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(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.
Wood Footings Continuous preserved wood footings are often used instead of concrete footings for preserved wood foundations. Wood footings and the required granular drainage layer act together to distribute loads from the structure to undisturbed soil. The sizes of interior and exterior footings are provided in the Canadian Wood Council publication entitled Permanent Wood Foundations.
31 Size of footings
wall thickness projection key polyethylene capillary break
depth not less than projection and never less than 100 mm (4 in.) width