Flat Roofing Q & A
FLAT ROOF GATHERS MOSS
Q What is the purpose of gravel on my garage flat roof? Silt and moss has built up around and under the gravel and some areas of the roof have no gravel as it has drifted. Should I remove and replace the gravel and sweep up the silt and moss?
A Gravel or pea shingle on a roof serves two purposes. One is to act as ballast to weigh down the covering surface and help prevent it being blown off in high winds. The second is to provide protection from the sun’s rays, which can both soften the covering material and make it brittle.
The shingle should not be free to drift around, however. It is supposed to be firmly bonded to the roof covering, either applied to mastic asphalt roofs while the material is still hot, or else stuck down to felt roofing using bitumen adhesive. If the shingle has simply been scattered dry across the roof, this is generally a sign of amateur work.
Build-up of silt, moss and other green growth is also usually an indication that the roof has been built badly, with insufficient falls to shed rainwater efficiently. A “flat” roof should really have a minimum incline of 1-in-40, and preferably more, otherwise rainwater will hang around long enough to support vegetative growth, and long-term settlement of timber joists and decking will result in ponding and potential leaks.
As long as the roof covering is not leaking, there would not seem much to be gained from adding an extra length of pipe to divert the main roof water. But at 20 years, any felt flat roof will be nearing the end of its life.
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