Is there excess humidity in your basement? Mould at the base of the walls, or water leaking onto the floor? If so, your foundation drain may no longer be doing its job.

What does a foundation drain do?
The foundation drain (often called a French drain or weeping tile) collects and drains groundwater from around the footing of a structure, to keep it from pooling there. It consists of a perforated pipe installed all around the footing that supports the foundation walls, which captures water and drains it either toward the storm sewer or a catch basin or soak pit (sometimes called a soakaway) at some distance from the building. 

The vast majority of homes built these days include foundation drains, but many of those built before the 1950s lack them, as the practice was not widespread.

Watch for the symptoms
If your foundation drain is not doing its job properly, there will be signs, such as significantly higher humidity levels in the basement. This may even lead to mould or rot on the floor and the lower parts of walls. On bare concrete surfaces, you might notice whitish crystalline powder, a phenomenon known as efflorescence. Eventually, water will leak into the basement between the footing and the base of the foundation walls or through cracks in the concrete floor slab.

In the latter case, before concluding that the foundation drain is faulty, you need to make sure the water isn’t leaking in for some other reason. The problem could be, for example, a crack in the foundation wall or a defect in the building’s exterior envelope (cladding, windows, doors, balcony, etc.). To be sure, laying bare a part of the basement wall will usually suffice. Another way is to wait for a dry spell, and extensively water the soil next to the foundation.

Problems and their causes
The most frequent cause of faulty drainage systems in houses more than 30 years old is accumulation of fine soil particles or root penetration inside the drain. In newer houses, the malfunction is usually related to improper installation (incorrectly sloped pipe, crushed pipe, insufficient pea gravel, etc.)

In areas where the soil is very sandy and rich in iron particles, drains can also be blocked by reddish muddy deposits (ferrous ochre) if the foundations are exposed to very wet soil (high groundwater or flood-prone area). 

Diagnosing the problem
The condition of the drain can be checked using a video camera connected to a monitor. To get the camera inside the drain pipe, the drain must have exterior access points (cleanouts), or be connected to a sump pit or a storm sewer fitted with a backwater valve. Also, there must not be too many bends in the system. 

If a camera can’t be inserted into the drain pipe, the solution is excavation. This method is obviously more intrusive, but the advantage is that it provides an assessment of the entire system, from the outside as well as the inside.

Planning the operation
If just one simple part of the drain is blocked, it may be possible to restore it. Partial repair or replacement may also be sufficient; for example, on one side of the building only. In this case, the total cost may be only a few thousand dollars. 

If the entire system needs replacing, the bill will be much higher: between $10,000 and $15,000, or even more. In this case it is essential that you obtain detailed estimates from two or three contractors who have valid, appropriate Régie du bâtiment du Québec licences and experience in foundation drain installation. 

The contractor will need to verify several things before starting the work, i.e.:

  • Check the local bylaws on drainage water discharge; e.g., connection to the municipal storm sewer;
  • Consul Info-Excavation for the location of the various public utilities’ underground lines and cabling (e.g., electrical, natural gas, telephone);
  • Inspect the premises with the homeowner to check the location of private underground lines (e.g., power lines, automatic sprinkler hoses).

Steps of the work
Regarding the details of the work, best practices require that the contractor:

  • establish the precise level of the basement floor slab so as to properly locate the drain;
  • remove all obstacles (e.g., fences, bushes, paving blocks) from the work area and install protection on the lawn (typically plywood sheeting) on which to lay the excavated soil;
  • dig down all around the building, as far as the footing; that is, the concrete base that supports the foundation walls (note that excavation work safety standards, e.g., the width of the trench, are governed by Quebec’s Commission de la santé et de la sécurité du travail);
  • achieve a compact, level bed at the bottom of the trench, since the drain requires no slope except for the discharge portion;
  • install a 4 in. (100 mm) perforated pipe around the entire foundation (the pipe can be flexible polyethylene or, if the soil is sandy, rigid, smooth-wall PVC); the upper part of the pipe must be lower than the bottom of the basement floor slab;
  • connect vertical sections (cleanouts) extending to ground level and providing easy access to the drain for inspection or cleaning;
  • connect the drain to the drainage-water discharge pipe leading to the municipal sewer or an area away from the house;
  • cover the sides and top of the drain with a layer of pea gravel at least 6 in. (150 mm) thick.
  • install, depending on the nature of the soil, a geotextile or polyethylene membrane to reduce the risk of drain obstruction due to fine particles in the backfill;
  • clean the outer face of the foundation wall to check for cracks, repair any cracks found and, if necessary, apply a waterproofing membrane or, of the soil is clayey, install a plastic vertical drainage membrane;
  • backfill the foundation perimeter and clean the site.

The useful life of the system can be made much longer (i.e., more than 40 years) if surface water from rainfall and snow melt runs away from the foundations. This is achieved by grading the lot so that there is a slope of 3/4 in. / ft (1 cm / 15 cm) over a width of 5 to 6 ft (1.5 to 2 m) all around the house, and making sure that downspouts direct water that same distance away from the house.

Our thanks to Patrick Alarie of Goudrons du Québec and Jean-François Sirois of Excavation Garco for their contributions to this instalment of Tips & Tricks.