Moss, Algae or Lichen? How to Identify What's on Your Roof

Moss: the one that needs removing

Moss is a true plant with visible structure. On a roof it forms raised, spongy cushions, usually bright to dark green, and often several centimetres thick along the bottom edges of tiles, in valleys and on north-facing slopes. Pressed with a finger it compresses and holds water like a sponge, which is precisely the problem.

Because moss is three-dimensional and holds moisture, it is the growth that genuinely affects a roof. It lifts tile edges, blocks the drainage paths a tiled roof relies on, and breaks off in clumps that fill gutters. Moss is removed by hand and then treated with a biocide to prevent it re-establishing.

Algae: the one that treats away

Algae presents as flat staining rather than growth you can grip. It is typically dark green, grey or black, and it follows water flow paths down the tiles, often appearing as streaks below chimneys or vents. There is no depth to it — run a hand across and the surface still feels like tile.

Algae causes no structural damage. It is a purely cosmetic issue, and because it is a surface film it responds extremely well to soft washing with a biocide, with no scraping required. This is the cheapest of the three problems to resolve.

Lichen: the stubborn one

Lichen is a symbiotic organism of fungus and algae, and it looks quite different: crusty, flat-to-slightly-raised patches, usually pale grey, white, sage green or occasionally bright yellow-orange, with a distinctive rosette or splash shape. It bonds chemically to the tile surface rather than simply sitting on it.

Lichen grows extremely slowly and does little harm. Removing it forcibly is what causes damage, because the bond is strong enough that scraping takes surface material with it. Biocide treatment will kill it and it will weather off gradually over one to three years. On a sound roof, patience is genuinely the correct advice.

• Moss: raised, spongy, green, compresses when pressed
• Algae: flat, streaky, black or dark green, no depth
• Lichen: crusty, circular, pale grey or yellow, bonded hard

Why they appear where they do

All three need moisture and reduced UV exposure, which is why north-facing slopes and elevations shaded by trees or neighbouring buildings are always worst. Overhanging branches make it worse still, because they drip, drop organic matter and prevent the roof drying between rain.

Porous surfaces hold more water, so older concrete tiles that have lost their factory finish colonise faster than newer ones or glazed clay. Anywhere water sits — behind a chimney, in a valley, along the bottom two courses — will grow the most.

The right response to each

For moss: manual removal followed by biocide. For algae: soft wash with biocide, no scraping. For lichen: biocide and time, and accept that a sound roof with lichen is not a roof with a problem.

In practice most British roofs carry all three in different proportions, which is why a competent survey looks at each elevation separately rather than pricing the whole roof off one photograph. It is entirely normal for one slope to need scraping while another needs nothing more than treating.

The bottom line

A useful shortcut when you are standing in the garden: if you could grab it, it is moss and it needs removing. If it is flat and streaky, it is algae and it will treat away. If it is crusty and circular and firmly stuck, it is lichen and it can usually be left to weather off after treatment. That three-second test gets most homeowners to the right answer without any equipment at all.

If you would rather be certain, send us a clear photograph of the worst elevation. Identification from a decent image is straightforward, and it means any quote you receive is based on what is actually growing rather than a generic assumption. It also frequently saves money, because roofs that look alarming from the road turn out to be algae far more often than people expect.

Frequently asked questions