Algae and moss growth on asphalt shingle roofs is common in Boston's humid, shaded conditions — and the wrong cleaning method can cause more damage than the growth itself. Here's what's safe and what isn't.
The dark streaking visible on many Boston area roofs is Gloeocapsa Magma — a cyanobacteria that feeds on the limestone filler in asphalt shingles. It's primarily aesthetic in early stages but accelerates shingle degradation over time as the bacterial colonies damage the granule-adhesive bond. Boston's humidity and frequent shade from mature trees create ideal conditions for algae growth. North-facing and heavily shaded roof sections develop algae significantly faster than south-facing sections. Asphalt Roofing Manufacturers Association →
Asphalt shingle granules are held to the shingle mat by an asphalt adhesive. Pressure washing — even at relatively moderate pressures of 1,000–1,500 PSI — strips granules from shingles at a rate that significantly shortens roof life and voids manufacturer warranties. GAF and CertainTeed — the two largest shingle manufacturers with significant Boston market share — specifically exclude damage from improper cleaning methods including pressure washing. A pressure-washed roof may look clean briefly while having lost years of useful life from granule loss. GAF roofing installation requirements →
The Asphalt Roofing Manufacturers Association recommends soft washing: a sodium hypochlorite solution applied at low pressure (under 500 PSI), allowed to dwell on the roof surface for 15–20 minutes, and rinsed with low-pressure water. This approach kills algae and moss at the root without damaging granules. Results develop over days to weeks as dead biological material weathers away with rain. A well-executed soft wash on a Boston roof should be clean within 2–4 weeks as the dead algae washes off naturally. PWNA soft wash certification standards →
Moss — three-dimensional plant growth, not just staining — is a more serious problem than algae. Moss holds moisture against shingles, accelerating granule loss and, in severe cases, lifting shingles along their edges as the moss roots grow under them. Established moss requires physical removal in addition to biocide treatment — but physical removal must be done carefully, brushing with the shingle direction (never against), to avoid granule damage. In Boston, roofs with significant moss growth need professional treatment. Amateur attempts with stiff brushes and high pressure do more damage than the moss itself. CertainTeed roof maintenance guidelines →
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