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Foundation Drains

April 30, 2018


Humidity in your basement: could it be your foundation drain?

Do you notice excessive moisture in your basement in the form of water infiltration or the appearance of molds or whitish marks at the bottom of your foundation walls? The foundation drain could be the cause!

Humidity problems in basements are more common than one might think. When basements are used as living space, a problem of excessive humidity can quickly become an irritant and cause significant health problems.

Utility and function of the drain

Houses built before 1950 often do not have a drainage system near the building, as it was not commonly installed at that time. Since 1963, the National Building Code of Canada requires the installation of a foundation drain in any new construction. However, the installation of a foundation drain (commonly known as a weeping tile) is not necessarily mandatory, depending on the applicable municipal regulations. In addition, it is estimated that the service life of a drain is about thirty years, after which its inspection is highly recommended.

A foundation drain serves primarily to prevent water accumulation near the foundation that might want to seep in, either by cracks or by capillarity. Proper drainage around your foundation, combined with proper properly leveled terrain around the building to keep surface water away, is the best way to prevent water infiltration into your basement. Moisture barriers are often applied to the exterior of the foundation walls to prevent capillary infiltration. Today, when a drainage system is installed, it often consists of a perforated polyethylene pipe installed at the perimeter of the footings, covered with clean stone [1] (Figure 1: schematic drain, source CMHC // Canadian Wood-Frame House Construction). In the case where there is a known history of problems of iron ochre, the installation will differ slightly.

When a weeping tile (drainage pipe) is installed, it must have its crown (upper end) below the level of the underside of the slab and be placed against the footing on the undisturbed or compacted soil. It must have a slight slope towards its outlet and have a minimum diameter of 100 mm (4 inches). Slight variations in slope or elevation are common weeping tile systems; the important thing is that the flow is not significantly obstructed or restricted, and the water level cannot rise beyond the underside of the slab. The drain must be connected to a sewer, ditch or drainage well in accordance with the applicable municipal regulations. It must be surrounded by a minimum of 15 cm (6 inches) of stone backfill. Clean stone should be used as backfill material on the drain (photography 1, legend: Drain with stone fill).

Vertical cleaning conduits can be installed to facilitate access and maintenance of the drainage pipe. Their installation will ensure the performance of the system in the long term.

Although it is common practice to install a drainage pipe, weeping tile is not mandatory and the code only requires a path for the water, which can also be provided by a clean stone backfill.

 Issues and symptoms: what are their causes?

Some signs, such as significantly higher relative humidity in the basement than in the rest of your home, may be the result of the inefficiency of your drain. Condensation, mold or rotting of wall or floor finishes, or efflorescence appearing (white crystalline deposit on exposed concrete) are also some possible consequences. Eventually, there may even be water seepage through cracks in the floor slab or through the junction between the foundation walls and footing.

!!! Beware, because water infiltration in the basement can come from a leaky pipe, cracks in the foundation walls, a rise of the water table level, or defects in the building envelope. All of these probable causes must be taken into account during the investigation before coming to the conclusion that your drain is defective.!!!

The accumulation of fine particles or roots inside a drain can cause blockages. Its diameter, thus reduced, will diminish its capacity. This is why it is possible for water infiltration to occur sporadically, for example during a heavy rainfall. Around older buildings, there is sometimes a terracotta tile drain (photography 2, legend: Damaged clay drain). Its sections can break down over the years reaching the end of their useful life, and rendering part of the drain completely ineffective.

In new construction, drain problems can be caused by a deficient installation. Sometimes, there can be slopes restricting the flow of water. The drain may have been installed higher than the footing, or may be deformed or crushed as a result of backfilling. Sometimes, when the soil is saturated and the backfill used contains a large proportion of fine particles or organic matter, the granular coating or the geotextile filter can get clogged quickly, reducing the efficiency of the drain.

Another source of malfunction of the drain may be the presence of iron ochre deposits that form inside or around it. The problem of ochre is not specific to a region, but rather depends on the hydrogeological characteristics of the soil.

 Diagnostics and Solutions

A first step of investigative process may be to check the condition of your drain by camera inspection. A specialist will use a camera paired with a monitor to inspect the drain for deficiencies or obstructions. The analysis and interpretation of the camera inspection should be done by an expert, as many factors can influence the effectiveness of the drain.

 If obstructions are discovered during the inspection, several solutions are possible depending on the extent of the problem. Sometimes cleaning with a pressure jet or a plumber’s snake is sufficient to extend the life of the installation.

 If the obstruction is greater or the pipe is damaged, a partial or complete repair of the existing drain should be considered. This type of intervention requires excavation work on the perimeter of the building, and may also include the installation of a sump pit with a pump inside the house that will receive water from the drain. The installation of a sump pit, to which the drain will be connected, can sometimes help to manage the water around the foundations of the building when subjected to frequent water infiltration by the basement slab. If the conditions are not favorable for draining the sump pit by gravity, a pump will be necessary to evacuate the water from it.

Conclusion

An adequately installed and maintained drain will allow water management around the foundations, and can prevent water infiltration into the basement of your building. Several verifications are necessary to identify potential issues before undertaking modifications or replacement of the foundation drainage system. To sum up, we advise you to do business with a reliable contractor, specializing in the field to ensure that the work performed is adequate and complies with applicable regulations.

If you have any questions or would like to learn more about this topic, please contact our Structural and civil Engineering team at info@cep-sintra.ca or 877 686-0240.

 [1]Clean stone is coarse crushed stone, in which the fine particles have been removed either by washing or sieving.

By Andréanne Labrecque, P.Eng., CEP-Sintra


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