About roof washing
Those black streaks running down an asphalt roof are not dirt or staining. They are a hardy blue-green algae, Gloeocapsa magma, and it spreads by airborne spores — which is why once one house on a street has it, the neighbours usually follow.
A roof must never be pressure washed
Asphalt shingles are protected by a layer of mineral granules bonded to the surface. Those granules are the shingle’s UV shield and a large part of its rated life. High pressure strips them, and once they are gone the asphalt underneath degrades quickly. The asphalt roofing industry’s own guidance is low-pressure chemical cleaning, and many shingle warranties are explicitly voided by pressure washing.
How low-pressure roof cleaning works
The roof is treated with a cleaning solution that kills the algae, then rinsed at very low pressure. The streaks lift over the following days as rain carries the dead growth away, so a roof often keeps improving for a week or two after the work is finished. Anyone promising an instantly spotless roof is either pressure washing it or overstating what is possible.
It is not only cosmetic
Algae holds moisture against the shingle surface and darkens the roof, which raises attic temperatures. Left long enough, moss follows the algae, and moss lifts shingle edges and lets water track underneath. Cleaning is far cheaper than the repair that follows.
What moves the price
- Roof area, pitch and number of storeys
- How far the growth has gone — streaking versus established moss
- Access for ladders and safe footing
- Landscaping and gutters that need protecting or flushing
- Roof type: asphalt shingle, metal and tile are all treated differently
This is why a photo prices most jobs without a visit.