Roof growths do not damage your roof. Furthermore they would be unlikely to make any significant difference to water absorption through the tile, even though they obviously will hold more water against the tile for a longer period.
How to remove growths from your roof
• Moulds and lichens can be removed by the home owner with copper sulphate ("bluestone") from a hardware store (2 kg to 45 litres of water to treat an average house).
• Lichens may also be removed by household bleach, but this does not have a long-term eradication effect, so it’s not as suitable.
• In both cases above, the solutions will corrode the coating on gutters. This can be minimised by running water through the gutters during the operation, but light corrosion could occur subsequently as the growths may retain some corrosive solution whilst dying and disperse it in rainwater run-off. In both cases, disconnect gutters from any rainwater tanks until after the first heavy rainfall to avoid poisoning.
• Be warned that whilst cleaning the tiles you may crack them, causing leaks. This may also occur if employing roof cleaners.
What is roof rejuvenation?
Roof “rejuventaors” clean, paint and re-cap tiles either in whole or in part. They clean the growths off roofs with high-pressure water sprays and they sometimes apply a fungicide paint or coating to inhibit regrowth. Home owners can obtain the same kind of sprayers roof rejuvenators use from hire companies.
Tips on roof rejuventation
• Sometimes these high pressure sprays can blast holes in your gutters and valley flashings if they are in the process of rusting out.
• You might consider repainting the roof for cosmetic/resale purposes, but there is no substance in the claim that this "waterproofs" or extends the life of the roof.
• Loss of the glaze on a tile does not make it significantly more porous. A tile which has lost its glaze can only absorb roughly 5% more water than a glazed tile. This extra weight of water is unlikely to add much more stress to the roof frame.
• If tiles are cracked during repainting, re-capping or cleaning, then the process may actually lead to more leaks. Removing old mortar and re-bedding ridge tiles in new mortar will, however, extend the life of a roof which is leaking from these locations.
• It is best to obtain a written statement from roof rejuvenators as to their obligation to replace tiles cracked whilst "rejuvenating". Some also offer guarantees on other aspects of the job. However, you should investigate the company behind the guarantee, as some have been known to go out of business before the guarantee expiry period.
• Since rejuvenating a roof can cost up to two thirds that of a new roof, which will be more resilient, you should carefully consider the benefits of rejuvenation versus re-roofing.
To find out more about re-roofing, read our factsheet on how to choose materials for roof replacements.