Direct answer: Black or dark roof streaks are commonly associated with blue-green algae, although moss, lichen, dirt, or another condition can look similar from the ground. Warm, humid conditions encourage algae discoloration. The roof should be inspected before treatment, and asphalt shingles should not be cleaned with high-pressure washing.
Answered by Eric Grooms, owner of Upper Cumberland Pressure Washing. Eric founded the Cookeville-based company in 2019 and serves residential and commercial properties across the Upper Cumberland.
Published: August 31, 2026
Last reviewed: August 31, 2026
From Eric’s firsthand job experience, added September 12, 2026.
What customers can expect from our roof cleaning
Eric Grooms: Customers often worry that we will use high pressure and damage their shingles. We do not pressure-wash asphalt-shingle roofs. We treat the affected areas, inspect the result, and rinse as needed for the roof and treatment. Manufacturer and product rinsing requirements still govern the work.
Why might some lichen remain when you leave?
In our experience, a roof usually looks much better by the end of the visit, but some brown areas or attached lichen may remain. Where appropriate for the treatment, subsequent rain gives remaining growth time to loosen and wash away. We often see improvement after a few good rains, although weather and roof conditions vary.
What if spots still remain?
Call us if the treatment has left spots that need attention. We handle follow-ups case by case and do not charge extra for needed treatment touch-ups. We often arrange those visits when we are working nearby; we do not promise a fixed return date or an unlimited maintenance program.
When would you recommend a roofer first?
If asphalt shingles are badly degraded and have extensive granule loss, we point that out and suggest a roofer’s assessment before the customer pays for cleaning. We do not want someone paying to clean a roof they may soon replace. Cleaning does not repair worn-out shingles.
How do you handle downspout runoff?
Depending on the drainage, we may bag downspouts to collect solution instead of allowing it to discharge into a flower bed. At yard-drain outlets we may use neutralizer as part of protecting the grass. We collect, neutralize, and dispose of captured runoff as appropriate for the products and site; neutralization alone does not establish that runoff is harmless or suitable for discharge anywhere.
Why the streaks appear
The Asphalt Roofing Manufacturers Association says algae discoloration occurs in many parts of the United States and is especially common in warm, humid conditions. It is easier to see on lighter-colored roofing.
GAF likewise notes that dark stains may be blue-green algae, moss, or lichen. A visual inspection helps distinguish ordinary staining from missing shingles, flashing problems, leaks, or other roof defects.
Should asphalt shingles be pressure washed?
No. ARMA warns that a pressure washer can cause granule loss and premature failure of an asphalt roofing system. It recommends gently applying an approved cleaning solution and using low-pressure rinsing where the shingle manufacturer calls for it.
Cleaning directions, mixture ratios, dwell time, and rinsing requirements can vary by manufacturer and roof condition. Landscaping, gutters, siding, and runoff areas also need protection.
Will cleaning permanently prevent the streaks?
No cleaning method can promise that algae discoloration will never return. ARMA notes that cleaning may be temporary and discoloration can recur. Tree cover, shade, debris, moisture, roof orientation, and the shingle product may influence recurrence.
Important limitation
Black streaks do not automatically prove algae, and cleaning does not repair leaks, deteriorated decking, lifted shingles, failed flashing, or storm damage. If the roof has damage or warranty concerns, consult the shingle manufacturer or a qualified roofing professional before cleaning.
Learn about our low-pressure roof-cleaning service or request an assessment. Browse more practical guidance in Eric Answers.