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

You've noticed dark streaks running down your roof like something's been dripping, and no amount of rain seems to wash them away. Maybe they started on the shaded side and have crept across the tiles over a couple of years. It looks like dirt or staining, but it isn't, and knowing what it actually is helps you deal with it properly.

Those streaks are a living thing, a type of algae, and it's one of the most common sights on roofs right across Dundee. It's not harmful in the way moss can be, but it's stubborn, it spreads, and it makes a decent roof look tired and neglected. Here's what the black streaking really is and what to do about it.

The quick answer: those black streaks are algae, a living growth feeding on the roof surface, and they need a proper soft wash and treatment to clear, not just a rinse.

1. It's algae, not dirt

The first thing to understand is that you're not looking at grime or soot. The dark streaks are colonies of algae, a living organism that settles on the roof and slowly spreads. That's why scrubbing at one patch or waiting for the rain to shift it never works, because you're dealing with something that grows back.

This particular algae shows up as those tell-tale dark, downward streaks, following the way water runs off the tiles. On roofs from Stobswell to Broughty Ferry it's become a familiar look, and once you know it's a growth rather than a stain, the reason it keeps coming back makes a lot more sense.

2. Why it loves a Dundee roof

Algae needs moisture, and Dundee provides plenty of it. Damp air blowing in off the Firth of Tay, the haar rolling up from the North Sea and long grey spells where the roof barely dries all give it exactly the conditions it wants. A roof here is damp far more often than it's dry.

North-facing slopes and shaded sections are always the worst hit, since they hold moisture longest. You'll often see one side of a roof heavily streaked while the sunny side stays clearer. In a city with this much cloud and coastal damp, algae finds a foothold on almost any roof left untreated.

3. What it feeds on

Part of why modern tiles streak so readily comes down to what the algae eats. Many roof tiles contain mineral fillers, and the algae feeds on these along with the general grime and moisture on the surface. That's why it can settle in and thrive rather than just sitting there like ordinary dirt.

As it feeds and spreads, it works its way across the whole slope over time. On the mix of tile types you get across Dundee, from old slate to newer concrete on the post-war estates, the streaking takes hold wherever there's a damp, textured surface to cling to. Left alone, it only gets darker and more widespread.

4. Is it actually damaging the roof

Algae streaking is more of a cosmetic problem than a structural one, at least compared to thick moss. It doesn't lift tiles or hold water in the same heavy way, so a streaked roof isn't usually in immediate danger. The main cost is to the look of your home rather than the fabric of the roof.

That said, it's rarely alone. Where you see algae you often find moss and lichen creeping in too, especially in Dundee's damp climate, and that combination does start to cause problems. Clearing the algae is also a chance to deal with whatever else has taken hold on the roof at the same time.

5. How to actually get rid of it

A quick blast with a pressure washer might shift the surface streaks for a while, but it damages the tiles and the algae returns. The proper way to clear it is a low-pressure soft wash paired with a biocide treatment, which lifts the growth gently and kills off what's left at the source.

The treatment is the important part. Because algae is alive, clearing the visible streaks without treating the surface just leaves it to grow back within a season. On a Dundee roof, where the damp never really lets up, a soft wash and biocide together are what give you a properly clear roof that stays that way.

6. Keeping it from coming back

Once the algae is gone, a bit of upkeep keeps it from returning quickly. Keeping the gutters clear so water drains cleanly, trimming back branches that keep the roof shaded and damp, and having the surface treated all slow the regrowth considerably.

Regular attention beats leaving it until the streaks are dark and spread right across the roof. A roof that's checked and treated now and then stays on top of the algae rather than letting it win. Across Dundee's cloudy, coastal setting, that steady care is what keeps a roof looking cared for instead of neglected.

• Living growth: the black streaks are algae, not dirt, which is why they spread and keep coming back.
• Damp-loving: Dundee's coastal air and haar give algae the constant moisture it needs to thrive.
• Feeds on tiles: it lives off mineral fillers and grime, settling on almost any damp, textured surface.
• Mostly cosmetic: algae harms the look more than the structure, but it often comes with moss and lichen.
• Soft wash to clear: a low-pressure wash plus biocide removes it properly, unlike a damaging pressure blast.

Frequently Asked Questions

Will the black streaks wash off in the rain?
No, because they're a living algae rooted on the surface, not loose dirt. Rain actually helps them spread by keeping the roof damp, which is the opposite of what you'd hope.

Is the algae dangerous to my roof?
On its own it's mostly a cosmetic issue rather than a structural threat. The concern is that it usually appears alongside moss and lichen, which do cause more real damage over time.

Why is only one side of my roof streaked?
The shaded, north-facing slope stays damp longest, which is just what algae wants. The sunnier side dries out between showers, so it holds off the growth far better.

Can I remove it with bleach or a jet wash?
Harsh home methods and pressure washers tend to damage the tiles and only clear it briefly. A proper soft wash and biocide treatment removes it safely and keeps it away much longer.

How long before it comes back after cleaning?
With a treatment and clear gutters you get a good stretch of clear roof, often a few years. In Dundee's damp air some regrowth is inevitable, just far slower once the surface is treated.

Tired of the streaks down your roof?
Ben's Roof Cleaning handles roof cleaning, moss removal and gutter cleaning across Dundee and Scotland, with over 15 years of experience. Get in touch or call 01382 236629 for a free, no-obligation quote.

Clear the streaks and give your roof its colour back

Once you know those dark streaks are a living algae rather than plain dirt, the way to deal with them falls into place. A gentle soft wash and a biocide treatment clear the growth and hold it back, giving your roof its proper colour again. On Dundee's damp, cloudy streets, that's the difference between a roof that looks neglected and one that looks well kept.