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Mould and Lichen

By: Hsin-Yi Cohen BSc, MA, MSt - Updated: 22 Sep 2012 | comments*Discuss
Moss Growth Lichen Growth Algae Growth

A common but unpleasant sight is algae, moss or lichen growing on your house, whether it is the walls, the roof or even any decking. Since fungi and lichen can use almost any material for food, they are able to colonise several areas of your home while their weather-proof spores can be easily distributed across long distances, meaning that new colonies can be set up quickly.

Moss growth is common in areas that are shaded and suffer damp, cool weather conditions. This is why shaded areas of the roof are often overgrown with moss and lichen. The material of the roof or wall can also make a difference – for example, many roof materials nowadays incorporate fungicides or algaecides. Parts of the roof that are made of galvanised metal, copper or aluminium are usually unaffected.

Is It Really a Problem?

In a word, yes. Moss or lichen on the roof retains moisture and keeps the roof “wet” longer, thus increasing deterioration and reducing longevity of the roof material. If you live in a climate where there are freezing conditions, moss and lichen can cause faster frost damage and cracking, whilst even in milder climates, the moss and lichen can penetrate and separate the roof materials and cause the roof shingles to wear out faster.

Lichen causes less problems than moss and, as the cleaning process for lichen can be quite aggressive and damaging to the roof materials, you may consider leaving lichen if it is not too much of a problem yet. Similarly, algae on the roof is less destructive than even lichen, although it can be an early indicator of an area that will later develop a lichen and moss problem.

Removing It

Make sure first of all that your roof is sturdy and in good condition. If the shingles are worn out or fragile, it will be impossible to clean the roof without damaging it. In this case, it is better to replace with a new roof. In many cases, if you are unsure, it may be best to consult a professional. It is possible to hire pressure cleaners yourself to clean the roof but this should only be considered if you are fairly confident about what you’re doing and the roof is in a reliable condition.

It is best to try and remove moss, lichen and other growths from your roof surface through GENTLE cleaning, using a soft brush or a power washer. But take extreme care that you are not breaking or loosening any shingles, or destroying the protective mineral granules from the shingle surface. It is also possible to use chemicals to “kill” the growths – however, these have the disadvantage of leaving the moss, lichen and other growths in place where they still hold on to moisture and cause roof damage. In addition, the chemicals can also damage roof materials and in many cases, cause contamination in the environment.

Prevent It

Since these growths thrive in damp and shade, it is best to prevent them by keeping the roof clean and uncovered as much as possible. Therefore, trim back any trees that overhang the roof and remove any organic debris like leaves, twigs and pine needles, regularly. Copper strips installed along the ridge of an existing roof can be very effective as it will slowly kill off any moss or lichen growing there when the rainwater washes over the metal and down the roof surface.

A bit of forward planning may be the best prevention so when you are considering replacing your roof, discuss the option of using chemically-treated roof shingles which are resistant to moss, lichens and algae growth, with your roofer.

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I read the entry for algae, lichen and moss with interest. However we have another factor which does not seem to be covered. We have a black lichen growing on our roof but we harvest the roof water as a our sole source of supply.Is the black lichen affecting the quality of our drinking water and if so how should we treat it? Regards
Lumpy - 1-Jun-11 @ 7:43 AM
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