Lichen, Algae or Moss? Identifying What Is Growing on Your Cambridge Roof

Look up at almost any roof in Cambridge and you will spot some kind of growth clinging to the tiles. It might be a dark streak, a spongy green cushion or a crusty patch that looks almost painted on. Most people lump it all together as moss, but there are actually three different culprits at work, and they are not the same thing at all.

Knowing which one you are dealing with is genuinely useful. It tells you something about the condition of your roof, how urgent the situation is, and what a proper clean will need to tackle. In this guide we walk through algae, moss and lichen, how to tell them apart, and what each means for a home in our damp corner of the country.

The quick answer: algae shows up as black streaks or a thin green film, moss forms thick spongy green cushions, and lichen appears as flat crusty patches in grey, white or yellow. All three thrive in the damp Cambridge climate, and all three are best removed with soft washing, which treats the growth at the root without harming your tiles.

1. Why It Helps to Know What You Are Looking At

It is tempting to treat everything green or black on a roof as the same problem, but the three main types of growth behave quite differently. Some are mainly a cosmetic nuisance, while others actively hold moisture against your tiles and can, over time, contribute to real damage. Being able to identify them helps you judge how worried to be.

Cambridge is something of a paradise for all three, sadly. The city sits low in the fens, the air is often damp, and mist off the Cam lingers over the rooftops on many mornings. Add in the mature tree lined streets that keep so many roofs in shade, and you have close to perfect conditions for growth of every kind. That is why local roofs so often show a mixture of all three at once.

2. Algae: The Black Streaks and Green Film

Algae is usually the first thing to appear and the most widespread. It shows up as a thin green film across the tiles or, more commonly, as dark grey and black streaks running down the roof. Those streaks are often mistaken for dirt or general weathering, but they are actually a living organism feeding on moisture and the surface of the tiles.

On its own, algae is more of a cosmetic problem than a structural one, but it should not be ignored. It makes a roof look dirty and neglected, and it thrives in exactly the damp, shaded conditions Cambridge offers in abundance. Have you noticed dark tiger stripes creeping down a north facing roof slope? That is almost always algae, and it tends to spread if it is left alone.

3. Moss: The Thick Green Cushions

Moss is the one most people picture, and the one that causes the most concern. It forms thick, spongy green cushions, usually starting in the gaps between tiles and along the shaded lower edges before spreading upwards. Unlike algae, moss has real bulk, and it holds a surprising amount of water like a sponge.

That water retention is what makes moss more than just an eyesore. Sitting damp against the tiles, it can work its way under edges, keep the surface permanently wet, and shed clumps that block gutters and downpipes. In freezing weather the trapped moisture can expand and stress the tiles. On the low lying, moisture holding ground around Cambridge, moss is especially common on older roofs and any slope that rarely sees direct sun.

4. Lichen: The Crusty Grey and Yellow Patches

Lichen is the odd one out and the most stubborn of the three. Rather than a film or a cushion, it appears as flat, crusty patches in shades of grey, white, pale green and yellow, almost as though someone has flicked paint at the tiles. It is a curious organism, a partnership of fungus and algae, and it grips onto surfaces remarkably tightly.

Because lichen bonds so firmly to the tile, it is the hardest of the three to shift and the most likely to leave marks if handled roughly. This is exactly why a gentle approach matters so much. Interestingly, lichen tends to favour cleaner air, so its presence is not a sign of pollution. What it does signal is a roof that has been left undisturbed for a good while, which is common on the quieter, established streets across the city.

5. What Each One Means for Your Roof

Taken together, these three tell a story about your roof. A light film of algae suggests growth is just getting started and the roof would benefit from attention before things progress. Thick moss points to a roof that has been damp and shaded for years and is now at risk of blocked gutters and trapped moisture. Established lichen usually means the roof has gone a long time without a clean.

None of these is a reason to panic, but none should be left indefinitely either. The common thread is moisture, and the longer any of them sits on the tiles, the more work it takes to remove and the greater the chance of knock on problems. Catching growth while it is still light always makes for an easier, cheaper job than waiting until the roof is completely covered.

6. How Soft Washing Deals With All Three

The good news is that you do not need a different method for each type of growth. Soft washing tackles algae, moss and lichen alike, using low pressure water and a biocide rather than a harsh high pressure blast. The biocide is the clever part, because it kills the growth at the root and keeps working after we have finished, slowing down how quickly it returns.

This gentle approach is particularly important for stubborn lichen and for older clay tiles and slate, which a pressure washer could easily damage. Rather than scouring the surface and risking cracked or loosened tiles, soft washing lets the treatment do the work. With over fifteen years of experience on Cambridge roofs, we know how to read the mix of growth on a roof and treat it thoroughly without causing harm.

• Algae: thin green film or dark black streaks, mainly cosmetic but spreads if left and thrives in shade and damp.
• Moss: thick spongy green cushions that hold water, block gutters and can work under tiles over time.
• Lichen: flat crusty grey, white or yellow patches that grip tightly and are the most stubborn to remove.
• The fix: soft washing with a biocide treats all three at the root without the risks of high pressure jet washing.

Frequently Asked Questions

Is moss worse for my roof than algae?
Generally yes. Algae is mostly a cosmetic issue, while moss holds moisture against the tiles, blocks gutters and can lift tile edges over time, so it tends to cause more practical problems if left.

Why is lichen so hard to remove?
Lichen bonds very tightly to the tile surface, far more so than algae or moss. That is why it needs a gentle biocide treatment and patience rather than aggressive scrubbing, which can damage the tiles.

Does having lichen mean my roof is old?
Not necessarily old, but usually undisturbed. Lichen takes a long time to establish, so its presence often simply means the roof has gone many years without a clean rather than that it is failing.

Can I just scrape the growth off myself?
It is not advisable. Scraping can crack tiles, and without treating the root the growth quickly returns. Working at height is also risky. A professional soft wash is safer and far more effective.

Why does my Cambridge roof grow so much of all three?
The low, damp fenland setting, frequent morning mist off the Cam and shade from mature trees create ideal conditions. Local roofs often carry algae, moss and lichen together, which is entirely normal for the area.

Not sure what is growing on your roof?
Ben's Roof Cleaning brings over 15 years of experience in safe, effective roof cleaning across Cambridge and the surrounding area. Get in touch or call 01223 320662 for a free, no-obligation quote.

Know the growth, treat it right

Whether it is streaky algae, cushiony moss or crusty lichen, all three thrive on Cambridge roofs thanks to our damp, shaded, low lying setting. The type of growth tells you how long the roof has been neglected and how much work it will take to put right, but the solution is the same gentle one in every case. A professional soft wash clears the lot at the root and leaves your tiles protected for years to come.