French Drain: What It Is, When You Need One, and What It Costs

A French drain is a gravel-filled trench with a perforated pipe that catches groundwater and carries it, on about a 1 percent slope, to daylight, a dry well, or a sump pump.

What is a French drain and how does it work?

A French drain is a buried trench lined with filter fabric, filled with washed gravel, and fitted with a perforated pipe. Groundwater and surface water seep into the gravel, drop into the pipe through its holes, and flow by gravity on a slight slope to an outlet: daylight at a lower spot, a dry well, a storm connection where allowed, or a sump pit. When a French drain’s outlet ties into a sump pit rather than daylight, pairing it correctly with a sump pump becomes the next decision.

The name comes from Henry Flagg French, an American lawyer and farmer who promoted the design in his 1859 book on farm drainage. Contractors also call it drain tile, weeping tile, or a perimeter drain.

The four parts that make it work

• Perforated pipe. Usually 4-inch corrugated or rigid PVC with rows of holes. Most installers lay the holes facing down so water rises into the pipe and sediment settles below it.
• Washed gravel. Clean stone with no fines. IRC R405.1 requires stone at least one sieve size larger than the pipe perforations so it cannot wash into the holes.
• Filter fabric. A nonwoven geotextile that lets water through and keeps silt out. Skip it and clay or fine soil fills the gravel within a few seasons.
• Slope and outlet. About 1 percent fall (roughly 1 inch of drop for every 8 feet of run, or 1 foot over 100 feet) carried to a point lower than the drain.

Do you need a French drain, or will gutters, grading, or a sump fix it?

You need a French drain when water moves through the soil, not just across it: a yard that stays soggy days after rain, water seeping at the base of a basement wall, or runoff from an uphill neighbor. If the water comes from your own roof or from soil that slopes toward the house, cheaper fixes usually come first.

Start at the roof. Clogged gutters and short downspouts dump the whole roof’s runoff right beside the foundation. Extend the downspouts away from the house before spending thousands on a trench.

Next, check the grade. IRC Section R401.3 requires the ground to fall at least 6 inches within the first 10 feet of the foundation, and where lot lines prevent that, a 5 percent slope toward drains or swales. Many older homes have settled backfill that now tilts toward the wall.

A sump pump and a French drain are partners, not rivals. The drain collects water, and the pump lifts it out when there is no lower ground to drain to. If your lot is flat, the French drain will almost certainly end in a pit, so read up on submersible and pedestal sump pump types before you price the job.

Interior vs exterior French drain: which one fits your problem?

An exterior French drain intercepts water in the soil before it reaches the house, and it costs less per foot because it is just digging and gravel. An interior French drain sits under the basement slab along the walls, catches water already pushing in, and pipes it to a sump. Interior work costs more because crews cut and replace concrete.

Exterior footing drains are the textbook fix, but on an existing house they mean excavating all the way down to the footing, past decks, walks, and landscaping. That is why most basement waterproofing crews install interior drains instead.

Interior drains do work. HomeAdvisor lists “Are interior French drains effective?” among its top questions, and the answer depends on the pump. An interior drain is only as good as the pump at the end of it, which is why a sump pump battery backup belongs in the same budget. The storm that fills the drain is often the one that cuts the power.

How much does a French drain cost per linear foot?

Expect $10 to $100 per linear foot installed, with HomeAdvisor’s average near $45. Shallow exterior and curtain drains sit at the low end, around $10 to $50. Interior basement drains run $40 to $100 because the crew breaks and repours concrete. A typical 100-foot job lands between $3,500 and $6,000, per Fixr.

Bars show the top of each installed range per linear foot. Installed ranges from HomeAdvisor; pipe material from Fixr.

DIY vs hiring a pro

Doing it yourself removes the labor line, which Fixr puts at $20 to $25 per linear foot for exterior drains and $35 to $40 for interior drains. Perforated pipe itself is cheap, about $0.70 to $3 per foot. You still buy gravel, fabric, and a trencher rental, and you supply the digging.

DIY makes sense for a shallow yard or curtain drain with an obvious low outlet. Hire out interior drains, footing drains, and anything that ties into a sump or storm system. Fixr flags that French drains generally should not go in without professional help, and a slope mistake on a 100-foot run is expensive to dig up.

What moves the price

• Length and depth. A footing drain dug down to the base of the foundation costs far more per foot than a shallow 18-inch yard trench.
• Concrete. Interior jobs include saw-cutting, jackhammering, and repouring the slab.
• Sump add-on. HomeAdvisor lists $500 to $2,000 to add a sump pump to a drain system.

How do you install a French drain, step by step?

Installing a French drain means planning the fall to an outlet, digging a lined trench, bedding gravel, laying perforated pipe holes down, covering it with more gravel, and folding the fabric over the top. The slope check is the step that decides whether it works.

1. Call 811. Call 811 or file online a few business days before digging. The utility locate service is free, and striking a gas or electric line is dangerous and may leave you liable for the repair.
2. Pick the outlet first. The outlet must sit lower than every point of the pipe. Options are daylight on a lower part of your lot, a dry well, a sump pit, or a storm connection where your city allows it. Never aim it at a neighbor’s lot.
3. Stake and string the route. Run a string line and a line level from the high end to the outlet. At about 1 percent, drop roughly 1 inch for every 8 feet, or 1 foot over 100 feet.
4. Dig the trench. NDS describes a typical yard drain at 6 to 12 inches wide and 18 to 24 inches deep. Foundation drains go at or below the area they protect, per IRC R405.1.
5. Line it with fabric. Lay nonwoven filter fabric in the trench with enough extra on both sides to fold over the top later.
6. Bed the gravel. Spread at least 2 inches of washed gravel on the bottom, the IRC minimum for foundation drains, and check the fall with a level.
7. Lay the pipe holes down. Set the perforated pipe on the gravel bed, holes facing down, and add a solid (non-perforated) section where the pipe exits to the outlet.
8. Cover with gravel. Add at least 6 inches of the same washed gravel over the pipe, again per IRC R405.1, then wrap the fabric over the top.
9. Add a cleanout. Put a capped riser at the high end so you can flush or snake the line later.
10. Backfill and test. Top with soil and sod or leave decorative stone, then run a garden hose into the high end and confirm water reaches the outlet.

For an interior drain, steps 4 through 9 happen inside a channel cut in the slab, and the pipe slopes to a sump pit instead of daylight. Setting that pit, the check valve, and the discharge line is its own job, covered in our guide on how to install a sump pump.

Do you need a permit to install a French drain?

Often not for a shallow yard drain on your own land, but many cities require one when the drain connects to a storm sewer, discharges near a property line, changes grading, or involves cutting a basement slab. Where permits apply, HomeAdvisor lists $50 to $200 and Fixr lists $50 to $100. Check with your local building department first.

Three rules catch homeowners most often:

• Discharge. In many areas you may not route collected water onto a neighbor’s property or the public sidewalk, and connecting to a sanitary sewer is commonly prohibited.
• Foundation drains on new work. IRC R405.1 requires drains around concrete and masonry foundations that enclose habitable space below grade, except where the soil is well-drained, and sets the gravel and fabric specs for them.
• HOAs and easements. Utility easements and HOA rules may limit where you can trench, even when the city does not need a permit.

How do you maintain a French drain so it lasts?

Keep the outlet clear, flush the line through a cleanout once a year, and watch for slow drainage or new wet spots. A well-built French drain with filter fabric lasts 30 years or more, per NDS. Fixr’s lifespan range runs from 3 to 40 years, and the low end is usually a drain built without fabric that silted up.

• Outlet check. Clear leaves, mulch, and critter nests from the daylight grate or pop-up emitter each spring and fall.
• Annual flush. Run a hose into the cleanout and confirm strong flow at the outlet. Fixr puts professional cleaning at $75 to $500.
• Roots. Keep trees and large shrubs away from the trench line.
• Pump check. For interior drains, test the sump pump by pouring water into the pit.

Put the outlet check and pump test on the same calendar as your other seasonal tasks. Our home plumbing maintenance checklist gives you a spring and fall routine that covers both.