Why Does Moss And Algae Form On Roofs

Almost no roof is free from the possibility of moss or algae growth. The problem is wind. It carries moss or algae spores and deposits them on a roof regardless of location.

Next time you’re out, look at the roofs of the homes and buildings you pass. Many will be sound and relatively clean. But you’ll also find a good number with moss or algae.

Moss and algae are inevitable in the UK climate. You can find them on the ground and on neglected outdoor surfaces such as tree stumps. However, they particularly thrive on roofs. They settle here and multiply undisturbed, unless you take action to remove them.

However, whether or not moss spores can live and spread on a roof depends on the environment. Frequent damp conditions help moss to develop and survive. A roof in the shade of a large tree also creates the type of climate moss responds well to.

Moss simply needs damp. It doesn't need earth in which to grow a root system. Other types of plant need roots to absorb water from the ground. Moss absorbs water through its leaves. A damp roof without too much sunlight is an ideal spot for moss to spread.

You can easily spot moss on a roof. It has small dense green leaves that stand out from the colour of most tiles.

Algae are also easy to see. But the algae on roofs are not what many people expect. They assume algae are always green like the thick greasy fronds you find in stagnant ponds.

If you lift green algae from a pond and place them in the sun, you’ll notice how they soon dry and die. The sun’s ultraviolet rays cause fatal damage.

Algae on roofs prefer shade but cannot always avoid sunlight. To cope, they've developed a defence mechanism.

Instead of remaining green, roof algae produce a pigment to turn them a dark blue or black. The pigment acts as a protective barrier against ultraviolet rays and enables the algae to survive.

Like moss, algae spores arrive on roofs carried by the wind. They stick to the surface of the tiles, and if the conditions are damp spread as far as they can reach.

As with moss, the algae draw nourishment from water. They dissolve the carbon dioxide in rain to produce the carbohydrates they require. The algae also use any phosphates and nitrates they can extract from rain.

Moss and algae form on a roof because the conditions suit them. If you have either or both on your roof, you should consider removing them. Over time, they can damage tiles, block gutters and cause dampness.

One approach is to use a water-based moss and algae remover. Once you spray it on to a roof it removes moss and algae effectively without staining your tiles. You can simply wait for the rain to remove any residue.

A water-based solution is also non-toxic. Any of the solution running off your roof won’t harm plants, fish or wildlife you have in your garden.