Take a walk into your local D.I.Y store and you will find a huge range of products aimed at killing moss and lichen from your roof surface. These products are usually sprayed on by the homeowner or roofing contractor and eventually eradicate the moss/lichen.
Whatever your view on the aesthetics of roof growth it might be useful to look at what harm if any is done by roof moss and lichen.
There are literally thousands of different types of Lichen but the one most encountered on roofs is Ascomycetes Foliose. Fungus and bacteria combine to make a symbiotic association resulting in the yellow, green coral like growth found on roofs. Lichen will survive in the most extreme conditions. Periods of extreme wet or drought are unlikely to have any effect.
As part of its growth cycle Lichen burrows into the roof surface. The lower cortex of the plant enters the roof substrate. This is where the roof damage begins. It will attach itself to paint and roof coverings eventually causing it to flake or blister. On chipped coated roofing tiles the lichen removes the coating and exposes the tile to corrosion.
If the growth is allowed to go unchecked on interlocking tiles there is a danger the channels of the roofing can become clogged causing leaks.
Furthermore, roof moss prevents the roof covering from drying out. This will deteriorate the paint on metal roofs and eventually make concrete and clay tiles more porous. The problem is made worse by frosts and cold weather conditions. The moss/lichen holds water which expands on freezing which can blister the paint. In extreme cases it will crack roof tiles or cause extreme pitting.
Eradication of moss can be achieved in a number of ways. If the moss or Lichen has taken a firm hold on the roof substrate, mechanical removal with a brush is not recommended. It is far better to kill off the growth first before attempting removal. If you try and remove deeply bedded Lichen you may cause more damage to the roofing.
The majority of moss killers contain Sodium hypochlorite. These solutions are usually mixed at 5 times the strength of household bleach. They work fine provided the moss is completely soaked in the solution. Optimum results will be had if it is applied during a dry spell. Even better results can be achieved if the moss killer is worked in using a soft brush.
Extreme care should be taken when handling chemicals and it is important to follow the manufacturer’s instructions. Consideration should also be given to the effects on the local environment. It is wise precaution to collect the solution at the bottom of the down pipe/guttering. The chemical can then be disposed of safely, or at least diluted to a safer level.
A slower but more permanent solution is provided by the use of copper or Zinc strips placed at intervals on the roof surface. As rain runs over the metal strips a fungicidal wash covers the roof area. The larger the area of exposed copper, the better the results. The strips need to be placed at intervals down the roof to ensure sufficient fungicidal wash covers the roof. For an average sized house one length along the ridge with a second half way down the roof is sufficient to keep the lichen down. After a few years the copper will gain a green patina and the fungicidal properties will be reduced. By scrubbing this patina away the copper can be “reactivated”.
These fungicidal metal strips are only suitable for concrete, clay or shingle roofs. They should not be used with metal roofing or spouting. The solution will cause corrosion if allowed to run onto metal surfaces.
If you would like to prolong the life of your roofing it really is important to prevent damage from roof growth. Any money spent on eradication will easily be recouped by the extended life of your roof.