Green algae
Is green algae on a roof the same as moss?
The short answer
No. Algae is a thin, slimy or powdery green film that lies flat on the surface, while moss is a plant with tiny stems that stands up from the roof and holds water. Both like damp, shaded tiles, and both are treated in much the same way. Algae does less physical harm than moss, but it keeps the surface wet and can be the first stage before moss moves in.
A green tinge on the tiles and a green cushion in the valleys can look alike from the pavement, and they are not the same thing.
How to tell algae from moss
Look at the texture. Algae lies flat, like a stain or a smear, and can feel slippery when wet and dusty when dry. It spreads in patches and streaks and rarely has any height. Moss grows in a cushion with tiny stems, has some depth and breaks into clumps when it dries. Lichen, a third common growth, forms crusty rings in grey, white or orange, and sits tight to the tile instead of standing up from it. Rub a little between finger and thumb and algae smears while moss crumbles, but a camera is the safer way to look when the roof is out of reach.
From the ground the colour can mislead, as a thick film of algae and a thin layer of moss both look green. A phone photograph, zoomed in, usually settles it. If you are unsure, send us what you have. Our answers on lichen on a roof and black streaks on roof tiles cover the other growths you may be looking at.
Why a film forms on a roof at all
Algae needs the same things as moss: moisture, light and a surface that gives it something to hold. Roof tiles are slightly rough, and a layer of dust, pollen and leaf residue gives algae something to feed on. A slope that stays damp, or that sits in the shade of a tree or a taller building, makes the ideal setting.
It tends to show first along the lower courses of a slope and on the sides that get least sun. It can spread across a roof in a season or two, then settle into a pattern of green streaks. Nothing about this reflects badly on whoever laid the roof. It is simply what damp surfaces do, and a roof in a sheltered garden will show it sooner than one on an open plot, where the wind dries the tiles faster. A cleaned roof is not immune either, because the next season's pollen and dust start the cycle again, which is why the pattern tends to repeat.
Does it do any harm?
Algae is mostly a matter of appearance, and the damage to a sound roof is slow. The film holds moisture against the surface, which gives moss and lichen a start, and a wet surface takes longer to dry after rain. On some coverings the growth can slowly mark the face of the tile and leave a stain that stays after the film has gone.
The bigger problem is slipperiness. A roof with algae on it is slick underfoot, so anyone working on it, including the person up there with a ladder, takes a greater risk. A film also hides the true condition of the tiles, so cracks and slipped pieces are harder to spot. That is the main reason to have it looked at: not because the roof is failing, but because a clean surface is far easier to inspect. Algae washed down by rain also leaves a green line along the back of the gutter and the face of the fascia, which is worth wiping away when you next look.
Should it be treated before it turns to moss?
There is no emergency. A light film can be left for a while without harm, and many homeowners choose to treat it when the colour starts to bother them or when the roof is next accessed for another reason. But treating early is easier than treating late, because a thin film responds to a gentle treatment without any need to lift growth first.
The method is the same family as for moss: a biocide suited to the covering, applied at low pressure and given time to work. There is no scraping, and no high-pressure jet. Our page on roof soft washing describes the method, and our guide to removing moss from a roof shows what happens if the growth has already thickened. The roof cleaning and moss removal page covers the rest.
What the survey photographs show you
We begin with a survey and photograph the roof so you can see which parts are algae, which are moss and where any damage is. A written quote follows with one figure for the treatment, the clearing of any gutters the growth has fallen into and the removal of waste from site.
If the film turns out to be purely cosmetic and the roof is sound, you will hear that plainly, and we will not recommend more than is needed. If you would rather wait a season, we will tell you what to watch for in the meantime. There is no obligation to act on a survey, and photographs of the roof as it stands today are handy to keep, whatever you decide to do about it. Two or three pictures, one wide and a couple close up, are enough for a first opinion.
Related questions
Algae questions
Is a green tinge on newly laid tiles a worry?
A new roof can show a light film within a season or two if the slope is damp and shaded. It is a surface growth and does not mean the tiles were faulty. A gentle treatment later will remove it.
Will treating the algae stop moss from arriving?
It lowers the chance for a while, because the food and damp that moss needs are reduced. It cannot change the shade and moisture around your roof, so growth may return in time, and cutting back overhanging branches helps.