Those Black Streaks on Your Roof: What the Algae Really Is

You've spotted them by now: those dark streaks running down your roof like something spilled from the ridge and never washed off. They tend to show up worst on the shaded side, and no amount of rain seems to shift them.

Most people assume it's dirt, or staining coming out of the tiles themselves. It isn't. Those black streaks are alive, and knowing what they actually are tells you exactly why scrubbing does nothing and what really clears them.

The quick answer: Those black streaks are a hardy algae that feeds on your roof and spreads in damp, shaded conditions, and it needs a biocide treatment to kill it rather than a quick scrub or a rinse.

1. What those black streaks actually are

The dark stains are a type of algae, a tough little organism that clings to the surface of your tiles and slowly feeds on the mineral content and the airborne grime that settles there. It starts as a faint shadow and thickens into those unmistakable vertical streaks as it multiplies.

Because it's living rather than just dirt, it grows back once it takes hold. That's why you can hose the roof down or wait for a heavy Leicester downpour and the streaks stay exactly where they were. You're not looking at a stain, you're looking at a colony that has made itself at home.

2. Why Leicester roofs get them so easily

Algae loves damp, and the East Midlands gives it plenty. While Leicester sits on the drier side of the country compared with the west, the moisture rising off the Soar valley keeps roofs damp enough for algae to settle in, especially where the sun never quite reaches to dry things out.

Older housing helps it along too. The porous surface of aged clay tiles and weathered slate on Highfields and Aylestone terraces holds moisture far longer than a smooth new roof, giving the algae the steady supply of damp it needs to keep spreading season after season.

3. How the algae spreads across your roof

Algae travels by tiny spores carried on the wind, which is why you'll often see neighbouring roofs streaking up around the same time. One colony quietly seeds the next, and a whole terrace can end up wearing the same dark stripes within a few seasons.

Once it lands, gravity does the rest. The streaks run downward because spores and dead growth wash down the slope with every shower, spreading the colony as they go. That downward smear is the real giveaway that you're dealing with algae and not simple surface dirt.

4. Why it always looks worse on the north side

Take a walk around your house and you'll almost certainly find the streaks are heaviest on the north-facing slope. That side gets the least sun, so it stays damp for hours longer after rain, and lingering damp is exactly what the algae is after.

On Leicester's tightly packed terraces, shade from neighbouring rooflines and the mature trees in areas like Clarendon Park makes this worse still. Any part of the roof that stays cool and wet becomes prime territory, while the sunnier southern slope often stays comparatively clear.

5. Why pressure washing is the wrong fix

It's tempting to think a powerful jet will blast the streaks straight off, and it might even seem to work for a moment. The trouble is that high pressure strips the surface off older tiles and drives water underneath, and it never kills the algae at the root, so the colony simply returns.

Worse, forcing the growth off with pressure can leave the tile surface rougher and more porous than before, which actually helps the next colony grab hold faster. On the aged slate common across Leicester, that kind of damage is both expensive and permanent.

6. How soft washing actually clears it

The method that works is a low-pressure soft wash with a biocide. The treatment kills the algae where it lives rather than just knocking the top layer off, so the colony dies and then weathers away naturally over the following weeks without any harsh scrubbing.

Because the algae is dead rather than merely disturbed, the results last far longer, and a follow-up treatment keeps it from coming straight back. For a damp, shaded Leicester roof, that's the difference between a real fix and a job you'll be repeating next year.

• It's alive: The streaks are algae, not dirt, so they regrow if the roof is only rinsed.
• Damp fuels it: Soar valley moisture and shade keep roofs wet enough for it to thrive.
• It spreads: Airborne spores seed nearby roofs and wash down the slope.
• North side first: Shaded slopes stay damp longest and streak the worst.
• Biocide clears it: A soft wash kills the algae at the root for a lasting result.

Frequently Asked Questions

Are the black streaks damaging my roof?
Over time, yes. The algae holds moisture against the tiles and can work into the surface, and it often shares the roof with moss and lichen that cause their own problems.

Will the streaks wash off in the rain?
No. Rain actually helps the algae by keeping the roof damp, and it spreads spores down the slope rather than rinsing the colony away.

Is it the same thing as moss?
Not quite. Algae is the flat, dark staining, while moss is the raised green cushion. They often appear together on damp Leicester roofs, and both respond to a proper soft wash.

How long before the streaks come back after cleaning?
A biocide soft wash keeps a roof clear for a few years, though a heavily shaded slope near the Soar valley may start to shadow again sooner than a sunny one.

Can I treat the algae myself?
Shop-bought sprays rarely reach the whole colony, and working at height is risky. A professional soft wash treats the entire roof safely and gets right to the root of the growth.

Tired of those dark streaks on your roof?
Ben's Roof Cleaning offers roof cleaning, moss removal and gutter cleaning across Leicester and Leicestershire, with over 15 years of experience. Get in touch or call 0116 216 7398 for a free, no-obligation quote.

Those Streaks Are Telling You Something

The dark stripes on your roof aren't just an eyesore, they're a sign that damp has settled in and something living is making the most of it. Left alone, algae keeps spreading and often invites moss and lichen to join it, so the roof only gets grubbier and holds more moisture.

Clearing it properly with a soft wash gives you back a roof that looks its age instead of decades older. On a shaded Leicester slope, staying ahead of the algae is well worth the effort.