Whether it is absent or non-functional, the French drain can cause a lot of worries to building owners. In fact, the French drain (or drainage of foundations) refers to thedrainage pipe installed at the base of the footing of the foundation walls. Normally, this pipe is found on the outside of the building. Originally, this drainage pipe consisted of terracotta cylinders laid one after the other.

French drain: the foundation drainage system

Nowadays, the drainage pipe of the foundations (French drain) is made of flexible plastic normally making 4 in diameter with perforations. This same pipe is connected to an inner retaining pit. In this pit is a gravity drain and / or a pump connected with the drain pipe that spills outside or into the municipal stormwater system. Sometimes the French drain is connected directly with the City's stormwater system. In this case, there is no containment pit in the basement.


The principle of the French drain is to evacuate groundwater when the groundwater rises and exerts pressure under the slab on the ground and around the footings of the foundation walls. Therefore, the drain acts as a safety valve to evacuate the hydrostatic pressure of the water beneath the building. The water chooses the easiest way to evacuate, the direction of the French drain, instead of entering the basement through the cracks and the joint between the slab and the wall of foundations.

The service life of the French drain is limited to about 25-30 years. On the other hand, it can close quickly depending on the type of soil. If the slope around the building directs the water towards the foundations, then the French drain is used to evacuate the water coming from the surface. It is possible that it is too heavy and does not manage to evacuate such a quantity of water. This water can then walk inside the basement. Surface water can therefore reduce the service life of the drain because it carries with it fine particles in the direction of the French drain. These particles accumulate on the geotextile membrane that surrounds the drain to cause rapid clogging of the drain.

Non-functional drain

In the case where the French drain (drainage of the foundations) is not functional, one can end up with problems of infiltration of water in the basement. There are symptoms that predict the non-functionality of the French drain, namely:

  • The presence of efflorescence on the slab on the ground and at the base of the foundation walls;

  • High humidity in the ambient air and concrete of the slab on the floor, in the foundation walls and in the floor finish wood;

  • Deterioration of the concrete;

  • Mold formation;

  • Cracks especially in the form of stars on the slab on the ground. (These cracks are star-shaped because of the hydrostatic pressure that pushes the slab upwards).

Installation in the rules of the art

As we said, the French drain should have a minimum diameter of 4 in and should be laid at the base of the foundation footings. He must also surround these soles from the outside and be covered, on the side and on the top, with at least 6 in crushed stones. A slight slope should be provided to the holding pit (1%). Thus, the connection pipe, between the French drain and the retention pit, passes under the sole. This hose must not be punctured. It is very important to level the soil (backfill) before laying the French drain.

The French drain can be clogged with fines particles du embankments that have migrated and crossed the crushed stone. In this case, it is advisable to put a geotextile membrane over the gravel, but not on the French drain.

When repairing the French drain, it is recommended to repair cracks in the foundation walls and to waterproof the foundation walls to prevent water infiltration caused by surface water. The latter can creep into the cracks inward before arriving at the French drain.

  • There are several good practices to prolong the life of a French drain and to ensure its proper functioning, it is necessary:

  • Correct the slope around the building, so that surface water runs off the building;

  • Install extensions for gutter downpipes to keep water away from foundation walls;

  • Do not connect the gutter downspouts with ducts spilling into the French drain, as this situation requires the drainage of foundations. In addition, leaves and roof debris accelerate clogging of the drain;

  • Make sure that the pump in the holding tank is properly adjusted and that the water is drained properly.

Here are some examples of problems encountered during expertise of drainage foundations or French drain :

  • Because of a very fine clay soil, the French drain is clogged after only a few months of installation;

  • The slope of the drain is not correct, which means that water can not escape into the holding tank;

  • A French drain that is installed above the footing of foundations;

  • It is installed far from the soles;

  • It is clogged with tree roots;

  • The French drain was crushed by a rock during backfilling;

  • The French drain was blocked by a piece of wood used during the installation;

  • There is absence of a geotextile membrane;

  • The French drain was laid outside, in one part of the building, and crosses inside in another part;

  • It lacks the layer of crushed stones;

  • The submersible pump is poorly fitted in the holding tank;

  • There is iron ocher in the drain;

  • The connecting pipe between the French drain and the retention pit is perforated, so the water always flows under the sole and travels over the slab;

  • The retention pit is not waterproof, so water seeps under the slab;

  • There is a pit in the basement, but the French drain is not connected to this pit.

Finally, a drainage system in poor condition leads to risks of water infiltration and a high humidity inside. This could cause deterioration of the finishing materials. In addition, the formation of mold could affect the health of the occupants, not to mention the unpleasant odors of humidity.

It is recommended that, when the building is old or if there are signs of problems, even if the building is not old, to carry out a verification of the drainage system of the peripheral foundations with the help of a drain camera.

The expertise of the French drain is an essential step when pre-purchase inspector recommend it in his report. Indeed, we must provide a substantial amount to change the French drain and think of the major drawbacks, because it is necessary to excavate the perimeter of the building, remove the terraces, the pavement, plants, etc. lying outside.

Foundation drainage, French drain

French drain