This Article establishes the support required for exterior concrete stairs. Larger concrete stairs need to have adequate structural support to reduce the risk of uneven movement or settlement caused by changes in soil moisture content or frost. Because of their weight, misalignment or settlement is more difficult to correct for these stairs than for smaller concrete stairs or wooden stairs.
Exterior concrete stairs with more than two risers and two treads are required to be supported on a foundation of unit masonry or concrete walls or piers not less than 150 mm (6 in.) in cross-section, or cantilevered from the main foundation wall, as shown in Figure 9.8.-25.
The depth below ground level for foundations for exterior concrete stairs must conform to the requirements in NBC Section 9.12. Because pier-type foundations can heave if the soil freezes to the sides of the piers, excavations for piers should be backfilled with drained granular material to reduce this risk (although this is not a requirement).
Quoted from Illustrated User's Guide - NBC2015
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