Clay soil conditions are a hazard of the Ottawa-Carleton area.  These expansive soils are highly susceptible to change as a result of moisture, and can lose moisture and shrink in volume, or absorb moisture (like a sponge) and swell in volume.  These changes in moisture content can cause disproportionate foundation settlement and damage.

Trees can be a contributing factor to the depletion of moisture from the soil, during long periods of  hot, dry weather.

In a City of Ottawa publication titled, "Trees & Foundation Damage", they identify the dominant soil in the Ottawa area as  "LEDA CLAY", a marine silty variety with a high water content.  When Leda clay loses water, it shrinks and doesn't regain its previous volume.  This shrinkage of Leda clay is a major contributor to building foundation damage in our area.

An excellent report titled: "Understanding and dealing with interactions between trees, sensitive clay soils and foundations", can be found on the Canada Mortgage and Housing Corporation website.

Another informative bulletin can be found on the website of The National Research Council of Canada, Institute for Research in Construction;  in their publication, Canadian Building Digest, titled: Trees and Buildings.