Is algae on my roof actually harmful?
Algae mainly stains, but it feeds on the limestone filler in asphalt shingles and keeps the surface damp, which can wear the shingle faster over time. Moss is more directly harmful because it lifts edges and holds water against the roof.
Dark streaks are an appearance problem first. Many homeowners treat them for that reason alone, and there is nothing wrong with that. The concern is that growth keeps the shingle damp, and that over years the surface can deteriorate in ways a clean roof avoids.
Moss is a more active problem. It forms thick mats that trap moisture and push under shingle tabs. Freeze-thaw winters make that worse, because water held in the mat freezes and widens the gap.
Why soft washing and not a pressure washer?
Sources recommend soft washing because high pressure can strip the granules that protect the shingle and force water under the tabs. A low-pressure wash treats the growth without that damage, and it is the method those contractor sources describe.
A pressure washer is strong enough to knock granules off a shingle, and a roof that has lost its granules has lost part of its protection. It can also drive water up under shingle edges, where it does not belong.
A soft wash puts a cleaning solution on the roof and lets it work, then rinses at a gentle pressure. Ask any contractor you speak to which method they use, and be wary of anyone who proposes a pressure washer for a shingle roof.
- Ask which method will be used
- Ask how plantings and gutters will be protected
- Ask what will be done about moss that is under the shingle tabs
Why is growth so common on Piedmont roofs?
Humid summers, steady rain and shade. Winston-Salem has rain through the year, humid heat in July and freeze-thaw winters, and trees shade many roofs. Those conditions keep shingles damp and give algae and moss what they need.
Monthly rain runs about 2.6 to 3.4 inches, and July has about 13.6 wet days. The city's hottest month is also the muggiest, so roofs rarely dry out completely in summer. Contractor sources from the neighboring area describe Davie County humidity as a known cause of roof algae, and Winston-Salem's weather is similar.
Shade matters as much as weather. A roof under oaks or pines stays wetter than one in the open, and the shadier slope usually shows the growth first. Trimming back limbs can help, though it is a separate job from cleaning.
When does growth mean the roof needs replacing?
When the shingles under the growth are worn through, curling or lifting, or when granule loss has exposed the mat. Cleaning cannot restore those. A roofer who looks at the shingles before washing can tell you which situation you have.
The city's median home was built in 1977, and many roofs are on a second or third covering. An old roof with heavy growth may have been losing granules for years, and cleaning can expose how little protection is left. The right call may be repair of a few spots or a new covering.
The guide to moss and algae on Piedmont roofs goes into that decision in more detail.
What can I do to slow regrowth after cleaning?
Reduce shade and keep the roof drying. Trim limbs that overhang it, keep gutters clear so water does not back up at the edge, and have the roof looked at every so often. These steps change the conditions growth needs, which cleaning alone does not.
Limbs close to the roof drop debris and keep slopes shaded and damp. Cutting them back lets sunlight and air reach the shingles, and the shadier slope is where regrowth usually starts again. Tree work is its own trade, and a limb over a roof is a job for someone equipped for it.
Clear gutters matter too, because water backing up at the eave keeps the lowest shingles wet. Ask the contractor what the roof looks like once clean, and what, if anything, they would watch for over the next year.
