What the black streaks actually are
Almost always a cyanobacteria called Gloeocapsa magma. It feeds on the limestone filler in concrete tiles and the moisture trapped in the porous surface.
It starts on the north-facing pitch (less sun, more damp) and spreads downhill with rainwater runoff — that's why the streaks always point down.
It is NOT mould, soot, or pollution. It's a biological organism with roots that hold onto the tile surface. That's why a pressure wash blasts the top layer off but it grows straight back.
Methods that don't work (or make it worse)
Pressure washing — strips the surface coating, exposes the aggregate, accelerates tile failure. Streaks back within 12 months on a damaged surface that's now more porous than before.
Bleach from a garden sprayer — kills surface growth temporarily but runs off into plants and gutters, and rinses away before doing the spore-deep work.
Roof tablets / zinc strips — preventative only. They do nothing for established streaks and take 2+ years to show any effect on new growth.
Manual scrubbing — slow, dangerous on a pitched roof, and the algae is back within a season because the spores live in the tile pores, not on the surface.
The method that actually works
Soft-wash with a sodium hypochlorite-based biocide blend. Low pressure, long dwell time, kills the organism at the spore level.
Rinse runs clear because the biocide dissolves the dead algae as it kills it — there's no scrubbing required.
A second biocide pass after the rinse seals in long-term protection. Streaks stay gone for 4–6 years on most UK roofs.
Done from a roof ladder or scaffold with proper plant protection. Pets and shrubs are masked, runoff is controlled.
When to deal with it
Cosmetically the streaks are harmless for a year or two. Structurally they hold moisture against the tile and accelerate moss growth — by year 3–4 you usually have visible moss as well.
Best time to treat is spring or autumn. Avoid summer (biocide dries before it can dwell) and deep winter (won't work below 5°C).