Does moss damage asphalt shingles?

Moss is the defining roof problem of Greater Victoria, more than rain. The region is the driest spot on the BC coast at about 608 mm a year at Gonzales, but the winters are mild and grey, and a roof under a Garry oak in Fernwood or a Douglas fir in Saanich stays damp from October to April without ever drying out. A 2026 Victoria guide notes moss here grows almost year-round, worst on north-facing slopes and under fir and cedar, which describes half the roofs in the Highlands, Metchosin, Cordova Bay and the treed parts of Oak Bay.

The damage shows up in a recognisable local pattern: the south slope of a Gordon Head split-level looks fine at 18 years while the north slope, three metres away under the neighbour's fir, has lifted edges, bald patches and a soft valley. Home inspectors here check moss extent as a standard item, and a mossy north slope is one of the first things a buyer's inspector photographs. It is the reason the same house can carry a roof rated 'fair' on one side and 'replace' on the other.

A green haze on the north slope after a wet winter is harmless if it is treated that spring; the shingles underneath are intact and the treatment kills it before rhizoids take hold. A mat you can lift with a finger is different: the granules under it are already loose, the shingle edge is already raised, and removing it will expose bald patches that were coming anyway. That is why the cheapest moss control is the earliest, and why a 2026 Victoria guide recommends treatment every 12 to 18 months on high-risk roofs rather than a big clean every five years.

Pressure washing is the classic mistake. It blasts granules off along with the moss and drives water under the tabs, and both Pacific Northwest extension guidance and local contractors advise against it on asphalt. Hard scraping with a metal tool does the same damage by hand. The correct method is a treatment that kills the moss, followed by gentle brushing or a low-pressure soft wash once it is dead, done from the roof by a crew with fall protection or from a ladder on a low roof. Anyone who walks on a mossy roof without a harness is taking a real risk; the surface is as slick as ice.

• Treat first, remove when dead
• Soft brush or low-pressure rinse only
• Downhill strokes, never against the tab edges
• Check gutters for granules afterwards; a lot of granules means the shingles were already failing

Bald shingles, lifted edges that will not lie flat, a soft feel at the valleys or persistent stains on the ceiling below the north slope all suggest the roof has moved from maintenance to replacement planning. On a roof past 15 years with heavy moss, a $150 to $400 inspection in our model tells you how many winters are left and whether a treatment now is a good spend or a delay. At replacement, an algae-resistant shingle, better venting, trimmed trees and zinc or copper strips give the new roof a better start than the old one had.