What is a French drain?

Two versions solve two different problems. An exterior or curtain drain sits in the yard or against the foundation, 18–36 inches deep and 12–18 inches wide, lined with filter fabric, filled with washed 3/4-inch stone, and sloped at least 1 percent (1/8 inch of fall per foot) toward a daylight outlet or dry well. An interior drain is a channel cut into the perimeter of the basement slab, piped to a sump basin. To spot one you already own, look for a pop-up emitter or a grated outlet at the low corner of the lot, a river-rock strip along the foundation, or a gravel channel behind a retaining wall.

The pipe does two jobs at once: it removes surface water that has nowhere to go, and it relieves hydrostatic pressure, the sideways push saturated soil exerts on a foundation wall. That pressure is what drives water through hairline cracks and the cove joint where the slab meets the wall, and it is why a basement can be bone dry for years and then seep every spring. A properly built drain also stops the soggy patch that stays spongy for three days after a storm. Failure is almost always silt or roots. The tells: the outlet dribbles nothing during a downpour while the yard puddles, the ground directly above the trench line stays wettest, or the surface gravel has turned to packed mud. Jetting a clogged line costs $200–500.

Flush the line every 1–3 years, at the short end of that range if maples or willows grow within 30 feet, and check the flushing interval against your own tree cover. Walk the outlet every spring: mowing debris, mulch, and grass creep over pop-up emitters until they stop popping. The rest of the routine sits in the French drain maintenance checklist. One thing to do before you ever dig: fix the gutters. Downspouts dumping at the foundation put hundreds of gallons into the soil per storm, and $50–200 of downspout extensions solves a surprising number of wet basements on its own.