Canada Builds


Foundations will generally come in one of three forms. Poured Concrete, Concrete block, or ICF (Insulated concrete forms). There are provisions in the building code for treated wood foundations but this type is not that common. Start first with the subgrade conditions and determine the bearing capacity of the soil. If no values are known, start with 75 KPa (1500 psf) to start the calculations. Generally, with a soils test, you may find that the soils will bear 150 KPa (3000 psf). The higher the bearing capacity of the soil, the lower the size of the footings.

For renovations, the footings are considered when lowering floors with two types or techniques to deal with the existing footings. Underpinning or bench footings are the two options. In a design sense before calculations are made, realize that underpinning will create more lowered floor area than bench footings. Also realize that bench footings will cost less than underpinning.

The section drawings above show a typical underpinning treatment to lower the basement floor slab of an existing 3 storey house. The length of the underpinning sections are a maximum of 4'-0" (1219 mm).
If the footings are made of brick, then the maximum length of the sections needs to be reduced to around 2'-0" or 3'-0" depending on the 'firmness' of the brick. The 4 foot maximum length will generally be OK for poured concrete. Less for brick.