Making a joist deeper to allow a bigger hole, is that allowed in Ontario?

The Ontario Building Code does allow increasing a joist's depth to compensate for a hole or notch, but the permission is tied to specific conditions, and the work still has to meet the Code's framing rules.

For solid wood floor, roof and ceiling joists, the Code sets a hole limit of not more than one quarter of the joist's depth, placed at least 50 mm from the top and bottom edges (9.23.5.1.). Notches on the top face are permitted only within half the joist's depth of the bearing point, and no deeper than one third of the depth (9.23.5.2.). Both clauses end with the same qualifier: these limits apply "unless the depth of the member is increased" to compensate. So yes, deepening the joist is a recognised way to create room for a larger hole or notch, provided the deeper member then satisfies the rest of the framing requirements for that span and spacing.

A few things to work through before going that route. First, a carpenter or designer needs to confirm the deeper member still works for the span in question. Second, if the joist in question is an engineered I-joist or LVL, we found no general hole or notch limit for those members in 9.23.5., so ask the designer before anything is cut or changed. Third, "structural repairs or alterations" are on the City of Ottawa's permit list, so confirm with the City whether this particular change triggers a permit before any work starts.

The repair or modification also has to respect the load path above and below: the deeper joist needs adequate bearing at each end, and the floor or ceiling assembly above it has to accommodate the change in depth. A carpenter or designer should look at the full picture, not just the hole size.

Before you agree to any scope of work, ask the carpenter exactly how the deeper joist will be sized and whether a registered designer or engineer is involved in that calculation.