If you've noticed dark vertical streaks running down your roof — particularly on north-facing slopes — it's almost certainly algae rather than dirt. Here's what's causing it and what your options are.
Will black streaks damage my roof?
Mostly cosmetic, but heavy algae and moss hold moisture against the tile surface — over years this accelerates degradation, especially on concrete tiles.
Can I clean black streaks off myself?
Specialist soft-wash chemicals (sodium hypochlorite-based) work well. Never use a pressure washer — it strips the granular surface off tiles and shortens their life by years.
How much does roof cleaning cost?
Typical 3-bed semi roof clean: £350–£600 depending on access and treatment. Add £100–£200 if biocide treatment is wanted to slow regrowth.
Airborne algae that thrives on damp, shaded surfaces. Almost universal on UK roofs over 10 years old, especially north slopes.
Soft-wash with biocide solution; never high-pressure clean (damages tiles).
Crusty grey-green patches alongside the streaks. Feeds on the same limestone filler but bonds harder to the tile surface.
Soft-wash + biocide; manual removal of large clumps; treat to prevent regrowth.
Green cushiony growth, usually at tile laps and gutters. Holds water against the tile and can lift tiles over time.
Scrape & soft-wash; treat with biocide; clear gutters of moss debris.
Less common — narrower streaks emanating from a chimney rather than full slope coverage.
Chimney sweep, cap repair, and roof wash for affected area.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.