Black streaks on your roof: what they are and what they mean
Black streaks on roof tiles are biological growth — an algal film, mould and lichen colonising the overlaps where tiles hold moisture longest. They are not dirt, and they do not rinse off. Left alone, the film keeps water pressed against the surface and spreads season on season. The fix is a soft-wash treatment that kills the growth at the root and keeps working after the visit.
The streaks are alive
The dark stains running down a tiled roof are not soot, not pollution, not weathering. They are a living film — a colony of algae, mould and lichen that has settled on the tile surface and started feeding. The black colour is the organism itself, darkening as the colony matures. That is why the streaks look worse every year rather than fading the way ordinary grime does. A stain sits there. A colony grows.
It also explains why rain never fixes the problem. Rain is not a cleaning event for growth — it is a watering event. The film is anchored to the tile, drinking from every shower and from the dew that settles overnight. Hosing it from the ground does nothing either. You are irrigating it.
They follow the water, and the water follows the laps
Look at where the streaks start: almost always at the tile laps, the horizontal lines where one course overlaps the next. Those overlaps are the last part of the roof to dry after rain. Moisture lingers in the shadow of each lap, and the growth establishes exactly there, then bleeds down the face of the tile below. The streak pattern is a map of where your roof stays wet longest.
The colony spreads season on season. Each wet winter extends it along the laps and down new courses; each humid summer thickens it. Shaded and south-facing sections go first because they dry slowest. A roof that is lightly marked now does not stay lightly marked. Growth compounds — that is the one thing it does reliably.
What the streaks mean — and what they don't
The real cost is not the look. A mature film of lichen and mould works like a fixed sponge, holding moisture against the tile surface long after the weather has cleared. Concrete and terracotta both age faster when they never properly dry — the surface stays damp, the growth digs in further, and the cycle feeds itself. The streaks are a symptom of a roof that has stopped drying out.
What they do not mean is that your roof is failing. Streaked tiles are usually sound tiles with a biology problem. Be equally clear about the limits: a clean cannot fix cracked tiles, failed ridge pointing or a rusted valley. What it does is remove the growth and the camouflage — after a proper treatment, anything that genuinely needs a roofer is finally visible.
Blasting is the wrong answer
The tempting fix is a pressure washer, and it is the wrong one. High pressure shears the visible growth off but leaves the roots bonded to the tile, so the colony returns quickly — often darker, on a surface the lance has now scoured open. Worse, driving pressurised water up under the laps pushes moisture exactly where the roof is designed to keep it out.
Soft washing works the other way around. Low pressure, a treatment dosed for the surface, and patience. The solution kills the growth to the root, and then it keeps working after the visit — sun and rain carry the dead material off the roof over the following weeks. The roof you see a month later is cleaner than the one you saw the day we left. That is the treatment doing its job.
A firm number before anyone climbs a ladder
Roof cleaning is quoted per job, because roofs vary in every way that matters — size, pitch, how established the growth is, access, and the tile or coating itself. Schmick's instant quote weighs those factors and returns a firm, bookable price in about 60 seconds. It holds unless the place is materially different from what you told us. No site visit to schedule, no vague range to argue about later.
If the roof is streaked, the gutters below it rarely escaped — and bundling services on one visit saves 5–15%. Get the number, look at it, decide. The streaks will not spread while you spend the 60 seconds. They will if you spend another season.