What causes white residue on composite decking in New Brunswick?

White residue on composite decking is almost always efflorescence — a mineral salt deposit that migrates to the surface when moisture trapped inside the boards evaporates. This is particularly common in New Brunswick due to the province's Maritime climate, where high humidity, frequent rain, and dramatic temperature swings between seasons create the perfect conditions for moisture cycling through composite materials.

Efflorescence typically appears during the first one to two years after installation, especially on capped composite boards that trap moisture between the polymer cap and the wood-fiber core. In New Brunswick, you will notice it most frequently in spring when snow and ice melt saturates decking that then dries rapidly during warmer afternoons. The white, chalky film can appear in patches or across entire boards, and it tends to be more visible on darker-coloured composites where the contrast is stark.

The good news is that efflorescence is cosmetic and does not indicate structural damage to your deck. In most cases, it resolves on its own after one or two full seasonal cycles as the residual manufacturing moisture works its way out of the boards. To speed up the process, wash the affected areas with warm water and a composite deck cleaner — avoid using a pressure washer above 1,500 PSI, as excessive pressure can damage the protective cap layer on the boards. A soft-bristle brush and a garden hose will handle most cases effectively.

If the white residue persists beyond the second year, there may be a drainage issue beneath the deck that is keeping moisture levels elevated. Poor airflow under the deck structure is a common contributor in New Brunswick builds, especially when decks are built close to grade on sloping lots where water pools against the house foundation. Ensure your joists have adequate spacing from the ground — a minimum of 12 inches of clearance is recommended — and that water drains away from the deck rather than collecting underneath.

Another less common cause of white residue is mould or mildew that appears whitish-grey rather than the typical green or black. New Brunswick's humid summers, where relative humidity regularly exceeds 80 percent along the Fundy coast, create ideal conditions for surface mould on composite decking, particularly in shaded areas that never fully dry. A composite-safe mould cleaner will resolve this quickly.

It is worth noting that some homeowners confuse pollen accumulation in spring with efflorescence. New Brunswick's birch and spruce pollen season in May and June can deposit a yellowish-white film on decks that looks similar. A simple wipe test will tell you which it is — pollen wipes away easily, while efflorescence requires scrubbing.

If you purchased your composite decking from a reputable manufacturer, check your warranty documentation. Many brands explicitly cover efflorescence under their surface warranties for the first few years. Keep your purchase receipts and installation records, as manufacturers may require proof of proper installation before honouring a claim. Proper gapping between boards (typically 3/16 to 1/4 inch) and correct fastener placement both affect moisture drainage and can be factors in warranty assessments.

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