Attic Mould and Rot Aren't a Roofing Problem.

This is the most common scenario we see that leads homeowners to question whether the new installation was the problem. In almost every case, the mould is not coming from the new roof. It’s coming from the same source it was before the replacement: air leakage from the living space. A new roof replacement typically doesn’t involve any work inside the attic. The ceiling penetrations, ventilation balance, and exhaust fan terminations that were causing moisture problems before the replacement are all still in the same state afterward. If the home performance issues weren’t addressed as part of the replacement scope, the new decking is experiencing the same moisture environment as the old one.

Not all attic mould carries the same health risk. Common surface moulds like Cladosporium and Penicillium are widespread and generally less hazardous than Stachybotrys (commonly called black mould), which produces mycotoxins with prolonged exposure. However, identifying mould species requires laboratory testing. The practical takeaway is that any mould in an attic indicates a chronic moisture problem that will get worse, and all attic mould presents a potential air quality concern if spores are reaching the living space through ceiling gaps. Treat it as a problem worth fixing regardless of species.

Get a professional assessment from a roofing or home performance specialist (not just a mould remediation company) before finalizing the purchase. The assessment should identify the moisture source, the extent of the mould growth, the structural condition of the decking, and the full remediation scope with cost. This gives you accurate information to either negotiate a price reduction, request that the seller remediate before closing, or decide whether the property is worth proceeding with at all. We regularly provide pre-purchase attic assessments for exactly this situation.

Improved ventilation helps flush moisture out of the attic faster, but if the air leakage pathways in the ceiling are still open (pot lights, bathroom fans, attic hatches), you’re still pumping warm, moist air in faster than the ventilation can remove it. The correct sequence is: air seal the ceiling penetrations first, then ensure ventilation is balanced, then remediate. Ventilation alone without air sealing is like trying to dry a leaking pipe by opening a window. It helps, but it doesn’t solve the problem.

The pattern tells you a lot. Condensation-driven moisture is usually distributed fairly uniformly across the underside of the decking, following the natural path of warm air spreading in the attic. It’s often worst in the centre of the attic and near the ridge, where temperature differentials are greatest. A roof leak, by contrast, produces localized water staining that tracks from a specific entry point (a chimney, skylight, valley, or failed flashing) and follows a distinct path downward from that source. If the staining is widespread and uniform rather than concentrated around a penetration, condensation is the most likely cause. An attic inspection in winter (where frost patterns are visible) makes the diagnosis very clear.

You can, but disclosure obligations apply. In Alberta, sellers are generally required to disclose known material defects, and attic mould is typically considered a material defect. Attempting to conceal it creates significant legal liability. The practical options are: remediate before listing (often the best approach for maximizing sale price and avoiding negotiation complications), disclose and price accordingly, or disclose and offer a price reduction or credit to cover remediation. A professional assessment and remediation with documentation is usually the most commercially sensible path if you’re planning to sell.