Cutting through a top plate for a duct in Ontario homes?

The Ontario Building Code (2024, O. Reg. 163/24) sets a clear limit for top plates: after notching or drilling, at least 50 mm of the plate's width must stay undamaged (clause 9.23.5.4.). That is the rule that governs what a carpenter can do when a duct has to pass through a plate.

A top plate is part of the wall's load path, so how much can be removed matters. If the duct would leave less than 50 mm of the plate's width undamaged, the cut is outside what that clause allows, so ask how the plate will be reinforced before anything is cut. The Code does not specify the reinforcement method in 9.23.5.4.; that is a question for the carpenter or a designer, depending on how the wall carries load.

A few things to sort out before the work starts:

• Whether the wall is load-bearing changes how carefully the plate repair is designed. Ask the carpenter to confirm the wall's role before anything is cut.
• Truss members are a separate matter: clause 9.23.5.5. says a truss member is not cut unless the truss design allows it. If the duct route crosses a roof truss, check the truss design first.
• For engineered members such as LVL or I-joists, we found no general rule in 9.23.5. Ask a designer before anything is cut in those members.
• The City of Ottawa lists "structural repairs or alterations" as permit work on ottawa.ca. A top-plate change in a load-bearing wall can be a structural alteration; confirm with the City whether a permit is needed before the work begins.
• HVAC work is out of scope here, but the carpentry side, meaning the framing repair and any reinforcement, is what a carpenter prices and executes.

A fair quote for this scope covers materials, labour for the cut and any reinforcement, patching of the wall finish afterward, and disposal. If a permit is required, the City of Ottawa's fee is $12.75 per $1,000 of construction value, minimum $117; the City sets the valuation.

Before you agree to any price, ask the carpenter exactly how they plan to keep 50 mm of the plate's width undamaged, and what reinforcement they will use if the duct opening leaves less than that.