What happens if my contractor discovers structural damage behind the walls during demo — who pays for the extra work?

In almost all cases, the homeowner is responsible for the cost of repairing structural damage discovered during bathroom demolition — this is considered a hidden or unforeseen condition that falls outside the original scope of work, and it is one of the most important reasons to maintain a contingency budget of 15–20% on any bathroom renovation in the GTA. Your contractor did not cause the damage and could not have predicted it before opening the walls, so it is not reasonable to expect them to absorb the cost.

Structural damage behind bathroom walls is more common than most homeowners realize, particularly in older Toronto homes where slow leaks from deteriorating plumbing, inadequate waterproofing, or poor ventilation have been causing hidden moisture damage for years or even decades. The most common discoveries during bathroom demo include rotted subfloor around the toilet flange (caused by a failed wax ring or cracked toilet base that has been leaking imperceptibly), rotted or weakened floor joists under the tub or shower (caused by waterproofing failure allowing water into the subfloor for years), mould growth in wall cavities behind tile that lacked proper waterproofing membrane, deteriorated stud framing in wet walls where plumbing supply lines or drain connections have been seeping, and corroded or compromised load-bearing elements in post-war homes where cast iron drain stacks have leaked at joints.

The cost of addressing these discoveries varies widely. Subfloor repair — removing rotted plywood or planking and replacing it with new 3/4-inch plywood — typically costs $500–$2,000 depending on the area affected. Floor joist repair — sistering new lumber alongside weakened joists — runs $300–$800 per joist plus the labour to access and install them. Mould remediation in a bathroom wall cavity, when properly contained and removed, costs $1,000–$5,000 depending on the extent. Stud wall framing repair — replacing rotted studs and adding blocking — is usually $500–$1,500.

How the Process Should Work

A professional GTA bathroom contractor will handle a structural discovery with a clear process. First, they stop work in the affected area and document the condition with photographs. Second, they notify you immediately — in person, by phone, and followed up in writing (text or email). Third, they assess the scope of repair needed and provide a written change order with the additional cost and any timeline impact. Fourth, you review and approve the change order in writing before additional work proceeds.

This is exactly why your renovation contract should include a change order clause — it establishes the process for handling unforeseen conditions so that both parties know what to expect. Without a change order clause, disputes arise about whether the contractor should have anticipated the damage, whether the repair method is appropriate, and whether the price is fair.

Protecting Yourself

The contingency budget is your primary protection. For bathroom renovations in Toronto homes built before 1970, a 20% contingency is strongly recommended because of the higher likelihood of hidden conditions. For homes built after 1990, a 10–15% contingency is usually sufficient. On a $30,000 mid-range bathroom renovation, that means setting aside $3,000–$6,000 for potential surprises.

If the discovered damage is extensive — for example, widespread mould behind multiple walls or severely compromised floor joists — get an independent assessment before approving the repair. A structural engineer's assessment costs $500–$1,000 in the GTA and provides an unbiased opinion on the scope of repair needed. This protects you from overpaying and protects the contractor from liability if the repair is later questioned.

Some homeowners ask whether their home insurance covers structural damage found during a renovation. Generally, insurance covers sudden and accidental water damage (a burst pipe) but not gradual deterioration (a slow leak over years). Check with your insurer, but do not plan on insurance covering most bathroom-related structural discoveries.

Finally, consider a pre-renovation inspection for older Toronto homes. A plumber can scope your drain lines with a camera ($250–$500) to assess their condition before demolition. Moisture meters can detect elevated moisture levels in walls and subfloors without opening them. These assessments cost a few hundred dollars but can reveal conditions that allow you to budget more accurately from the start.