soggy ground and carries it somewhere it can drain away. Most yard French drains are 16 to 24 in. deep, slope at least 1% (about 1-1/4 in. per 10 ft) and empty to daylight, a dry well or a legal storm drain.
This guide covers when you need one, how deep and how sloped the trench should be, what it costs, and how to install it step by step. Product specs come from NDS, the manufacturer of the EZ-Flow gravel-free French drain we sell.
What Is a French Drain?
Water always takes the easiest path. A French drain gives it one. Groundwater and surface water seep into the loose, open material in the trench, enter the pipe through its slots, and flow downhill to the outlet. The fabric or mesh around the drainage material keeps soil from washing in and clogging it.
The name comes from Henry French, a 19th-century Massachusetts farmer who wrote about the method, not from France.
Do You Need a French Drain?
A French drain is the right fix for water that sits in the ground or seeps across it. Good fits:
- A lawn that stays soggy for days after rain
- Water seeping toward a foundation, basement or crawl space
- Runoff coming down a slope from a neighbor or a hill (an interceptor or curtain drain)
- Wet soil behind a retaining wall
- Downspout water that needs to travel across the yard
It is the wrong tool for water that sheets across a hard surface. For a driveway or patio, a channel drain catches the water at the surface. For a low spot where puddles collect, a catch basin collects it and feeds a drain line. Many yards use both: a catch basin in the low spot, piped into a French drain or a dry well.
How Deep, How Wide and How Much Slope?
These numbers come from the NDS EZflow design and installation manual. They also work as a sanity check for a gravel drain.
| Where the drain goes | Minimum cover over the bundle | Typical trench depth at the high end* |
|---|---|---|
| Lawns and beds (residential mowers) | 6 in. (4 in. in low-permeability clay) | About 16 in. |
| Athletic fields with steady use | 8 in. | About 18 in. |
| Occasional light vehicles | 12 in., compacted | About 22 in. |
| Driveways, roads, parking | Not rated for EZ-Flow | Use a channel drain |
- Width: NDS says the trench should be about as wide as the bundle, so 10 in. for the 4 in. pipe bundle. A gravel drain is usually dug 12 in. wide or more so stone fits around the pipe.
- Slope: at least 1%. That is 1-1/4 in. of drop every 10 ft, or 12 in. over 100 ft. Check it with a level and a string line, not by eye.
- Length: in a grid of drains, NDS keeps side branches under 50 ft and main lines under 100 ft before moving up a pipe size. With more than 200 ft of pipe feeding one line, NDS recommends the 15 in. bundle with 6 in. pipe.
- Foundation drains: go down near the footing so water is caught before it reaches the wall.
Gravel vs. Gravel-Free French Drains
| Gravel and pipe | NDS EZ-Flow (gravel-free) | |
|---|---|---|
| What goes in the trench | Perforated pipe, filter fabric, washed stone | Pre-made 10 ft bundles: slotted pipe inside Poly-Rock aggregate and mesh |
| Material to move for 100 ft | Roughly 5 to 6 cubic yards of stone (about 7 to 8 tons) for a 12 in. x 18 in. trench | 10 bundles a person can carry |
| Flow | Baseline | 30% better flow rate than stone and pipe, per NDS |
| Trench width | Usually 12 in. or more | About the bundle width (10 in.) |
| Water storage in the trench | About 40% of the stone volume | About 21.5 gal per 10 ft bundle, per NDS |
| Best for | Very long commercial runs with equipment on site | Yards, foundations, downspout lines, retaining walls, sports fields |
Stone is cheap to buy but heavy to move. On a residential job most of the labor is wheelbarrowing it into the trench. EZ-Flow puts the drainage material and the pipe in one sleeve, so you dig, lay, connect and backfill.
How Much Does a French Drain Cost?
Here is the material cost for an EZ-Flow French drain at our current prices (September 2026). Orders over $2,000 ship free.
| Run length | EZ-Flow bundles | Material cost |
|---|---|---|
| 100 ft | 10 | $722.50 |
| 200 ft | 20 | $1,445.00 |
| 400 ft (online minimum) | 40 | $2,890.00, ships free |
| 1,000 ft | 100 | Request a quote for freight to your site |
Each bundle is $72.25, about $7.23 per foot with the pipe, aggregate and mesh included. Bundles come in sets of five, and the online minimum is 8 sets (400 ft). For a shorter run, request a quote with your length and ZIP code and we will tell you the options.
For comparison, NDS puts professionally installed French drains at about $20 to $30 per foot. Depth, soil, access, the outlet and any hardscape you have to cut through all move that number.
How to Install a French Drain
- Call 811 first. It is free, and utility lines get marked before you dig. Check whether your town needs a permit or limits where the water can go.
- Plan the route and the outlet. Mark the line from the wet area to an outlet that sits lower than every part of the drain. For a whole yard, NDS suggests a herringbone or grid pattern of branches feeding a main line.
- Dig from the outlet up. Start at the outlet end so water can drain as you work. Grade the trench bottom evenly at 1% or more, checking with a level as you go.
- Lay the bundles. Set EZ-Flow bundles end to end in the trench and join the pipe ends with couplers. Cap the upstream end. To cut a bundle, push the aggregate to both sides of the cut, cinch it with two zip ties and cut between them.
- Clay soil? If the soil is more than half silt or clay, NDS says to wrap the bundles in a second fabric matched to the soil. Ask an engineer or landscape architect which one.
- Connect the outlet. Run solid pipe from the end of the drain to daylight, a dry well or an approved storm drain.
- Backfill and finish. Put at least 4 in. of loose permeable backfill over the bundles without compacting it, then add soil to the cover depth in the table above. Mound it slightly so the trench does not sink, then seed or sod.
Download the NDS EZflow design and installation manual (PDF) for drawings of French, curtain, foundation and retaining wall drains.
Where Should a French Drain Empty?
A French drain only works if the water has somewhere lower to go. Your options:
- Daylight: the pipe comes out on a downhill slope, ideally onto rock or a pop-up emitter so it does not erode the lawn.
- Dry well: when there is no downhill spot, an underground dry well stores the water and lets it soak in. See our dry well sizing and installation guide.
- Storm drain or curb: only where your town allows it.
Do not aim the outlet at a neighbor's property, and do not tie into a sanitary sewer line. Most codes prohibit both.
Common French Drain Mistakes
- Too little slope. Water and silt sit in a flat pipe. Keep at least 1% the whole way.
- No real outlet. A drain that ends in the same soggy soil just moves the puddle.
- Skipping the fabric in clay. Fine soil clogs unprotected stone and slots over time.
- Compacting right on the drain. Leave the first 4 in. of backfill loose so water can reach the bundle.
- Running it under a driveway. EZ-Flow is not rated for regular vehicle traffic. Use a channel drain there.
- Leaving bundles in the sun. NDS warns that more than 60 days in direct sunlight can damage the mesh, so store bundles covered until you install.
What to Buy
- NDS EZ-Flow French drain, 4 in. pipe x 10 ft: the drain itself, sold in sets of five (400 ft minimum online, free shipping).
- NDS Flo-Well dry well: somewhere for the water to go when there is no downhill outlet.
- Catch basins: collect surface puddles and feed them into the drain line.
- Channel drains: stop water sheeting off a driveway or patio.
Frequently Asked Questions
How deep should a French drain be?
Deep enough for the drain bundle or pipe plus the cover on top. With a 10 in. NDS EZ-Flow bundle and the 6 in. of cover NDS calls for under lawns, the trench starts at about 16 in. deep and gets deeper toward the outlet as it slopes. Foundation drains go deeper, down near the footing.
How much slope does a French drain need?
At least 1%, which is about 1-1/4 in. of fall for every 10 ft of trench, or 12 in. over 100 ft. That is the minimum NDS gives for EZ-Flow. More slope moves water and silt out faster.
Can I install a French drain myself?
Yes. A French drain is a trench, a slotted pipe wrapped in drainage material, and backfill. Call 811 to have utility lines marked before you dig, plan an outlet that is lower than the whole run, and check local rules on where the water may go. Gravel-free bundles like EZ-Flow remove the heaviest part of the job, which is hauling and shoveling stone.
How much does a French drain cost?
Materials for a gravel-free NDS EZ-Flow French drain are $72.25 per 10 ft bundle at our prices, about $7.23 per foot with the pipe, aggregate and mesh included. NDS puts professionally installed French drains at about $20 to $30 per foot. Your cost depends on the length, depth, soil and outlet.
Do you need gravel for a French drain?
Not with a gravel-free system. NDS EZ-Flow bundles surround the slotted pipe with Poly-Rock, a lightweight engineered aggregate, in place of stone. A traditional French drain uses washed stone wrapped in filter fabric.
Where should a French drain empty?
To a spot lower than the whole drain: daylight on a downhill slope, a dry well such as the NDS Flo-Well, or a storm drain where your town allows it. Do not send it onto a neighbor's property or into a sanitary sewer line, which most codes prohibit.
Can a French drain go under a driveway?
Not EZ-Flow. NDS rates it for lawns, landscapes and occasional light vehicles with at least 12 in. of compacted cover, but not for driveways, roads or parking areas with regular traffic. To stop water running across a driveway, use a channel drain.
What are the downsides of a French drain?
It needs a lower outlet, it takes digging, and fine silt or clay can clog it over time if the drainage material is not wrapped in fabric. In heavy clay soils NDS recommends an extra fabric wrap chosen by an engineer or landscape architect.
Related guides: dry well sizing and installation, catch basin installation and channel drain installation.