How do I know if rot in my Blackburn Hamlet house is structural?

Rot becomes structural the moment it has reached a member that carries load: a joist, a sill plate, a stud in a load-bearing wall, a beam, a rim joist or the structural subfloor. The only way to know for certain is to have a carpenter probe and expose the affected area, because rot behind finishes can run further than the visible damage.

A few things a carpenter looks at when assessing rot:

• Which member is affected. A piece of interior trim or a non-structural nailer rotting is a different situation from a floor joist or a wall plate rotting at its bearing point.
• How deep the decay goes. Surface discolouration is not the same as soft, crumbling wood that a probe or awl sinks into easily.
• Whether the rot has reached a bearing point, where the member transfers its load.
• Whether engineered members are involved. We found no general limit in 9.23.5. for engineered joists or beams, so if the affected member is an I-joist or LVL, ask the carpenter whether a designer needs to assess it before anything is cut or sistered.

On the source of the moisture: a carpenter can tell you where the rot is and how far it has gone, but finding why water got there in the first place is a separate step. Ask the carpenter to describe what they found and whether they can see where the water entered, so you know whether the repair will hold.

If the rot is in a joist or sill at or below ground level, the Ontario Building Code (9.23.2.2.) requires that the replacement member either be treated against decay or have a 12 mm air space at its end where it bears in masonry or concrete.

On repair scope: we found no Building Code rule specifying how much decayed wood must be removed or exactly how a sistered repair is assembled. Ask the carpenter how the repair is designed and whether a designer is involved, particularly if more than one member is affected or if the damage is near a bearing point or a beam pocket.

The City of Ottawa lists structural repairs and alterations as permit work, so ask Building Code Services whether the scope of your repair requires a permit before work begins.

If at any point a floor, ceiling or wall is sagging, cracking or moving in a way that feels sudden or worsening, leave the area, keep people out, and call emergency services from a safe place.