|Underpinned Basement - Typically Basemnent Floors Are|
Lowered By 2 to 2 and a half feet (60 to 75 cm).
Toronto Open Data provides statistics on building permits and description of works that may include underpinning. As house prices have increased in Toronto, homeowners are clearly motivated to increase the amenity value of their basements, whether as second suites or elaborate "man-caves". Trends from 2001 to 2017 are shown below. Several hundred more permits were issues for projects including 'benching' but are not shown. Overall the number of lowered basements increased consistently from less than 200 properties in 2001 to over 1700 properties per year in 2017 - in total 14,000 properties were lowered (adding benching projects to underpinning projects shown).
|Toronto Basement Underpinning Permits 2001 to 2017 - Lower Basements Can Result in Higher Flood Risks|
The following cross sections illustrate how lowering a basement reduces the 'freeboard' safety factor, or clearance, between the finished basement floor and the municipal sewer system.
Where sewer elevations are not favourable, ejector pumps for sewage may be proposed which would create a good hydraulic break between the municipal sewer and the lowered / underpinned basement.
In any case, where a basement is lowered, especially when the new space is then finished and filled with valuable contents, a backwater valve should be installed to reduce back-up risk. This can only be considered where downspouts and weeping tiles drains (foundation drains) are separated from the sanitary lines, to avoid flooding the home upstream of the backwater valve with local drainage.