· by Michael Blair, Nashville's Pressure Washing
Most asphalt roofs in Middle Tennessee develop dark streaks on the north and shaded slopes within a decade, and older roofs under trees grow moss and lichen. The streaks are a blue-green algae that feeds on the limestone filler in the shingles and spreads in our humidity. Homeowners see a dirty roof and reach for the pressure washer. The pressure washer is the one tool that must not be used, because the thing it strips off the shingle, the granule layer, is what protects the roof from the sun.
What the streaks are
The dark streaking is Gloeocapsa magma, an algae with a dark protective coating that shows against the shingle. It arrives as airborne spores, takes hold where the roof stays damp longest, and spreads downhill with the rain. It does not eat the shingle directly, but a roof covered in it holds more moisture and heat, and the growth eventually loosens granules. Moss is worse: it roots into the shingle surface and lifts the edges.
Lichen, the flat, crusty patches, bonds to the granules and takes them with it if it is pulled off. All three come off with the right chemistry and patience; none of them should be scrubbed or blasted.
What pressure does to a shingle
Asphalt shingles are a fiberglass mat with asphalt on both sides and a layer of ceramic granules pressed into the top. The granules are the UV shield. A pressure washer strips them in sheets, and the bare asphalt underneath dries, cracks and fails within a few years. Pressure also drives water under the shingles and into the deck, and lifts the sealed edges so the next wind lifts the shingles. A roof that has been pressure washed looks clean for a season and ages a decade.
Manufacturers say so in the warranty terms, and an inspector or an adjuster who sees granule loss in a pattern of wand strokes knows exactly what happened.
How roofs are actually cleaned
Soft washing: a solution of sodium hypochlorite, a surfactant to help it cling, and water, applied at roughly the pressure of a garden hose and left to work. The algae dies within minutes and turns white; moss and lichen die and release over the following weeks and wash off in the rain. Nothing is scrubbed. The roof is rinsed at low pressure, and the runoff is managed so the solution does not reach plants and gutters undiluted.
The person doing it should be walking the roof as little as possible, or working from the edge and a ladder, and should be pre-wetting and rinsing the landscaping below. Ask what solution they use and at what strength, how they protect plants, and whether they rinse the gutters afterward. A company that answers with a pressure rating rather than a chemistry is the wrong company.
- Low-pressure application, no scrubbing, no wand strokes
- Sodium hypochlorite solution, not a general-purpose degreaser
- Plants pre-wetted, covered or rinsed, and runoff diverted
- Moss and lichen left to release, not pulled
Keeping it clean afterward
A soft-washed roof stays clean for a few years, longer on a sunny slope and shorter under trees. Zinc or copper strips installed along the ridge release ions with each rain that suppress new growth downslope. Trimming back branches that shade the roof and drop debris helps more than anything else. And when the roof is eventually replaced, algae-resistant shingles, which have copper-coated granules mixed in, are worth the small premium in this climate.
One caution: if the roof is old, brittle or already losing granules, cleaning it does not make it younger, and the moss coming off can take the last of the surface with it. A roofer’s opinion on the roof’s remaining life is worth having before paying to clean it.