Between July 21 and August 2, 2026, three storms that the City of St. Catharines describes as 100-year rain events dropped more than 300 mm of rain on the city. St. Catharines and Lincoln both declared states of emergency, and thousands of Niagara basements took on surface water, sewage, or both.
Two months later, most of those basements look fine. The water is gone, the fans have been returned, and the curbside piles have been collected. But flood damage has a second act. Mould that started in the first few days can keep growing quietly behind drywall, under flooring and inside insulation, and it usually announces itself in the fall, when windows close and the furnace comes on. Here is what to check now, what you can safely handle yourself, and how the insurance side works in Ontario.
Why mould shows up weeks after the water is gone
Mould can start growing on wet drywall, carpet, wood and cardboard within 24 to 48 hours. In a normal leak, fast extraction and drying stop it. The July and August storms were not normal. Many homes waited days for help while restoration crews, plumbers and drain companies across Niagara worked through thousands of calls, and a lot of basements were dried with household fans and open windows in humid summer weather. That leaves three common problems:
- Hidden moisture. The face of a wall can feel dry while the bottom plate, the insulation behind it and the back of the drywall are still wet. Under laminate, vinyl plank and carpet underpad, water can sit for weeks.
- Contaminated water treated like clean water. Floodwater and sewer backup are what the IICRC S500 standard calls Category 3 water. Porous materials it touched, such as drywall, insulation, carpet and underpad, should be removed and the area disinfected, not just dried in place.
- Fall conditions. Once the house is closed up for the season, the humidity a basement still carries has nowhere to go. Mould that grew slowly over the summer can take off.
10 signs to check in your basement this month
Walk your basement slowly with a flashlight. Look low, because most flood mould starts in the bottom 30 to 60 cm of walls and anything that sat on the floor.
- A musty or earthy smell, especially when the furnace first runs.
- Tide lines, yellow or brown staining, or dark speckling along the bottom of drywall.
- Baseboards that are swelling, separating from the wall or growing spots on the back.
- Laminate or vinyl plank flooring that is cupping, lifting or soft at the seams.
- Paint that is bubbling or peeling low on the wall.
- White, chalky deposits on concrete foundation walls (efflorescence), a sign moisture is still moving through.
- Spots or fuzz on stored boxes, the backs of furniture, or the underside of stairs.
- Condensation on basement windows or cold water lines.
- Allergy, asthma or sinus symptoms that are worse downstairs.
- Relative humidity above 50 to 60 percent. A basic hygrometer costs under $30 at a hardware store and tells you more than your nose can.
What you can clean yourself, and when to call a professional
Health Canada’s general advice is that a small patch of mould, under about one square metre on a hard surface, can usually be cleaned by a homeowner with water and dish soap, wearing gloves, eye protection and a disposable N95 mask. Skip the bleach: it does not reach mould growing inside porous materials like drywall, and it adds fumes to a closed-up basement.
Call a professional when any of these apply:
- The mould covers more than about one square metre, or keeps coming back after cleaning.
- It is on drywall, insulation, carpet or wood that was soaked by floodwater or sewage.
- You can smell it but cannot find it, which usually means it is inside a wall or under the floor.
- Someone in the home has asthma, a respiratory condition or a weakened immune system, or is very young or elderly.
- Your home was built before the 1990s. Drywall joint compound, plaster, textured ceilings and vermiculite insulation from that era can contain asbestos. Have suspect materials tested before anyone cuts, sands or tears them out.
Professional mould remediation and asbestos testing starts with finding the extent of the problem using moisture meters and thermal imaging, not just what is visible. Affected materials are removed under containment with HEPA air scrubbing so spores do not spread to the rest of the house, the structure is cleaned and treated, and the area is dried and verified before anything is closed back up.
How flood and mould insurance claims work in Ontario
Every policy is different, so your policy wording and your adjuster have the final say. These are the points that cause the most confusion after a storm like this one:
- Sewer backup and overland water are usually separate coverages. On most Ontario home policies, both are optional add-ons (endorsements). Two neighbours with identical basements can have very different claims depending on how the water got in and which coverage each one bought.
- Mould coverage is often limited. Many policies limit or exclude mould unless it results directly from an insured event. If you find mould now, tie it back to your July or August loss: give your adjuster your claim number, photos, and the date you found it.
- Report new damage promptly. Policies require notice of damage as soon as practicable. Do not wait until spring to mention what you find this fall.
- Keep records. Photos before anything is removed, receipts, moisture readings and contractor reports all help. Your adjuster may ask to see removed materials before they are thrown out.
- If you were uninsured or underinsured, complete the City of St. Catharines’ Flood Impact Form. The city is using it to help the Province decide whether Disaster Recovery Assistance for Ontarians (DRAO) applies. DRAO generally does not cover sewer backup, with some exceptions for low-income households.
Details on the Flood Impact Form and the city’s flood programs are on the City of St. Catharines July 2026 flooding page.
Protect the basement before the next storm
Once the basement is clean and dry, the best investment is keeping water out next time. St. Catharines’ Emergency Flood Alleviation Program (Emergency FLAP) reimburses eligible homeowners for 90 percent of the cost, up to $5,000, of flood protection such as a mainline backwater valve, disconnecting weeping tiles from the sanitary sewer, and a secondary sump pump. Check the city’s page for current eligibility before you book the work. Beyond that:
- Keep downspouts extended at least two metres from the foundation and clear of the driveway.
- Test your sump pump before winter and consider a battery backup.
- Store basement items in sealed plastic bins on shelves, not cardboard on the floor.
- Run a dehumidifier to hold the basement under about 50 percent humidity.
How DKI Miller can help
DKI Miller has restored Niagara homes from our base on Dunkirk Road in St. Catharines since 1995. If you are worried about what is behind your walls after the summer floods, we can map moisture, test suspect materials for asbestos, remove and remediate mould safely, and give your adjuster the photos and moisture logs a claim needs. We also handle the rebuild, so one team takes you from wet basement to finished space. If sewage came up through a floor drain, our sewer backup and sewage cleanup crews remove and disinfect contaminated materials safely.
Call our 24/7 line at 905-688-9224, report a claim online, or read more about our water and flood damage restoration in St. Catharines and across the Niagara Region.
Frequently asked questions
My basement was dried in August. Can mould still be growing?
Yes. If drywall, insulation or flooring that touched floodwater was dried in place instead of removed, or if the drying was never checked with a moisture meter, mould can keep growing inside the wall or under the floor. A musty smell when the heat comes on is the most common early sign.
Will my insurance cover mould I find now?
It depends on your policy and on whether the mould can be traced to an insured event, such as a covered sewer backup or overland flood. Report it to your adjuster as soon as you find it, with photos and your original claim number, rather than treating it as a new problem.
Should I test for asbestos before I renovate my flooded basement?
If the house was built before the 1990s, yes. Disturbing asbestos-containing drywall compound, plaster or insulation releases fibres that are a far bigger health risk than the mould. Testing takes a few days and tells you whether the tear-out needs asbestos precautions.