Guide · Roof Cleaning
Those dark streaks running down your shingles aren't dirt, and that moss isn't just cosmetic. Here's what's actually growing on your roof, why the obvious fix — blasting it — is the one thing you should never do, and how professional roof cleaning really works.
The vertical dark staining on north- and shade-facing roof slopes is a cyanobacteria called gloeocapsa magma. It eats the limestone filler inside asphalt shingles, spreads downslope with every rainfall, and darkens as its protective coating builds up against sunlight. That's why the streaks always run top-to-bottom, always get worse, and never rinse off — you're not looking at a stain, you're looking at a colony.
Beyond looks, the darkened areas absorb more heat, and the bacteria is slowly consuming the shingle itself. On a roof that should last 25 years, established growth is quietly shaving off some of them.
Simcoe County's tall pines and maples keep a lot of roofs shaded and damp — exactly what moss wants. Moss doesn't just sit on shingles; it roots into them and holds moisture against the surface like a wet sponge. Through our freeze–thaw winters, that trapped water expands and contracts against the shingle edge over and over. Lifted, curled, and cracked shingles follow, and every lifted edge is a place for wind and meltwater to get underneath.
Asphalt shingles are protected by a layer of embedded mineral granules — that's the roof's armour against UV and weather. A pressure washer strips granules off by the thousands (check the gutters after anyone blasts a roof; they'll be full of them). The roof looks briefly cleaner and is measurably closer to failure. It's also the fastest way to void a shingle manufacturer's warranty — the major manufacturers explicitly warn against it.
The same goes for aggressive scraping of live moss: it's rooted into the granule layer, so tearing it off tears the shingle surface with it.
The professional method is chemistry, not force. A purpose-made detergent solution is applied at low pressure — gentler than rain — and left to do the work: it kills the bacteria, algae, and moss at the root. Streaks typically vanish or fade dramatically at treatment. Moss dies in place, then dries, loosens, and sheds naturally with weather over the following weeks and months.
That last part surprises people, so we say it plainly: if your roof has heavy moss, it will not look perfect the day we leave — it will look better every week after. Patience is the price of not wrecking the shingles, and it's the approach roofing manufacturers actually recommend.
At Nord, roof cleaning starts from $500, quoted from satellite imagery of your actual roof — no site visit, no ladder in your yard just to get a number. Scope (full roof versus affected slopes) and complexity set the final price, and you approve it before anything is booked.
The treated growth is dead at the root, so nothing regrows from it. New spores can eventually recolonise shaded, damp roofs — trimming back overhanging branches to let in light and air slows that dramatically, and zinc or copper strips fitted near the ridge release a trace of metal in the rain that discourages regrowth down the slopes.
With proper technique, yes — we pre-soak and rinse surrounding plantings throughout the job, the same way we protect gardens on a house wash.
Typically years, not months — because the growth is killed rather than knocked back. Heavily shaded roofs under pine benefit from an occasional maintenance treatment.
Reference: the Asphalt Roofing Manufacturers Association (ARMA) endorses low-pressure chemical cleaning and explicitly warns against pressure washing or scrubbing asphalt shingles — see ARMA, “Algae & Moss Prevention and Cleaning for Asphalt Roofing Systems.”