How much rotten wood gets cut out in an Ottawa framing repair?

How much rotten wood a carpenter removes depends on what they find once the framing is exposed, and that decision belongs to the carpenter or designer on site, not a formula.

The goal is to get back to sound wood. Rot can run further than the visible damage, so the extent of the cut is confirmed by probing and inspection, not by measuring from the surface damage. We found no Building Code rule specifying how many millimetres of rotted material must come out or exactly how a sistered repair is sized; the carpenter assesses the member and decides how the repair is designed.

A few Code facts do shape the repair. The 2024 Ontario Building Code (O. Reg. 163/24) requires that the ends of joists and beams in masonry or concrete at or below ground level be either treated against decay or given a 12 mm air space (9.23.2.2.), so if the rot started at a bearing point, the repair addresses that condition as well as the damaged wood. For holes drilled in floor, roof or ceiling framing, the Code sets a limit of one quarter of the member's depth, at least 50 mm from the edges (9.23.5.1.); for notches on the top of a joist, the limit is one third of the depth, within half the joist depth of the bearing (9.23.5.2.). Those limits apply to the repaired member going forward, not to how much rot is removed. For engineered joists or beams, we found no general limit in 9.23.5., so ask the designer before anything is cut or drilled in those members.

Whether a designer needs to be involved depends on the scope. A single sistered joist may be straightforward; a repair that affects a bearing point, a beam, a sill or multiple members in a row is worth having a qualified, registered designer (BCIN), an engineer or an architect look at, so the repair is sized for the loads it carries.

The carpenter or designer is the right first call, because they can probe the framing, trace how far the rot has travelled and tell you whether the repair stays within what a carpenter sizes on site or whether a designer should specify it.