Is 1.2 m the minimum deck footing depth in Ottawa clay?

In clay or soils not clearly defined, the 2024 Ontario Building Code table (9.12.2.2) sets 1.2 m, but not less than the depth of frost penetration, for a foundation with no heated space. The Code says frost penetration depth "shall be established on the basis of local experience," and we found no City of Ottawa page that sets a single number for the whole city.

What this means in practice is that 1.2 m is the table's starting point, not a ceiling. If local experience shows frost goes deeper than 1.2 m on your lot, the footing must go deeper. The Code also allows shallower depths where experience with local soil conditions shows they are satisfactory, or where the foundation is designed for lesser depths, but that is a site-specific determination, not a general rule.

When the depth rule does not apply at all

The 2024 Ontario Building Code includes a floating-deck exemption at section 9.12.2.2(7). If all five of the following hold, the frost-depth minimums in Table 9.12.2.2 do not apply:

• the deck is not more than one storey
• its area is not more than 55 square metres
• the distance from finished ground to the underside of the joists is not more than 600 mm
• it does not support a roof
• it is not attached to another structure, unless differential movement can be shown not to affect that structure

If any one of those conditions fails, the full frost-depth rules apply and the footing design goes back to Table 9.12.2.2.

Soil type adds another layer. Ontario's Building Code, at sections 9.15.1.1(3) and (4) and 9.4.4.1(3), requires engineered design where a foundation sits on filled ground, peat or sensitive clay, including Leda clay. We found no City of Ottawa map for homeowners, so the builder or a geotechnical engineer is the right person to confirm whether it applies to your lot. If it does, the footing design moves to Section 4.2 of the Code, which means an engineer sizes it, not a prescriptive table.

Helical piles are one foundation option. The City's permit submission requirements ask for a helical pile layout and specifications where piles are used. Because piles are deep foundations, they also fall under Section 4.2 engineered design.

Before any digging starts, whoever is doing the work must submit a locate request through Ontario One Call at least five business days in advance. That applies to homeowners and contractors alike, and there is no charge for the request.

The permit drawings the City requires must show the footing or foundation type, size and location, so the depth question gets answered on paper before a shovel goes in the ground.

Ask the builder how they will determine the required footing depth on your specific lot, and whether they expect soil conditions that would require an engineered design.