Foundations for Decks and Concrete Steps
Frost action can cause supports for exterior decks, balconies and stairs to shift. Movement can cause structural damage and may allow water to drain towards the house. For this reason, balconies with roofs, concrete steps with more than three risers, and decks more than 600 mm (2 ft.) above the ground require their supports to be founded on rock, on coarse-grained soil with good drainage, or, for other types of soils, the supports must extend below the frost line (Figure 175).
Source : Canada Mortgage and Housing Corporation (CMHC)