Moss and Lichens are a common defect that is often noted by surveyors across the propertysurveying network. Below we explain the causes, the dangers and removal methods concerned with Moss and other growths upon different roofing materials.
Moss growth can begin with just a single spore that has been blown onto the roof or been carried or deposited by a bird. This settles into the dirt which the tiles have filtered out of rain water. The growth can be assisted by a number of factors including lack of sunlight, low pitched roofs and damp, which is why it is commonly noted to a greater extent on the north facing side of a roof, where direct sunlight is at a minimum.
Concrete tiles with sanded or granule facing attract the growth of moss because of the increased amount of dirt that is harboured in the grooves. Because mosses have a shallow root system, the damage to the concrete itself is usually minimal; however the moss does obtain and hold the water preventing it from flowing to gutters. Moss roots will exploit any damage in tiles and in frosty weather cause further damage by freeze thaw, cracking or delaminating tiles and in some cases lifting them.
Almost all mosses are unable to grow on natural slate, but Lichens can. The biological make up of Lichens mean that they are unlikely to do any structural damage to tiles, although, similarly to moss they will retain water and put additional strain on the roof. The retention of water by the Lichens will create an atmosphere for more to grow, so the problem should be addressed in the short term to avoid costly escalation.
Artificial Slate/Asbestos Tiles/Wooden Shingles
Having moss on any of these roof materials may be more of an immediate concern. As these materials are weaker and more permeable, the moss is more likely to do damage structurally and cause leaks due to the retention of water. The damp will also create a habitable atmosphere for wood-rotting organisms which will gradually erode roofing materials such as wooden shingles and the structural timbers below.
There are a number of methods that can be used to get rid of and prevent the growth of moss and lichens.
A toxic wash of the roof can be a solution, but usually the effects last no longer than 2-3 years. It should be applied during a dry spell so that it is not washed off in the rain before it gets a chance to react with the moss. The pressure of the wash can also cause damage to the roofing tiles, reducing their lifespan.
A more permanent method of preventing the re-growth of moss is by installing copper wires across the surface of the roof. With every shower of rain, the copper will oxidise creating copper sulphate, a natural moss killer that will run down the roof.
Each property and each instance of heavy moss growth is individual and, if you have concerns, should be assessed by a qualified and experienced Chartered Surveyor. To find your local surveyor, click on this link.