How do I prevent mould growth under flooring in a NB home that was flooded during spring snowmelt?

Flood-damaged subfloors in New Brunswick require complete drying, antimicrobial treatment, and moisture barrier installation before any new flooring goes down. Installing flooring over wet or contaminated subfloors guarantees mould growth, health problems, and expensive re-work within months.

Immediate Assessment and Drying

First, determine the extent of water penetration into your subfloor assembly. Spring snowmelt flooding in NB often saturates wood subfloors, floor joists, and insulation for weeks after surface water recedes. Use a moisture meter to check subfloor moisture content — wood should read below 12% before any flooring installation. Concrete slabs need 60+ days to dry after flooding, even with dehumidifiers running continuously.

Remove all existing flooring materials immediately, including carpet, pad, vinyl, and any underlayment that contacted flood water. These materials cannot be salvaged and will become mould sources if left in place. Cut and remove any wet drywall at least 12 inches above the flood line — drywall acts like a sponge and wicks moisture upward through capillary action.

Professional Remediation Requirements

For flooding that lasted more than 24-48 hours or involved contaminated water (sewage backup, river overflow), hire a certified mould remediation company. They'll assess whether structural lumber needs replacement, apply antimicrobial treatments to salvageable materials, and ensure proper drying before reconstruction begins. This typically costs $3,000-$8,000 for a flooded basement but prevents $15,000+ in future mould damage.

Subfloor Preparation and Moisture Management

Once everything is completely dry, apply a penetrating antimicrobial sealer to all wood surfaces that contacted flood water. Products like Concrobium Mold Control or Microban are designed for this purpose. For concrete slabs, use a concrete densifier/sealer that penetrates the surface and reduces future moisture transmission.

Install a proper moisture barrier system before any new flooring. Over concrete, use 6-mil polyethylene sheeting with sealed seams, or better yet, a dimpled drainage membrane like DRIcore or BARRICADE that creates an air gap between the concrete and your flooring. Over wood subfloors, replace any water-damaged sections and install a vapour-permeable underlayment that allows the subfloor to continue drying while blocking moisture from reaching your new flooring.

Flooring Material Selection

Choose 100% waterproof flooring for flood-prone areas. Luxury vinyl plank (LVP) with SPC or WPC cores, porcelain tile, or polished concrete are your best options. These materials won't support mould growth and can be easily cleaned if future flooding occurs. Avoid solid hardwood, standard laminate, or carpet in any area that has flooded — these materials will fail again when NB's next major spring melt or storm surge occurs.

Long-Term Prevention

Address the root cause of flooding to prevent recurrence. Install proper drainage around your foundation, ensure gutters direct water away from the house, and consider a sump pump system if your area is prone to spring flooding. Many NB communities experience regular spring flooding due to ice jams, rapid snowmelt, and poor drainage — your flooring choice should account for this reality.

When to Hire a Professional

Any flood damage beyond surface water requires professional assessment. Mould remediation, structural drying, and subfloor replacement are not DIY projects. Find contractors experienced with flood restoration through the New Brunswick Construction Network — proper remediation now prevents years of indoor air quality problems and protects your family's health.