· by Michael Blair, Nashville's Pressure Washing
Asphalt roofs in Middle Tennessee can develop dark streaks on their shaded slopes, and older roofs under trees can grow moss and lichen as well. The streaks are something growing on the shingles: CertainTeed’s page for its Landmark shingles puts black streaks down to airborne algae. Homeowners see a dirty roof and reach for the pressure washer. That is the one tool we will not put on a shingle roof, because what it can strip off the shingle, the granule layer, is the surface that faces the weather. It is why our roof cleaning service is a soft wash and nothing else.
What are the dark streaks on a shingle roof?
The dark streaking is that airborne algae, and it shows as stains running down the slope from the ridge. Moss and lichen are a separate matter. Moss is the soft green cushion that roots into the shingle surface, and lichen is the flat, crusty patches that look painted on.
We never pull or scrape either one off a roof. We kill the growth, leave it to loosen in its own time, and let the rain carry it 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
An asphalt shingle has a layer of granules pressed into its top surface, and that layer is what takes the sun and the rain. A pressure washer can strip those granules, drive water up under the shingle courses and lift edges that had sealed down. Shingles are one entry on a longer list, and our post on what pressure washing can permanently damage goes through the rest of the house.
Read the shingle warranty before anyone cleans the roof. CertainTeed’s Landmark page describes algae-streak warranty coverage ranging from 25 to 30 years and lifetime limited transferable warranties on most residential applications, so the terms on a roof like that are worth checking for anything they say about cleaning. If a company tells you pressure washing will not affect your warranty, ask them to show you where the warranty says so.
How is a shingle roof cleaned without pressure?
We soft wash: 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. Nothing is scrubbed, and the roof is rinsed at low pressure afterward. Why the same chemistry suits siding as well as shingles is laid out in our post on soft washing and when it is the right call.
Runoff is the part we plan most carefully. Metro Water Services’ stormwater page notes that stormwater receives no treatment before it is released into the environment, and the city’s pollution prevention page reminds residents that only rain belongs in the drain, citing Metro Code 15.64.205, which makes environmentally hazardous dumping illegal and subject to a fine. So we pre-wet and rinse the plantings below, keep the solution out of downspouts that empty toward the street, and flush the gutters before we leave. Wash water from commercial work raises its own questions, covered in our post on where wash water goes in Nashville.
Ask any company how its crew is kept from falling. OSHA’s fall protection standard for construction work requires a worker on a steep roof with unprotected sides and edges 6 feet or more above lower levels to be protected by guardrails with toeboards, safety nets or a personal fall arrest system. We work from the edge and a ladder wherever we can and walk the roof as little as possible. Ask, too, which chemical goes in the tank, how strong they mix it, and what they do about the beds below. 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 kept out of the storm drain
- Moss and lichen left to release, not pulled
Keeping it clean afterward
We don’t promise a number of years for how long a cleaned roof stays clean, because shade, overhanging trees and the direction a slope faces differ from one house to the next. Trimming back the branches that hang over the roof is the part within your control: more sun reaches the shingles and less debris piles up on them. When the roof is eventually replaced, ask for shingles sold as algae-resistant; CertainTeed, for one, says its StreakFighter technology protects against black streaks caused by airborne algae.
One caution: if the roof is old, brittle or already losing granules, cleaning it does not make it younger. A roofer’s opinion on the roof’s remaining life is worth having before paying to clean it. If the roof has good years left and the streaks are what bother you, ask us for a free estimate.
Sources
- CertainTeed, Landmark Series shingles — StreakFighter protects against black streaks caused by airborne algae; algae warranty coverage of 25 to 30 years; lifetime limited transferable warranties on most residential applications
- Metro Nashville Water Services, Stormwater — Stormwater receives no treatment before it is released into the environment
- Metro Nashville Water Services, Stormwater Pollution Prevention — Only rain belongs in the drain; Metro Code 15.64.205 makes environmentally hazardous dumping illegal and subject to a fine
- OSHA, 29 CFR 1926.501 Duty to have fall protection — Steep roofs, 1926.501(b)(11): fall protection for workers 6 feet or more above lower levels
FAQ
Roof Washing: Why It Is Never Done With Pressure — questions
Can you pressure wash an asphalt shingle roof?
We will not. Pressure can knock loose the granules that coat the shingle, push water under the courses and lift sealed edges. Whether it affects your shingle warranty depends on the warranty’s own terms, so read them before anyone cleans the roof. Roofs are cleaned by soft washing: a cleaning solution applied at low pressure and allowed to work.
What causes the black streaks on my roof?
Algae. CertainTeed, which makes algae-resistant shingles, describes the black streaks as caused by airborne algae. It comes off with a soft wash, and if the roof is due for replacement, shingles sold as algae-resistant are worth asking your roofer about.
How long does a roof wash last?
It varies with shade and tree cover, so we do not quote a fixed number of years. Trimming back branches that hang over the roof lets it dry and keeps debris off it between cleanings, and algae-resistant shingles are an option when the roof is replaced.
Is soft washing a roof safe?
It is the only way we clean a shingle roof, because the solution does the work at about garden-hose pressure and nothing gets scrubbed or blasted. The risks sit elsewhere: the beds underneath, the runoff and the crew. So we pre-wet and rinse the plantings and keep the solution out of downspouts that drain toward the street.