What works better for rainwater seeping into my basement, an interior or exterior French drain?

For most New Hampshire basements, an interior French drain is the better long-term fix. It catches water at the floor wall joint, routes it to a sump pump, and keeps working for life. We do not do exterior excavation drainage, because in our field experience those buried systems clog and fail in about 2 to 3 years. Interior drainage stays accessible, and ours comes with a guarantee.

Here is how the two compare.

An exterior French drain means digging down the outside of your foundation, all the way to the footing. That is a big dig. The pipe sits in gravel against the wall, wrapped to keep silt out. The problem is what happens later. That trench gets backfilled with soil, and over time silt and roots find their way into the pipe. Once it clogs, you cannot clean it without digging the whole thing up again. Nik, one of our owners, has seen plenty of these. Poor fix. They fail fast for the money.

An interior French drain solves the same water from inside. We cut a channel in the basement floor along the perimeter, set perforated pipe in stone, and feed it to a sump pump that throws the water back outside, away from the house. If it ever needs service, it is right there under an access port. No backhoe required.

Why water shows up at all

Your basement is wet because the dirt around it lets water sit. When the house was built, the crew dug a hole, poured the foundation, then shoved the dug-out soil back in. That disturbed backfill is looser than the packed ground beyond it. It drains slowly and holds water against your wall. Add a few days of New England rain, and that water pushes through tiny cracks and the floor wall joint where the wall meets the slab.

NH frost makes it worse. When that wet backfill freezes, it expands and pushes on the wall. Over winters, that pressure can move a wall inward. So the goal is to give the water a path to a pump before it ever touches your living space. A wall coating will not do that. Interior drainage will.

What this actually costs

A full interior system starts around $3,000 to $30,000 for a typical basement, and the real number depends on a few things. How much perimeter we run. Whether your floor is poured concrete or something harder to cut. How many sump pumps the layout needs. We size the pump at 1/2 hp per 120 feet of drain, so a bigger basement needs more pump.

We will not quote an exterior dig, because we do not sell one. If a salesperson is pushing a buried outside drain as a permanent fix, ask them to put the lifespan in writing.

If your water is only a damp corner after a hard storm, you may not need a full perimeter yet. We will tell you that to your face during the inspection. We would rather size it right than oversell you.

The full Forever Dry System

Interior drainage is one piece of how we keep a basement dry. The complete Forever Dry System adds a 12-mil wall vapor barrier so wall moisture drains down into the drain, a sump pump, and a dehumidifier to pull the damp out of the air. It is 100% guaranteed to be dry for life! That guarantee is transferable if you sell, as long as no other contractor touches the work.

You can read how we stand behind it on the 603 guarantee page, and see the full setup on basement waterproofing. Got a crawl space instead of a full basement? That is a different build. See crawl space services. If the wall is also leaning or cracked from that frost pressure, that is structural and foundation repair, not waterproofing.

We back the pump too. The sump pump system carries a 3-year warranty, batteries excluded. If you add a battery backup, a charged unit can pump roughly a residential pool’s worth of water, about 15,000 to 20,000 gallons, when the power goes out during a storm.

For the building science here, the U.S. Department of Energy’s Building America program covers foundation moisture and drainage, and the Insurance Information Institute explains how water damage and seepage are treated on a homeowners policy. Worth a read before you decide.

Bottom line for your house

If rainwater is seeping into your basement, an interior French drain tied to a sump pump is the fix we trust across New Hampshire, Maine, and Massachusetts. It works, you can service it, and we guarantee it. An exterior dig costs more, hides the pipe where you cannot reach it, and tends to give out in a few years.

Want eyes on it? Call us at 603-610-1770 or book your free inspection. The inspection and estimate are free, and we get your quote back within 24 hours.

Related questions

Can I just paint the walls with waterproof coating instead? Coatings slow surface dampness, but they do not stop water pushing in from the backfill outside. The water finds the floor wall joint or a crack and gets in below the paint. For real seepage, you need a drain and a pump, not a coating.

Will an interior drain crack my basement floor? No. We cut a clean channel along the edge of the slab, lay the pipe, then patch over it. The bulk of your floor stays untouched, and the system sits under an access point you can reach later.

Do you fix the outside grading too? Keeping gutters clear and sloping soil away from the house helps, and we will point out quick wins during the inspection. But grading alone rarely stops a basement that is already taking on water. The interior drain is the durable fix.

What if my wall is bowing, not just wet? That is frost and soil pressure, and it is a separate repair. Carbon-fiber straps or other wall stabilization handle the lean, carrying a 25-year warranty, while the drain handles the water. See foundation repair.