How to dig a French drain around my house

A French drain around your house is a gravel-filled trench with a perforated pipe at the bottom that carries groundwater away from your foundation. You dig a trench sloped at least 1% (about 1 inch of drop every 8 feet) toward a low daylight spot or a dry well, line it with filter fabric, add a perforated pipe holes-down on a few inches of washed gravel, backfill with more gravel, and fold the fabric over the top. That is the basic exterior method, and we will walk it through. But first, the part most homeowners miss. If your problem is a wet basement, an exterior French drain is the wrong tool here in New Hampshire.

The exterior method, step by step

Call Dig Safe (811) before any shovel goes in the ground. Mark gas, water, and electric. This is free and it is the law.

Find your low point. Water moves downhill, so you need somewhere for it to go. A spot that daylights on a slope works best. A dry well works if you have no daylight.

Dig the trench. For surface runoff, 12 to 18 inches deep is common. To protect a foundation, the trench has to reach down near the footing, which can mean 4 to 8 feet of excavation against the wall. Keep a steady slope the whole run.

Line it with non-woven filter fabric. This keeps fine soil from clogging the pipe.

Add washed stone, then the perforated pipe with the holes facing down. Cover with more stone. Fold the fabric over. Backfill.

That is the textbook job. Now the hard part for our region.

Why exterior drains fail around here

The soil right against your foundation is disturbed backfill. It is the loose dirt that got shoveled back in when the house was built. It never packs down like the ground next to it. So it drains slowly and holds water, and water pools right where you do not want it. Dig an exterior trench and you disturb that fill again.

We have watched exterior excavation jobs fail in about two or three years. That is our field experience, not a study. The trench settles, the fabric clogs, frost heave shoves on the wall, and the water finds its way back in. So we do not do exterior excavation waterproofing. We will not sell you a fix we do not trust.

The durable fix for a wet basement is interior. Our Forever Dry System puts a full-perimeter drain inside, under the floor, at the floor wall joint where most water actually enters. It feeds a sump pump (we size it at 1/2 hp per 120 feet of drain), adds a 12-mil wall vapor barrier and a dehumidifier, and it is 100% guaranteed to be dry for life! The warranty is transferable, with conditions. Most jobs run $3,000 to $30,000, depending on how much perimeter you have, how deep we go, and whether the floor needs concrete work.

What if you just have a soggy yard

If the water is in your lawn or your driveway and not your basement, a true exterior French drain can be the right call for surface drainage. That is a landscape and grading job. It is a different problem from a leaking foundation, and you may want a landscaper or excavator for it. We will tell you plainly when your issue is outside our lane.

A little NH framing

Frost heave is real here. Ground freezes, expands, and pushes inward on foundation walls every winter. A shallow exterior drain does little against that. Interior drainage keeps working below the slab where it never freezes.

One more thing on pumps. We back every sump with a battery backup, not a water-powered one. A charged backup battery can pump roughly a residential pool’s worth of water before it quits, about 15,000 to 20,000 gallons. That is your safety net when the power goes out in a storm.

You can learn more on our basement waterproofing page, and read exactly what we stand behind on the 603 guarantee. If your wet space is a crawl, see crawl space services instead, since the fix there is encapsulation, not a French drain.

For grading and drainage standards near a foundation, the U.S. Department of Energy’s Building America program (basc.pnnl.gov) is a solid neutral reference. Your homeowner’s policy and water backup coverage are worth a read too; the Insurance Information Institute (iii.org) explains how flooding and water backup are treated.

Want us to look at your situation? Call us at 603-610-1770 or book your free inspection. We give you a free estimate and a quote within 24 hours.

Related questions

Will a French drain stop my basement from flooding? An exterior French drain alone usually will not, and around here it tends to fail in a couple of years. An interior perimeter drain tied to a sump pump is the durable fix for a flooding basement.

Do you install exterior French drains? No. We have seen exterior excavation waterproofing fail too often, so we install interior drainage with the Forever Dry System for basements. For pure yard drainage, a landscaper is the better fit.

How much does it cost to waterproof a basement instead? Most full-perimeter interior waterproofing jobs run $3,000 to $30,000. The price depends on how much wall we drain, how deep we go, and any concrete work needed. The free inspection gets you an exact number.

Do I need a French drain if my crawl space is wet? No. A wet crawl space gets crawlspace encapsulation, a 12-mil wall and 20-mil floor vapor barrier with drainage matting, a dehumidifier, and a sump. That work runs $3,000 to $25,000.