We have investigated a high number of flooded and damp basements following the unusually wet summer and fall of 1992.
The increasingly popular use of basements as living space means that flooding is becoming more destructive and more costly. Fortunately, preventative measures are available.
Old Homes, Wet Homes
Many older homes were constructed with partial or less than full-depth basements. These basements were originally referred to as "cellars"; dark moist places with exposed earth floors.
Homes built prior to the Second World War often have no foundation drains or damp-proofing. These basements will leak. Leaking is worse when grading or down spouts direct water against the foundation wall.
Homes constructed following the Second World War often have foundation drains. However, the older drains were usually constructed from clay tiles. Over time, these drains become clogged with soil, leading to flooded basements.
Wet basements are best avoided by providing drainage away from the house. Municipalities typically have specifications that require a minimum 2% ground slope away from the foundation wall. The Canada Mortgage and Housing Corporation (CMHC) recommends a slope greater than 2% to allow for settling of the ground next to the foundation walls.
Another preventative measure is to ensure that run-off from the roof is not directed onto the ground beside the foundation wall. This can be prevented by the use of rain water leader extensions or splash blocks.
Several measures are taken below grade (below the ground) to prevent water from infiltrating the basement. Foundation drains positioned around the building perimeter are used to carry water away from the walls. Walls are also parged (coated) with mortar and damp-proofed to prevent infiltration.
Masonry walls, usually concrete block construction, must be parged (coated) with a layer of mortar to prevent water infiltration. Poured concrete walls do not require parging, but small holes where form ties were located must be filled.
Both masonry wall and poured concrete walls must be damp-proofed with an asphalt compound prior to backfilling. Damp-proofing prevents soil moisture infiltration due to capillary action.
High Water Table
Damp-proofing does not prevent water leaks caused by a high water table. If a high water table exists, a sump pump is normally used to keep the water level below the basement floor. Pump failure during electrical storms is a frequent cause of basement flooding.
Unfortunately, there are few solutions to the flooding problems associated with a high water table. Construction of a waterproof basement is one solution. To do so, the walls are waterproofed with bitumen saturated membranes, similar to the construction of a built-up roof. However, this is an expensive task. In addition, ground water exerts large forces on waterproofed foundation walls and they must be reinforced to prevent collapse.
Walters also offers Construction Claims Analysis and Commercial and Residential Property Assessment.
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Walters Forensic Engineering | 277 Wellington
Street West, Suite 800 | Toronto, ON M5V 3H2