Quick answer
Black streaks are often Gloeocapsa magma, an algae that feeds on limestone filler in shingles and some tiles. It spreads in humid, shaded conditions and is different from moss buildup.
What you need to know
Algae streaks are mainly cosmetic on many tile types but can hold moisture.
Treatment products target organic staining without unnecessary mechanical damage.
Improving airflow and trimming overhanging branches slows recurrence.
How David White can help
We assess each property individually before recommending work. For what causes black streaks on roofs, that means checking access, roof or gutter condition, and any risks to neighbouring properties.
You can send your postcode and photos for a remote quote in many cases. If the job needs a closer look, we will say so upfront rather than guessing on site.
What to expect on the day
Our team works safely from ladders, towers or MEWPs depending on the building. We protect gardens, paths and driveways where moss or debris may fall, and we clear up when the job is done.
We will confirm timing, access and parking before we arrive so the visit runs smoothly for you and your neighbours.
Next steps
Call 0330 056 7751 or use the contact form with your postcode, photos and a short description of the issue. We cover roof cleaning and gutter work across London, Surrey, Sussex and Kent.