How does a French drain work properly with the holes?

A French drain works because water flows downhill into a perforated pipe, and the holes face down so groundwater rising from below can enter the pipe and get carried away to a low point or a sump pump. Holes down, gravel around, slope toward the drain. That is the whole idea.

Most people get the holes backward. They picture rain falling from the top, so they point the slots up. That is not how the water gets to a basement.

Water in your soil sits low and rises. When the ground around your foundation fills up after a storm or a thaw, the water table climbs. It pushes up against the footing and the floor wall joint. A French drain catches that rising water near the bottom, before it reaches your slab. So the holes need to face down and to the sides, where the water actually is.

Here is the full path. You dig a trench beside or under the footing. You lay a bed of clean stone. You set a perforated pipe on that stone with the holes pointed down. You cover the pipe with more stone. Groundwater rises through the gravel, finds the holes, drops into the pipe, and the pipe carries it by gravity to a sump pit. From there a pump lifts it out and away from the house.

The gravel matters as much as the pipe. Clean stone gives the water an easy road to travel. Without it, fine soil packs against the pipe and clogs the holes. A fabric sock or filter fabric keeps silt out while still letting water through. Skip the gravel and you get a buried pipe that does almost nothing.

Why does water pool at the foundation in the first place? When your house was built, the crew dug a big hole, poured the foundation, then pushed the dirt back in. That disturbed backfill is looser than the packed ground around it. Loose soil drains slowly and holds water. So your foundation sits in the wettest spot on the lot. In New Hampshire, frost heave makes it worse, pushing walls inward over the winter.

There are two kinds of French drain, and people mix them up. An exterior French drain sits outside, in the dirt, against the foundation. We do not install those. In our field experience they fail in about two to three years once the trench settles and clogs, and digging the whole yard up again is a hard sell. The fix that lasts is an interior French drain, set inside along the perimeter of the basement, under the slab edge. That is the heart of our Forever Dry System: full-perimeter interior drainage, a sump pump sized at 1/2 hp per 120 feet of drain, a 12-mil wall vapor barrier, and a dehumidifier. It is 100% guaranteed to be dry for life! You can see the full setup on our basement waterproofing page.

If your trouble is a crawl space, the drainage works a little differently. There we run dimpled drainage matting under a 20-mil floor vapor barrier, with a sump, as part of crawlspace encapsulation. Same goal, different build.

You do not always need a full system. A single seeping crack might just need crack injection, not a perimeter drain. If you only see water at one spot after heavy rain, say so when we come out, and we will tell you plainly what the job really calls for. We are not going to sell you a perimeter drain to fix one crack.

What drives the cost of an interior French drain? The length of the perimeter, how many corners, whether we are cutting through a finished area, and the pump setup. A standard interior waterproofing job runs $3,000 to $30,000, and a crawl space encapsulation runs $3,000 to $25,000. Every job starts with a free inspection and a free estimate, and we get you a quote within 24 hours.

One more piece that ties to the holes. The sump pump is what makes the whole drain useful. Pipe and gravel move the water to the pit. The pump moves it out of the house. We add a battery backup so the system keeps pumping during a power outage, which in a storm is exactly when you need it. A charged backup battery moves roughly a residential pool’s worth of water, about 15,000 to 20,000 gallons, before it runs down (pool volume reference [verify]).

The Insurance Information Institute notes that most homeowners policies do not cover groundwater seepage damage, which is why keeping water out in the first place beats cleaning it up after. See our 603 guarantee for what we stand behind.

Talk to a real person about your basement

Call us at 603-610-1770 or book your free inspection. We will come look, point the holes the right way, and tell you what your house actually needs.

Related questions

Should the holes in a French drain face up or down? Down. Groundwater rises from below and enters through the bottom holes. Pointing them up means the pipe fills last instead of first, which defeats the drain.

Do I need gravel around a French drain? Yes. Clean stone gives water a clear path to the pipe and keeps soil from packing against the holes. A filter fabric or sock keeps silt out. No gravel means a clogged, useless pipe.

Is an interior or exterior French drain better for a basement? For a basement, interior. Exterior drains in the soil tend to clog and settle in a couple of years and cost a fortune to redig. An interior perimeter drain is reachable, serviceable, and part of our Forever Dry System.

Can a French drain fix a leaking foundation crack? Not by itself. A single crack usually needs crack injection. A French drain handles rising groundwater along the whole perimeter. Tell us what you see and we will match the fix to the problem.