The size of wall footings should comply with building code requirements. Table 4 (p. 265) shows minimum concrete sizes for average, stable soil and the number of stories supported. If the distance to the water table from the bearing surface is less than the width of the footings, the footing sizes in Table 4 must be doubled.
Footings should project beyond each side of the wall by at least 100 mm (4 in.), and the thickness of unreinforced footings should be no less than their projection beyond the wall. Footings must never be less than 100 mm (4 in.) thick (Figure 31). If the soil has low load-bearing capacity, wider reinforced footings may be required. Local building officials can provide advice on local conditions affecting footings.
Forming a key on the top of the footings (Figure 31) is a good practice that helps the foundation wall resist earth pressure at the bottom of the wall. It is good practice to place a polyethylene sheet over the footing for the width of the foundation wall. This will act as a capillary break, preventing moisture in the footing from wicking up into the foundation wall. The polyethylene can also be connected to the air barrier system in the basement floor and walls.
If the footing excavation is uneven and too deep in places, compacted granular material, not excavated material, should be used to level the excavation. Pipe trenches directly under wall footings should be encased with concrete.
Source : Canada Mortgage and Housing Corporation (CMHC)