Foundation drainage

Whether it's missing or not functioning properly, a French drain can cause a lot of problems for building owners. In fact, a French drain (or foundation drainage) refers to the drainage pipe installed at the base of the foundation walls. This pipe is usually located on the exterior side of the building. Originally, this drainage pipe was made up of terracotta cylinders laid one after the other.

French drain

Nowadays, foundation drainage pipes (French drains) are made of flexible plastic, typically 4 inches in diameter, with perforations. This pipe is connected to an indoor sump pit. Inside this pit are a gravity-fed discharge pipe and/or a pump connected to the discharge pipe. Sometimes, the French drain is connected directly to the city sewer system. In this case, there is no sump pit in the basement.

Role of the French drain

The principle of a foundation drainage system (French drain) is to remove groundwater when the water table rises and exerts pressure under the slab and around the footings of the foundation walls. Therefore, the drain acts as a safety valve to release the hydrostatic pressure of the water under the building. The water chooses the path of least resistance, which is the direction of the French drain, instead of entering the basement through cracks and the joint between the slab and the foundation wall.

Useful lifespan

The lifespan of a French drain is approximately 25-30 years. However, it can clog quickly depending on the soil type. If the slope around the building directs water towards the foundation, the French drain is designed to remove surface water. It may become overloaded and unable to handle such a large volume of water. This water can then seep into the basement. Surface water can reduce the drain's lifespan because it carries fine particles towards the French drain. These particles accumulate in the drain, causing it to clog rapidly.

Non-functional French drain

If the French drain is malfunctioning, you may experience water infiltration problems in your basement. There are some telltale signs of a malfunctioning French drain, including:

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

2- A high level of humidity in the ambient air and in the concrete of the floor slab, in the foundation walls as well as in the finishing wood of the floor;

3- A deterioration of the concrete;

4- The formation of mold;

5- Cracks, particularly star-shaped, on the floor slab. (These cracks are star-shaped due to hydrostatic pressure pushing the slab upwards).

Installation according to standards

As mentioned, the French drain must have a minimum diameter of 4 inches and be installed at the base of the foundation footings. It must also surround these footings and be covered on the sides and top with at least 6 inches of crushed stone. A slight slope (1%) must be provided towards the retention pit. The connecting pipe between the French drain and the retention pit runs under the footing. This pipe must not be perforated. It is very important to level the ground (backfill) before installing the French drain.

The French drain can become clogged by fine particles from the backfill that have migrated through the crushed stone. In this specific case, it is therefore advisable to lay a geotextile membrane over the gravel, but not over the French drain itself.

When repairing a French drain, it is recommended to: 1) repair any cracks in the foundation walls; and 2) waterproof the foundation walls to prevent water infiltration caused by surface water. This water can seep inward through cracks before reaching the French drain.

Good practices to adopt

There are several best practices to extend the lifespan of a French drain and ensure its proper functioning; therefore, it is necessary to:

1- Correct the slope around the building, so that surface water runs away from the building;

2- Install extensions for downspouts to divert water away from foundation walls;

3. Do not connect downspouts to pipes that empty into the French drain, as this puts stress on the foundation drainage system. Furthermore, leaves and debris from the roof accelerate drain clogging.

4- Ensure that the pump in the holding pit is properly adjusted and that it discharges the water adequately.

Here are some examples of problems encountered during French drain inspections :

1- Due to very fine clay soil, the French drain is blocked after only a few months of installation;

2- The slope of the drain is not correct, which means that the water cannot drain into the retention pit;

3- A French drain which is installed above the foundation footing;

4- It is installed far from the soles;

5- It is clogged by tree roots;

6- The French drain was crushed by a rock during backfilling;

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

8- There is an absence of a geotextile membrane;

9- The layer of crushed stone is missing;

10- The submersible pump is poorly fitted in the retention pit;

11- There is presence of iron ochre in the drain;

12- The connecting pipe – between the French drain and the retention pit – is perforated, so the water always flows under the footing and travels above the slab;

13- The retention pit is not watertight, so water seeps under the slab;

14- There is a holding tank in the basement, but the French drain is not connected to this tank.

Ultimately, a poorly maintained drainage system leads to water infiltration and high humidity levels inside. This can cause deterioration of finishing materials. Furthermore, mold growth can be detrimental to the health of the occupants, not to mention unpleasant musty odors. French drains are frequently cited in cases of hidden defects.

It is recommended, when the building is old or if there are signs of problems, even if the building is not old, to carry out a check of the perimeter drainage system using a drain camera.

A French drain inspection is an essential step when a pre-purchase inspector recommends it in their report. Indeed, a substantial amount must be budgeted for replacing the French drain, and significant inconveniences must be considered, as it involves excavating the perimeter of the building near the foundation walls and removing patios, paving stones, plants, etc.