How effective are French drains?

A French drain works well at one job: moving water that has already gotten close to your foundation. The most durable version for a wet basement is an interior French drain set in the floor along the wall, tied to a sump pump. That setup catches water at the floor wall joint and pumps it out, and we back ours for life as part of the Forever Dry System. Exterior trench drains in the yard help with surface water, but in our field experience the buried exterior kind around a foundation tends to clog and fail in about 2 to 3 years.

So the real answer depends on which French drain, and where.

What a French drain actually does

A French drain is a trench with a perforated pipe in gravel. Water flows into the pipe and runs downhill to somewhere safe. Simple idea. It does not stop water from showing up. It gives water a path once it is there.

That matters in New Hampshire. When your house was built, the crew dug a hole, then put the dirt back against the foundation. That backfill stays looser than the packed ground around it. It drains slow and holds water. So every heavy rain or spring melt, water collects right against your wall. Frost heave in winter pushes those walls inward too. A drain set in the right spot gives that water a way out before it ends up on your floor.

Interior vs exterior, and which one lasts

There are two French drains people mean, and they are not equal.

An interior French drain goes inside, under the basement floor slab, along the perimeter. It catches water at the floor wall joint where it usually seeps in, routes it to a sump pit, and a pump sends it outside. You never have to dig up the yard, the deck, or the driveway. It stays accessible. This is the version that holds up.

An exterior French drain buried around the outside of the foundation works at first. Then it sits in soil, fills with silt and roots, and stops draining. We have pulled out enough failed exterior systems to stop installing them. We do not do exterior excavation waterproofing for that reason. The interior fix is the durable one.

If your only problem is rainwater pooling in the middle of the yard, a shallow surface drain out there is fine and cheap. You do not need a full interior system for that. We will tell you so.

What makes an interior system effective

A pipe in the floor is only half of it. What makes it work for life:

• A sump pump sized to the run. We use a 1/2 hp pump per 120 feet of drain.
• A backup. Power goes out in storms, exactly when you need the pump. A charged battery backup can move about a residential pool’s worth of water, roughly 15,000 to 20,000 gallons (pool volume reference [verify]).
• A way to keep the air dry. A 12-mil wall vapor barrier and a dehumidifier stop the damp that a drain alone leaves behind.

Put together, that is the Forever Dry System. It is guaranteed to keep your basement dry for life, and the guarantee transfers if you sell. See the 603 guarantee for what that covers.

A whole-basement interior waterproofing system runs $3,000 to $30,000. Cost moves with how many feet of wall you need to protect, whether the slab is thick, and how much access we have to the floor wall joint. We give you the number after a free inspection, in writing, within 24 hours.

When a French drain is not the fix

Sometimes water on the floor is a crawl space problem, not a basement one. If you have a dirt or fieldstone crawl, the fix is crawl space encapsulation, which runs $3,000 to $25,000, not a basement drain. And if your wall is cracked and leaking at the crack, that is foundation work. Crack injection seals it from $1,000 to $3,000. You do not need a full drainage system for a single hairline crack. We will say so when that is the case.

For more on drying out a damp basement, see our basement waterproofing hub.

Ready to find out which fix you need?

Call us at 603-610-1770 or book your free inspection. We will look at where the water is coming in, tell you whether you need a French drain at all, and put a number in writing within 24 hours. Free estimate, no pressure.

Related questions

Does a French drain need a sump pump? Inside, almost always. The drain catches the water, but the slab usually sits below the line where water can run out on its own, so a pump pushes it up and out. We size it at 1/2 hp per 120 feet of drain and add a battery backup for outages.

How long does an interior French drain last? A properly installed interior system is built to last the life of the home. That is why our Forever Dry System carries a transferable lifetime guarantee. The exterior buried kind is the one that fails early, often in 2 to 3 years, which is why we stopped installing those.

Will a French drain stop my walls from leaking? It handles water at the floor wall joint, where most seepage shows up. A crack in the wall itself is a separate fix. That is crack injection or wall stabilization, not drainage.

Is an exterior French drain ever worth it? For surface rainwater pooling out in the yard, a simple surface drain is fine. For a wet basement, no. The buried exterior version around the foundation clogs and quits, so we put the drain inside instead.