How often should I treat my roof for moss in Victoria?

The interval is set by the tree next door more than by the postal code. A north-facing slope under a Douglas fir in the Mount Tolmie or Cedar Hill parts of Saanich, a cedar-ringed lot in Metchosin, or a Highlands roof that sees no direct sun from November to February will be green again within a year of a clean, and 12 months is the honest interval. The same shingle on a south-facing Fairfield roof, a wind-scoured Sidney bungalow or a Royal Bay subdivision house with no mature trees may show nothing for three years.

Roof material shifts the schedule too. Cedar shakes in Oak Bay and the Uplands hold moisture in their grain and need attention at least as often as shaded asphalt; a 2026 BC cedar guide puts maintained shakes at 18 to 25 years and unmaintained at 12 to 15, which is the cost of skipping the interval. Metal roofs in the same neighbourhoods shed needles and never root moss; a rinse every few years is enough. Victoria home inspectors check moss extent as a matter of course, so sellers should time a treatment for the spring before listing.

Treatment on a light haze is a spray and a rinse; treatment on a two-year mat is a removal job with bald patches underneath and lifted tabs to reseal. In our pricing model a routine visit is at the low end of $400 to $1,200 and a heavy first clean at the top, and a 2026 Victoria guide puts annual contracts at $300 to $600 against $500 to $1,200 for a one-off. Over a roof's life the shorter interval usually costs less and the shingles last longer.

From the ground with binoculars in March, look at the north and east slopes, the strip below any tree, the shaded side of dormers and the valleys. A uniform green tint means growth started over the winter and a spring treatment will catch it before rhizoids take hold. Thick cushions you could lift mean the interval was too long. Granules in the gutter or pale patches mean the shingles are already suffering. Photograph the same slopes each year; the comparison is the schedule.

• North and east slopes first
• Below every overhanging branch
• Valleys and the uphill side of skylights and chimneys
• Gutters: granules or moss fragments

Three things lengthen the time between treatments: trimming trees back to open sky above the roof, installing zinc or copper strips after a clean so rain washes a mild moss inhibitor down the slope, and choosing an algae-resistant shingle at the next re-roof. Better attic ventilation helps at the margin by keeping the deck a little warmer and drier. None removes the need to look each spring, but together they can move a 12-month roof to 24.