What are the uses of a French drain?

A French drain moves water away from where you don’t want it. It’s a gravel trench with a perforated pipe inside that catches groundwater and carries it off to a lower spot or a sump. Around a basement, the version that lasts is the interior one we install as part of our Forever Dry System: a full-perimeter drain set at the floor wall joint, feeding a sump pump that pushes the water out. Cost for that system runs $3,000 to $30,000.

So what does a French drain actually do? A few jobs.

It dries out a wet basement. Water finds the floor wall joint, the seam where your slab meets the wall, and seeps in there. An interior perimeter drain sits right at that seam and collects the water before it pools on your floor. That’s the main use we see in New Hampshire homes.

It pulls down a high water table. After heavy rain or spring melt, the ground around your foundation fills up. The soil right against your house is disturbed backfill. That’s the looser dirt the builder put back when the house went in. It drains slow and holds water, so pressure builds against the wall. A French drain gives that water a path out.

It protects a footing or a crawl space. In a crawl, we run drainage matting and tie it to a sump as part of crawl space encapsulation, so groundwater under the floor barrier has somewhere to go.

It handles a soggy yard. An exterior French drain in the lawn or along a driveway carries surface water and downspout runoff away from the house. That’s the landscape version. We don’t do exterior excavation waterproofing on foundations. In our field experience it tends to fail in about two to three years, and then you’re digging again. For keeping a basement dry, the durable fix is the interior drain.

What drives the cost

The biggest driver is how much drain you need. A full basement perimeter is more pipe than a single wet wall. The sump matters too. We size a pump at roughly 1/2 hp per 120 feet of drain, and a battery backup keeps pumping when the power’s out, about 15,000 to 20,000 gallons on a charge. Access, depth, and whether we’re cutting concrete or laying matting all move the number. We give you the real figure after a free look, not a guess over the phone.

You don’t always need a full system. If you’ve got one weeping wall and a dry rest of the basement, we’ll tell you. Sometimes the fix is smaller. We’d rather size it right than oversell you a perimeter you don’t need.

A quick note on what a French drain won’t fix. It moves water. It doesn’t hold back a wall that’s bowing or cracking from frost heave or settlement. NH frost pushes foundation walls inward over the winters, and that’s a structural job: carbon-fiber straps, a power brace, or piers, depending on what we find. Drainage and structure are two different problems. We check for both when we come out.

If water’s the issue and you want it gone for good, the interior drain plus sump plus vapor barrier is the package, and it’s 100% guaranteed to be dry for life!

For the science on why water collects at a foundation, the DOE Building America program covers soil and foundation moisture. For groundwater and flooding basics, see the Insurance Information Institute.

Ready when you are

Call us at 603-610-1770 or book your free inspection. We serve New Hampshire, Maine, and Massachusetts, and we’ll have a quote to you within 24 hours.

Related questions

Does a French drain need a sump pump? An interior basement drain almost always feeds a sump. The drain collects the water, the sump pushes it out. We size the pump to the length of drain and add a battery backup so it keeps running in an outage. Our sump system carries a 3-year warranty.

Will a French drain stop my basement walls from cracking? No. A drain handles water, not structure. Cracking and bowing usually come from soil pressure or frost heave, and those need carbon-fiber straps, a power brace, or piers. We look at both when we inspect.

Interior or exterior French drain for a basement? For keeping a basement dry, interior. Exterior foundation drains mean digging, and in our field experience they tend to fail in about two to three years. The interior perimeter drain is the durable fix and it’s what our lifetime guarantee covers.

Can a French drain dry out a crawl space? Yes, paired with the rest. In a crawl we run dimpled drainage matting under a 20-mil floor barrier and tie it to a sump, all part of crawlspace encapsulation. That carries 25-yr warranties on the encapsulation and barrier.