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Black streaks on a roof are algae, not dirt, and the moss in shaded sections is actively shortening the roof's life by holding water against shingle edges. Both come off easily with low-pressure soft washing, and both get far worse if you attack them with a pressure washer, which removes the ceramic granules that shield the asphalt from UV. This guide covers the gear, the solution, dwell times, working safely at height, protecting the planting below and how zinc strips stop regrowth.
Roof cleaning removes algae, moss and lichen. Done correctly it is a low-pressure chemical process. Done with a pressure washer it strips the granules off the shingles and takes years off the roof.
Never pressure wash an asphalt shingle roof. The granules are the shingle's UV protection and its fire rating, and a pressure washer removes them. Manufacturers void warranties over it, and the damage does not show immediately — it shows as premature failure five years later.
A steep or two-storey roof, a tile roof that cracks underfoot, or heavy moss growth that has lifted shingles. Also any roof where you would be working near a power line. Professional soft washing is not expensive relative to a fall.
Not an asphalt shingle roof. The granules are its UV protection and its fire rating, and a pressure washer strips them. Manufacturers void warranties over it, and the damage shows years later as premature failure.
A low-pressure application of a cleaning solution, left to dwell, then rinsed gently. The chemistry does the work rather than the pressure, which is why it removes algae without removing granules.
Zinc or copper strips near the ridge, so rain carries a trace of metal down the roof. Also cut back overhanging branches, keep the gutters clear, and specify algae-resistant shingles at the next reroof.
A wet roof with cleaning solution on it is extremely slippery — that is the biggest hazard of the job. Work from a ladder or the ground with an extension wand wherever possible, never alone and never in wind.