What is french drainin construction?

A french drain collects groundwater that seeps into a gravel trench through perforated pipe, moving slow subsurface flow away from a wall or foundation. A storm drain carries surface runoff in solid pipe between inlets, manholes, and an outfall. They meet on many sites, since french drains commonly discharge into a storm structure, but they are different assemblies with different details and different unit prices.

Almost every commercial detail includes geotextile, either wrapping the full trench or as a sock on the pipe, because the fabric keeps soil fines out of the washed stone. Without it the voids in the stone silt up and the drain stops flowing, which is the most common failure mode. Take off the fabric from the detail including laps, and follow the geotech report where it specifies the fabric type.

There is no standard dimension; the trench section comes from the project detail. A pavement edge underdrain might be 18 in deep, while a foundation or wall drain is set by the footing elevation it protects, which can put the trench several feet down. Depth and width drive the excavation and stone volumes, the two biggest numbers in the assembly, so read the detail before pricing a single foot.