Most people say "moss" about anything growing on a roof. The marks on a roof around Basingstoke are rarely just moss. A soft green cushion in the gaps is moss. A dry grey or orange crust stuck to the tile face is lichen. A dark stain that runs down the slope like soot is algae or, more exactly, a colony of bacteria. They need different things to thrive, they leave different marks, and they call for different judgement about whether to act. This guide helps you work out what is on your roof from the ground, and what that means for the tiles.
Four things that get called moss
Four growths turn up on UK roofs. It helps to meet them one by one.
Moss
Moss is a plant with no true roots, only fine anchors called rhizoids. It forms soft green or brown cushions, usually in the gaps between tiles, along the lower edge of a slope and in the channels of interlocking tiles. It holds water like a sponge and spreads where the roof stays damp. It is the growth most likely to affect how a roof works, because it can block laps, lift tiles and fill gutters when it falls.
Lichen
Lichen is a partnership between a fungus and an alga living together. It grows as flat, crusty patches, small leafy rosettes or tiny branching tufts, and it comes in grey, green, white, yellow and orange. It clings to the face of the tile rather than the gaps, and it grows slowly. A patch that looks fixed for years may only have grown a few millimetres. Our short answer on the orange growth on a roof deals with the commonest orange kind.
Algae
Algae are simple green organisms that coat a surface in a thin film. On a roof, the colour is often a green slime on damp tiles, mostly seen in shade and on north-facing slopes. They are easy to wipe off in the early stages and come back quickly.
The black streaks
Dark stains running down a roof in vertical streaks are generally caused by a bacterium, often called Gloeocapsa magma, which forms a dark film in the shaded, damp parts of a slope. The streaks run in the direction of the water, because the colony spreads where rain flows. They look like soot or mould, and they are not dirt. They stain more than they damage, and they spread more where the roof gets less sun.
How to tell which you have
Stand in the garden and look across at the roof in daylight. Use binoculars if you have them. Moss shows as thick, soft lumps in the joints and along the bottom edge. Lichen appears as flat, dry patches that sit on the tile face and do not lift away. Algae shows as a green tinge over a whole area of damp tile. Black streaks are long, dark, vertical stains, often more obvious on one slope than the other.
The side of the house matters. North-facing slopes stay damp longest, so they carry the most growth of every sort. Shaded roofs under trees stay damp too. Our short answer on why moss grows on one side of the roof explains the physics. A roof that is green on the north and clean on the south is behaving normally.
| Growth | What it looks like | Where it grows | Effect on the roof |
|---|---|---|---|
| Moss | Soft green or brown cushions | Laps, gaps, lower edge, valleys | Holds water, lifts tiles, blocks gutters |
| Lichen | Flat grey, green or orange crusts | Tile face, ridge, exposed edges | Mostly cosmetic, slow to spread |
| Algae | Thin green film | Damp, shaded tile | Makes tiles look dirty, keeps them wet |
| Black streaks | Dark vertical stains | Along the path of water down a slope | Stains the surface, can darken the roof |
What each one does to a roof
This is where honest advice matters, because the marketing around roof cleaning treats every growth as a threat. They are not equal.
Moss: the one to watch
Moss is the growth with consequences. It keeps tiles wet, which in a hard frost speeds up surface flaking on porous clay and concrete. In thick growth it can lift the tail of a tile and let wind-blown rain run behind it. When it dries and falls, or is raked down, it blocks gutters and downpipes. Our page on moss, gutters and drainage explains what that does. Moss is not usually the cause of a leak alone, but our answer on whether moss can cause a roof to leak sets out when it can contribute.
Lichen: slow and mostly cosmetic
Lichen is slow growing. On sound tiles it is generally a cosmetic matter and many owners are happy to leave it, especially on an old slate or clay roof where it adds to the look. Heavy, thick lichen can hold moisture against the tile, and some types etch the surface over decades. It is also hard to remove by force, because it grips the tile, and scraping it can scratch the surface more than the lichen ever did. Treatment is the gentler way.
Algae and black streaks: mostly appearance
These stain and darken the roof. A dark roof absorbs more heat and the staining can make a roof look neglected, which matters if you are selling. They do not generally damage the tiles themselves. On concrete tiles, a dark algal film can be a sign that the face is wearing, because a rougher surface holds growth more easily. The streaks are an indicator of shade and damp more than a defect. If appearance matters, our page on roof cleaning before selling a house explains what buyers notice.
Why growth takes hold
Four conditions favour every one of these growths: moisture, shade, a rough or porous surface and a supply of spores. The spores are in the air everywhere. Moisture and shade are the variable factors. A roof under overhanging trees, close to a wall that blocks the sun, or with a shallow pitch will hold water longer and grow more. Rough concrete and handmade clay absorb water, which feeds growth. Smooth glazed tiles and dense slate are less welcoming.
Other things add to it. Gutters that overflow keep the lower edge wet. A flat or low-pitched section next to a slope delivers water to the tiles below. Air quality plays a part, and some lichens only appear where the air is clean, so a heavy lichen cover is not necessarily a bad sign for the area.
What can be done
The approach depends on the growth, the material and what you want from the roof.
Leaving it alone is a real option for light lichen and algae on sound tiles, with an eye on the gutters. Hand removal of thick moss is the first step where it fills laps and valleys. A biocide treatment, applied low-pressure and left to work, kills moss, lichen and algae at the root and lets them release over weeks, and is the usual choice for black streaks. We explain how it works in roof biocide treatments. Pressure washing is the method to avoid, and our page on whether pressure washing damages roof tiles shows why. The broader comparison is in moss removal methods compared.
Zinc or copper strips along the ridge are another approach. Rain running over them carries metal ions down the slope, which discourage growth. They help on the area below the strip and not elsewhere, and their effect fades. See our answer on whether zinc strips stop roof moss for the limits.
No treatment stops regrowth for good. The spores return and the conditions that favoured them remain. A roof that is trimmed back from overhanging trees, has clear gutters and a good fall, and is treated again when growth reappears, stays clean longest. Our answer on how long before moss grows back gives a realistic expectation.
When a stain means something else
Not every dark mark on a roof is a living thing. A brown or rust-coloured stain below a metal flashing can be corrosion. A dark patch on tiles around a flue may be soot or condensate. Black marks on the ceiling inside, as opposed to the tiles, are mould from damp, which can be a leak or condensation. White powder on slate or lead is a different issue again. If you are not sure, a survey can look at the roof, the loft and the ceilings together. Our page on investigating damp and condensation in roofs explains how we separate the causes.
A seasonal view of growth
Growth on a roof has a rhythm. Moss and algae spread fastest through the wet months, from autumn to spring, when tiles seldom dry out and the sun sits low. In a dry summer, moss browns and shrinks, and a roof can look much better than it does in February, which is why a roof that seemed fine in August can disappoint in the new year. Lichen does not follow the seasons so closely and simply keeps expanding at its own slow rate. Black streaks darken in damp spells and fade a little in sun but do not disappear.
This matters for timing. A treatment applied in spring or early autumn, when the weather is mild and rain is not about to wash it off, has the best start. Our answer on the best time to remove roof moss sets out the reasoning, and whether a roof can be cleaned in winter covers the exceptions.
Roof type and pitch
Low-pitched roofs and the shallow slopes of older extensions hold water longer and grow more. A steep pitch sheds rain quickly and is less hospitable, though shade can cancel that advantage. Pantiles and interlocking concrete tiles have channels that trap debris and moisture, so moss gathers in them first. Flat roofs have their own growth and their own treatment, which our short answer on how to clean a flat roof explains separately.
How we approach it
We identify what is on the roof before suggesting anything. The free survey records the growth, the tile material, the state of the gutters and any damage, with photographs, so you can see the roof as we do. If the growth is lichen on sound tiles, we may tell you to leave it. If moss is lifting tiles or blocking drainage, we price the clean. The price you are given is one figure on paper, and anything we carry out is covered by our written ten-year guarantee on workmanship.
Start with our roof cleaning in Basingstoke page, or tell us what colour you are seeing when you call 01256 213956 or write on WhatsApp, and we will know what to look for when we arrive.
