A French drain is a perforated pipe that is designed to redirect stormwater to areas where it can soak into the soil. French drains are best suited to areas with pooling water, such as the bottom of a downspout.
- Typical ‘problem areas’ include exits of downspouts, bottoms of sloped areas or depressions.
- The perforated pipe will require a downhill slope of 1%.
- The exit area should ideally be vegetated and exposed to the sun to aid in evaporation.
- Save on costs by choosing an exit location close to your ‘problem area,’ keeping the system as short as possible.
- Do not direct runoff towards your neighbour’s property, erosion-prone zones (dirt slope with no protective vegetation), or hard surfaces such as driveways or sidewalks where it could freeze in the winter.
- Before beginning any major projects or alterations, make sure you are aware of the locations of any underground pipes, gas lines, or electrical wires and be sure to obtain any permits that may be necessary.
- Dig a trench with the slope away from the problem area. The size of your trench and pipe depends on the amount of runoff that will be regularly diverted. Typically the trench will be 30 cm deep.
- Line the trench with landscaping fabric.
- Partially fill the trench with clean gravel. Gravel sizes between ½-1 inch are preferable; however, gravel can be larger depending on trench depth and drain length. Using larger gravel creates more void space, which increases water permeability and drainage rates. However, larger gravel is not suitable for short, shallow drains.
- Place a perforated pipe in the trench.
- Cover the pipe with more clean gravel. To avoid seeing exposed gravel at the surface, use slightly less gravel to cover the pipe, then add a layer of landscape fabric and cover the fabric with topsoil and seed or sod.
French drain maintenance involves removal of debris from the gravel surface and cleaning of the perforated pipe. Clean-outs are easy-access points that are installed during construction to allow the pipes to be flushed and cleaned with a jet of water.