The short answer
Black streaks on a roof are usually algae, a thin living growth that spreads down the slope as rain runs off. They stain more than they damage, but they hold moisture against the surface and often sit beside moss. Gentle treatment works better than force, because Marley advises against powerful jet washes, which can damage slate and tile.
The streaks follow the path water takes off the roof. They are worth treating, but how it is done counts for more than how fast.
What the streaks are
Look at the pattern. Dark bands run down the slope along the line the rain takes, often widest on the lower courses. That shape comes from a growth carried and fed by run-off, which is why the marks point downhill like the tracks left by a leaking pen. The growth is algae, a simple organism that settles on damp surfaces and spreads wherever water keeps them wet. It needs no roots and no soil, only moisture and a surface to cling to.
Are black streaks on a roof always algae?
Usually, yes. Soot, a stain below a lead flashing and old coatings can mark a roof too, and they look different. A stain below lead follows the metal, and soot sits around a chimney. If the streaks start near the top of a slope or an eaves line and fan out along it, algae is the likely cause.
Algae often sits alongside moss and lichen, and our answer on lichen on roof tiles covers one of the other growths. Moss behaves differently, because its roots lift the edges of tiles, while algae is a surface film that does not.
Does it damage the tiles?
Algae is a cosmetic problem at first, and a roof can carry a light film for years without leaking. The tile still sheds water. The growth changes how the surface behaves, though, because it holds moisture against it, and a damp tile is more exposed to frost.
Should the staining worry me?
Not in a panic, and not to be ignored either, because the streaks tell you the roof is staying damp. A light film on a sound roof is no emergency, and there is usually time to book a survey and choose the method calmly. Streaks that arrive with moss, cracked tiles or stained ceilings suggest the roof has more going on, so look in the loft for damp and in the gutters for grit and flakes.
Shaded slopes, north-facing slopes and those beneath trees stay wet longer and tend to grow more. Trimming back overhanging branches helps as much as any treatment, since a roof that is cleaned and then left under the same wet canopy will mark again. Our page on whether moss grows back after cleaning explains what to expect, and why a clean is maintenance rather than a one-off cure. Clear gutters matter too, because overflow keeps the lower courses wet and feeds new growth at the eaves, where streaks often begin.
Treating it gently
A soft approach protects the tile. Marley advises against powerful jet washes, which can damage the face of slate and tile and cut its working life. Force can also push water under the laps and into the loft, so a roof that looked tidy outside can start dripping inside a week later.
What does a gentle clean involve?
Soft washing applies a treatment at low pressure, lets it work, then rinses lightly, so the tile surface is left as it was and only the growth is lifted away. The product is chosen for the job. Under the Biocidal Products Regulation, products have to be used in line with their authorisation, and HSE tells householders to steer clear of biocides sold for trade use only. That is a good reason to leave the choice of product to whoever is doing the work.
Our page on soft wash roof cleaning describes the method in detail. Nesting birds and bats are a further check before work starts, because the law protects both, and a roofer asks about them at the survey.
When a clean is not enough
Cleaning takes the growth off. It does not mend a tile, seal a crack or replace a nail that has rusted through. If the survey shows cracked, flaking or slipped tiles, those are dealt with separately, either before or after the clean.
When does a stained roof need more than a wash?
When the surface is failing, the tiles are soft or the underlay has perished. Cleaning a roof that has little life left can show how little is holding it together, which is why we look before we treat. You see what we see, in pictures, before anything is agreed.
Access matters too. Working from a roof ladder, tower or scaffold is part of the plan, and the written price names it. Our roof cleaning and moss removal page lists the work in full. The survey is followed by a single written total, covering the method we judge right for your roof and the access it needs. If the survey turns up repairs, they are written into the same total, so you can decide on the whole job at once.
More on roof staining
Can I clean the streaks myself with a hose and brush?
Climbing a roof to scrub it is risky, and tiles crack underfoot. Spraying from the ground with a household product may not reach the growth, or may use something meant for professionals only. A roofer has the access and the right product, so it is safer to leave the job to them.
Will the streaks return after cleaning?
They can, particularly on shaded or damp slopes, because algae comes back wherever moisture lingers. Trimming overhanging branches and keeping gutters clear slows it. The survey tells you how quickly your own roof is likely to need attention again, given its aspect and its surroundings.
The full picture: Roof cleaning and moss removal in Taunton ยท More short answers