Mould found by an Ottawa home inspector, how serious is it?

A home inspector's mould note tells you where to look, not how serious the problem is. The next step is to measure the affected area and find the moisture source, because both of those things decide what comes next.

Health Canada sets three size bands. Small is one to three patches, each less than one square metre. Medium is more than three patches, or patches greater than one square metre but less than three square metres. Extensive is a single patch larger than three square metres. Health Canada says you can generally clean small and medium sized areas yourself, and recommends getting professional help with extensive mould growth.

Size alone does not tell you where the water comes from, and mould returns if the moisture source is not repaired. A professional assessment looks at both the visible growth and what is feeding it.

One thing worth raising before any wall or ceiling is opened: if the house was built before 1990, the materials behind that surface, such as drywall compound, plaster or textured coatings, may contain asbestos. Health Canada advises hiring a professional to test suspect materials before any renovation work that could disturb them, and a qualified asbestos removal specialist if asbestos is found. Cutting into a mouldy wall in an older home can become an asbestos job at the same moment it becomes a mould job, so it is worth asking about that before work starts.

We found no Ontario statute or regulation that licenses mould remediators or sets indoor mould limits. Industry guidelines exist, but none of them carry the force of law. A written scope should say what material comes out, how the moisture source is repaired, and that no visible mould remains when the job is done. Ask the contractor to put that scope in writing before signing anything.

A few things worth confirming with whoever you hire: whether they will identify and repair or refer out the moisture source, what they will do if they find suspect materials behind the wall, and whether the quote covers disposal of the removed material.

The contractor is the right first call, because they can measure the affected area in person, assess what is behind the surface, and tell you whether the moisture source is something they handle or whether a plumber or waterproofing specialist needs to come first.