Can I pressure wash my roof to remove moss?

The pressure-washer temptation is strongest in the parts of Greater Victoria where moss is thickest and driveways are already being washed every spring: the fir-shaded streets of Saanich around Mount Douglas, the acreages of Metchosin and the Highlands, and the older cedar-and-asphalt mix of Colwood and View Royal. The result is visible from the street a year later: clean-looking roofs with pale, bald streaks where the washer ran, shedding granules into the gutters faster than the moss ever did.

The local market makes the alternative easy. Victoria has an unusual number of dedicated roof moss companies, and most now advertise soft-wash or treat-and-brush methods precisely because pressure-washed roofs became a warranty argument between homeowners and roofers. On a 1990s Langford roof still under a manufacturer warranty, a pressure-wash is also the kind of 'improper maintenance' that can void coverage; ask before anyone puts a wand near it.

Asphalt shingles are a fibreglass mat coated in asphalt and covered in ceramic granules; the granules are the sunscreen and the wear layer. Moss rhizoids have already loosened the granules under a mat, so the jet removes both at once and leaves bare asphalt that will dry, crack and curl within a few summers. The jet also travels uphill under the tabs and into the underlayment, and on a roof past 15 years it can tear brittle shingles outright. A clean-looking roof with a decade taken off its life is the usual trade.

A professional visit begins with a treatment applied on a dry day; the moss browns over the following weeks and loosens its grip. On a second visit, or after some rain has done the work, the crew brushes downhill with a soft broom or rinses at garden-hose pressure, clears the valleys and gutters, and hand-seals any tab that was lifted. On steep or high roofs the crew is tied off; on a low bungalow some of it can be done from a ladder on firm ground. Expect $400 to $1,200 in our pricing model for a typical house, with a heavy first clean at the top of the range.

• Treatment first, removal when dead
• Soft broom or low-pressure rinse, downhill
• Gutters and valleys cleared in the same visit
• Photos before and after; a note on shingle condition

Check the gutters for a layer of granules and look at the roof from the ground for pale streaks or exposed black asphalt. Light loss on a young roof is survivable; keep it moss-free and it will still make its expected life. Heavy loss on a roof past 15 years means the shingles are now ageing fast and a $150 to $400 inspection will tell you how many winters are left. Either way, do not wash it again.