Removal. Treatment. Prevention
Flat roofs construction from timber and with a felt (bitumen based) covering are popular the world over but not more so than in the United Kingdom.
They are often laid with a green mineral finish or with a standard bitumen finish with stone chippings laid on top.
It is very important not to remove the surface material as doing so will allow direct sunlight onto the felt and prolonged exposure will result in drying and cracking of
the bitumen based covering.
If there is excessive moss build up then it should be lightly swept off with a stiff brown. Always take extra care on flat roofs as they are prone to leaks and subsequent timber rot.
First consider laying some copper strips on the roof. I have seen them placed underneath stone chippings and it has worked very well at keeping moss growth at bay. This page has more information on how to best use copper as a moss killer. This option is great for flat roofs that are not seen, such as dormers.
Copper is very much a safe product and is used quite often on roofs, such as a replacement for lead or as a material from which guttering is made from. Drinking water is also supplied to many homes via copper pipes.
Another option is to use chemicals directly onto the roof. Because you cannot be so rigorous at removing existing moss you may need to apply more of the chemical than you would onto a patio or other surface.
Copper is a virtually harmless metal that can be used around the home quite safely, however it is a metal that should not be used in conjunction with aluminium as it will cause a mild corrosive chemical reaction. This means that laying copper onto a roof can cause degradation to aluminium gutters.
If you have aluminium gutters then consider using a chemical moss killer instead.