Bathroom subfloor mold in an older Calgary house, how is it fixed?

Bathroom subfloor mould needs the water source fixed first, then the damaged material removed and replaced. Published Calgary-area prices for mould remediation range from roughly $1,800 to $6,000 for a medium-affected area (a patch larger than one square metre but smaller than three square metres), or $3,000 to $15,000 or more if the damage has spread into the subfloor framing, wall cavities or an adjacent crawlspace.

What drives the cost and the decision

Three things move the price: how far the mould has spread beyond what is visible, whether the subfloor is plywood or OSB over a crawlspace or a concrete slab, and whether the framing underneath is also affected. Porous materials such as drywall, carpet and ceiling tiles typically require removal rather than surface cleaning, according to Alberta Health Services. Cleaning a hard, non-porous surface with water and dish detergent works for small patches (each under one square metre, no more than three patches), but a bathroom subfloor that has softened or discoloured is likely past that point. The CMHC guide puts medium areas (more than three patches, or up to three square metres) with qualified and trained maintenance staff, and anything larger with qualified and trained mould remediation contractors.

Before anyone touches the subfloor, raise one question: was the house built before 1990? Alberta OHS bulletin ASB004 notes that pre-1990 buildings carry a higher risk of asbestos-containing materials, and vinyl sheet flooring and vinyl floor tiles are on that list. The Alberta Asbestos Abatement Manual adds floor mastic and levelling compound. If the bathroom has original vinyl flooring or adhesive underneath it, have bulk samples tested before cutting or prying anything up. Published Calgary-area lab costs run $65 to $145 per sample. Alberta Environment's guidance is not to disturb friable asbestos-containing materials and to restrict handling to qualified personnel.

What a fair scope of work includes.

Ask any contractor quoting this job to cover the following:

• Written confirmation of the moisture source and how it will be corrected before remediation begins (no fix to the leak means mould returns)
• A clear description of what gets removed, what gets treated and what gets tested after
• A post-remediation check before the floor is closed up, and who carries it out
• Confirmation that any asbestos-containing flooring or adhesive will be assessed and handled separately under Alberta OHS Code requirements if the house predates 1990
• A written prepaid contract if any deposit is taken, as required under Alberta consumer protection rules for prepaid contracts of $200 or more

There is no Alberta licence for mould remediation, and we found no City of Calgary permit specifically for mould work. Alberta Health Services notes that air testing for mould is usually not recommended when you can already see it; put the money into the repair and the post-remediation check instead.

Ask for a WCB clearance letter confirming the contractor has an account in good standing before work begins; anyone can request one online from WCB-Alberta.

If the subfloor has already been disturbed and you are not sure what is under the flooring, stop the work right now, leave the area, keep everyone including pets out, close the door, and do not sweep or vacuum until the material has been tested.