What Is Actually Growing Up There
Black streaks are a blue-green algae, Gloeocapsa magma. It feeds partly on the limestone filler used in modern asphalt shingles and spreads by airborne spores, which is why it moves down a street one house at a time. Green moss needs standing moisture and colonizes north slopes, valleys, and anywhere debris holds water. Moss lifts shingle edges as it thickens, and lifted edges let wind and water under the course. Lichen shows up as flat crusty spots, roots into the shingle mat, and is the hardest of the three to remove cleanly.
Why Pressure Washing a Roof Is a Mistake
An asphalt shingle is a mat coated in asphalt and covered with ceramic granules. Those granules are the UV protection. Strip them and the asphalt below starts breaking down in sunlight. Roofing manufacturers are consistent on this: low pressure and appropriate cleaning solutions only. Many shingle warranties explicitly exclude damage from high-pressure cleaning. Soft washing kills the organism at the root, low-pressure rinse carries it off, and the roof keeps its granules.
See also: Soft Washing vs Pressure Washing · Gutter Cleaning Main Line PA
Roof Types on the Main Line
Asphalt architectural shingle is the default on postwar and newer construction in Paoli, Malvern, Blue Bell and Newtown Square — straightforward soft wash. Original slate is common on pre-1930 Haverford, Gladwyne and Wynnewood homes. The rule on slate is that you do not walk on it — a cracked slate is a leak and foot traffic is how most of them crack. Cedar shake is rarer and the most sensitive surface — gentle mixes, low volume, no aggressive dwell.
By Town
Gladwyne (19035): the heaviest moss loads we see — wooded acre-plus lots, north slopes that never fully dry. Haverford (19041): original slate under specimen-tree shade — no-foot-traffic work, ground or drone based. Wynnewood (19096): street trees sitting directly over the roofline so streaking is uniform across the block. Ardmore (19003): twins with shared roof planes — cleaning one half and not the other is visible from the street. Narberth (19072): steep Victorian roof pitches on tight borough lots. Conshohocken (19428): river humidity plus hillside elevations, the worst combination for drying time. Paoli (19301): three build eras means slate, asphalt and everything between within a few blocks. Malvern (19355): historic borough roofs and large modern township roofs, different jobs entirely. Newtown Square (19073): large roof areas on open lots, moderate growth, longer intervals. Blue Bell (19422): full-sun lots and the longest roof cleaning intervals in our service area.