Many buildings, built in the past or today, extend one or more floors below grade level.
These below grade floors often provide functional spaces for building owners such as mechanical rooms, office spaces, storages, parking garages, extra living spaces or recreational spaces.
If the subsurface or ground water table is close to the underside of the basement floor slab, water might be rising through the slab by the capillary action producing leaks and dampness.
Very often subsurface or ground water table is higher than the basement floor slab, leading to water leaking through the walls and floor slab causing standing water or dampness.
Two basic approaches to keep below grade spaces dry are: interior drainage systems or exterior drainage systems. The installation of an exterior drainage systems requires the excavation of soil around basement space perimeter, all the way down to the bottom of the foundation footing.
This process in many cases can be disruptive due to surrounding buildings, sidewalks, landscaping, porches or other obstacles.
The installation of an internal drainage system, so called French Drain should be used when installation of exterior drainage system is impossible and/or is not sufficient enough to keep below grade spaces dry.
Installation of a French Drain requires opening the floor around the perimeter of basement space, digging a trench and installing polyethylene perforated pipe.
Perforated pipe is covered with gravel and wrapped with filter fabric to keep dirt away and prevent clogging.
In most cases perforated pipe is connected to collection tank sunk into the floor and water is discharged by the submersible pump to storm drain or elsewhere.
When installation process of interior French drain is complete, the basement slab is re-cemented to original level.