What is a French drain?

A French drain consists of a trench containing a perforated pipe surrounded by coarse gravel, installed along the perimeter of a foundation to intercept and channel groundwater away from the building. The gravel bed acts as a filter while allowing water to flow freely toward the pipe, which then directs water to a sump pit, daylight outlet, or municipal storm drain. Both interior and exterior installations are common in Rock County. Exterior French drains are placed at the footing level outside the foundation wall during construction or excavation, while interior drains run along the basement floor perimeter on the inside, typically preferred when exterior work is not feasible. Both reduce hydrostatic pressure against the foundation and prevent water from pooling in crawl spaces or basements.

French drains are often confused with drain tile, though the terms are sometimes used interchangeably. Drain tile historically referred to solid clay or concrete pipe laid under or around a foundation, while modern systems use perforated pipe to actively collect water. A French drain emphasizes the gravel-and-pipe assembly as a unified water management system, making it the standard approach for residential and light commercial waterproofing in the region. Proper installation requires slope toward the outlet, appropriate pipe sizing, and protective fabric to prevent gravel from clogging. When correctly designed and maintained, a French drain significantly extends foundation life by managing the volume of water reaching below-grade walls.