What is the best way to remove mould from a deck in New Brunswick's humid climate?

The most effective method for removing mould from a New Brunswick deck is an oxygen bleach treatment using sodium percarbonate followed by a power wash at 1500 PSI. This two-step approach kills the mould at its root structure, lifts the staining from the wood fibres, and does so without the environmental damage or wood degradation that chlorine bleach causes. Given that maritime humidity is the primary driver of deck mould throughout the province, understanding why it forms and how to prevent its return is just as important as the removal itself.

New Brunswick's climate creates near-perfect conditions for mould growth on wood decks. The province averages over 1100 millimetres of annual precipitation, summer humidity regularly sits above 70 percent, and the cool overnight temperatures in river valleys around Fredericton, Moncton, and Saint John create morning dew that keeps deck surfaces damp well into the day. North-facing decks are especially vulnerable because they receive limited direct sunlight to dry the surface, and decks surrounded by trees or close to the ground with poor air circulation underneath are almost guaranteed to develop mould within a season or two of being built.

To begin the removal process, sweep the deck clear of all loose debris, leaves, and pine needles. Mix the sodium percarbonate oxygen bleach according to the manufacturer's directions, typically about 250 grams per 4 litres of warm water. Apply the solution generously across the deck surface using a pump sprayer or watering can, making sure to saturate any areas with visible black or green mould growth. Let the solution sit for 10 to 15 minutes. You will see it fizzing slightly as the oxygen release breaks apart the mould structure. Do not let it dry on the surface — if it is a warm day, mist the area with plain water to keep it damp during the dwell time.

After the dwell period, power wash the entire deck at 1500 PSI using a 25- or 40-degree fan tip nozzle. Work with the wood grain in smooth, overlapping passes, keeping the nozzle about 15 to 20 centimetres from the surface. The combination of the oxygen bleach pre-treatment and the mechanical action of the washer will remove mould that has penetrated into the wood surface. For deeply embedded black mould stains, a second application of oxygen bleach followed by scrubbing with a stiff-bristle brush before the final wash may be necessary.

Once the deck is clean, allow it to dry completely for 48 to 72 hours before applying any protective finish. This drying period is critical in New Brunswick's climate because sealing in residual moisture creates the exact conditions that led to the mould in the first place. When the deck is dry, apply a penetrating deck stain or sealant that contains mildewcide. Semi-transparent stains offer a good balance of protection and appearance, and products specifically marketed for maritime or coastal climates tend to include higher concentrations of mould inhibitors.

Preventing Recurrence

Prevention in New Brunswick's humid climate requires ongoing effort. Keep the deck surface clear of leaves and organic debris that trap moisture. Trim back any overhanging branches to maximize sunlight exposure and airflow. Ensure the gap between the ground and the underside of the deck is sufficient for air circulation — at least 300 millimetres is ideal. Clean the deck twice per year, once in spring after snowmelt and once in early fall before leaf drop, even if mould is not yet visible. These lighter cleanings can be done with a garden hose and oxygen bleach solution without the power washer. On north-facing decks, a mid-summer cleaning may also be warranted. Consistent maintenance is the only reliable defence against mould in this climate.

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