Black streaks running down your roof tiles — what they actually are
Dark vertical streaking down a roof is one of the most common things people call us about, and it is almost always misdiagnosed as dirt, pollution or 'the tiles wearing out'.
It is biological. The streaks are colonies of airborne algae that establish on the tile surface and spread downwards with the run-off, which is why they always run vertically and always start at the ridge.
Why it looks the way it does
Algae land on the roof as airborne spores, take hold where moisture lingers, and then spread down the pitch every time it rains as the run-off carries cells with it. That is why the streaking is directional, why it is worse below ridge and hip lines, and why it appears on some slopes and not others on the same house.
Pale tile ranges show it earliest because of the contrast. Dark tiles carry exactly the same colonisation and just conceal it for longer.
Why scrubbing and pressure make it worse
- Pressure washing strips the weathered surface off concrete tiles, leaving a rougher, more porous face that re-colonises faster.
- Mechanical cleaning removes the visible cells but not the spore load, so the streaks return within a season or two.
- Force drives water under laps and into the roof space, which is a far worse problem than staining.
What actually works
Killing the organism rather than removing the stain. A low-pressure softwash with biocide treats the whole covering, including the areas that still look clean but carry the spore load that would reseed the rest. The roof then keeps lightening for weeks afterwards as the dead growth weathers off.
Get the streaking treated properly
Send a photo of the affected slope and we can usually tell you what you are dealing with before we quote.
Sky Scrapers Ltd · info@sky-scrapers.co.uk
Our own photographs
Images from Sky Scrapers Ltd work showing the methods described on this page.
Where this applies locally
Towns where this comes up most often on our patch:
- Softwashing & Biocide Treatment in Milton KeynesBuckinghamshire · MK1
- Softwashing & Biocide Treatment in BiggleswadeBedfordshire · SG18
- Softwashing & Biocide Treatment in St NeotsCambridgeshire · PE19
- Softwashing & Biocide Treatment in BicesterOxfordshire · OX25
- Softwashing & Biocide Treatment in Houghton RegisBedfordshire · LU5
- Crusty lichen on roof tiles and why it is harder to shift than mossCommon problem
- Why the north-facing slope is always the one covered in mossCommon problem
- New build roofsBuilding type
Questions we get asked
Are black streaks a sign my roof is failing?
No. It is surface colonisation by algae, not a structural problem. It affects appearance rather than the integrity of the covering.
Why is only one slope streaked?
Aspect and shade. The slope that dries slowest holds the moisture the algae need, so colonisation is uneven across the same roof.
Will the streaks disappear the day you finish?
Usually not entirely. Biocide keeps working after we leave and the dead growth weathers off over the following weeks.
Will they come back?
Eventually, everywhere does. How fast depends on shade, aspect and surrounding trees, which is what determines a sensible re-treatment interval.
Short answer
It is algae, not dirt. Pressure washing removes the stain and speeds up its return; low-pressure biocide treatment removes the organism and slows it down.