When does an Ontario deck design need Part 4 engineering?

When Part 4 Engineering Applies to an Ontario Deck

The 2024 Ontario Building Code covers decks serving a single dwelling unit in Part 9, its prescriptive residential rules. Part 4, the engineered-design section, is required when the deck's conditions fall outside what Part 9 covers. The trigger points are specific, and it is worth knowing them before drawings are started.

The clearest trigger is soil condition. Ontario's Building Code, at sections 9.15.1.1(3) and (4) and 9.4.4.1(3), requires a design that conforms to Section 4.2 where a foundation sits on filled ground, peat or sensitive clay, which includes Leda clay. If the soil at your lot falls into any of those categories, the footing design moves out of Part 9 and into engineered territory. A builder or geotechnical engineer can tell you whether that applies to your specific lot.

The second trigger is the structural framing. Section 9.17.1.1(1)(b) of the Building Code sets the boundary for Part 9 column design: beams carrying loads from not more than two levels of wood-frame decks, where the supported joist length does not exceed 5 metres and the combined snow and occupancy load does not exceed 4.8 kPa. A deck that exceeds any of those limits needs Part 4 design for that element.

Deep foundations are a third trigger. Section 9.4.4.1(3) requires Section 4.2 design where deep foundations are used, or where the footing size falls outside the scope of Part 9. Helical piles are deep foundations, so a deck on helical piles calls for engineered design.

A few other conditions push a deck toward Part 4:

• The deck supports a hot tub. Part 9 has no provision for sizing a deck to carry a filled hot tub's load, so the structure needs a design that accounts for it, confirmed with an engineer and the City's Building Code Services before purchasing the tub.
• The deck supports a roof. The floating-deck footing exemption and the post-anchoring exemption in the Code both exclude decks that support a roof.

Separately, where the guard is a pre-engineered or manufactured product outside the Code's prescriptive requirements, Ottawa's permit list asks for the manufacturer's information and attachment details.

On the permit side, the City of Ottawa asks every applicant to name on Schedule 1 who is taking design responsibility: a BCIN designer, an architect, an engineer or the homeowner exemption. A homeowner may take design responsibility for their own house under the Building Code's homeowner exemption, but that exemption does not apply where more than two dwelling units exist in the building. Where Section 4.2 design applies, that means an engineered design on the drawings.

If your deck will be attached to the house and its walking surface will be more than 600 mm (24 in.) above adjacent grade, or if it is a raised deck providing principal access to the building, the City of Ottawa requires a building permit: confirm the engineering question with the City's Building Code Services when you submit.