French Drain

A French drain is a subsurface drainage system consisting of a trench—usually 18 to 36 inches deep—filled with gravel and a perforated drain pipe, typically 4-inch PVC or perforated plastic. Water percolates through the gravel and into the pipe, which then carries it away from the problem area (foundation, basement, crawlspace) to daylight, a storm drain, or a dry well. The perforations allow water to enter from all sides, and the gravel acts as a filter to keep soil particles out of the pipe.

In the field, French drains handle continuous seepage and rising water tables better than surface swales. They're essential around basements prone to hydrostatic pressure, under downspout extensions, and along property lines where neighbors' water runoff is a problem. Proper slope—minimum 1% grade—is critical; a flat drain becomes a mud-filled ditch. Many contractors backfill with landscape fabric to separate the gravel from topsoil and prevent clogging.

Code-wise, outlet placement matters most: the drain must discharge clear of the foundation (usually 10+ feet) and away from neighboring properties. In areas with poor drainage or high water tables, a French drain alone may not solve basement leaks—interior weeping systems or sump pumps often work alongside it. Don't confuse it with a perimeter drain system installed during new construction, which is typically a rigid pipe at the foundation footer.