Moss on roofs

A rolling stone gathers no moss they say, but your roof might.  Chartered Surveyor Stephen Cornish PhD MA BSc FRICS FFPWS takes a look at the damage moss can lead to ...


With the on-set of winter and the likelihood of consistent rainfall, we should be checking our properties for any possible source of damp penetration. 

Have you become aware of clumps of moss which appear to have been deposited, rather than grown on your paving and driveway? Look up and take a close at your roof covering. Can you see moss growing on the tiles/slates?  Extensive moss growth can lead indirectly to rainwater penetration into your property.

Widespread moss growth will impede the flow of rainwater from the roof slopes towards the gutters. In such situations, rainwater retained on relatively absorbent roof tiles can, in extreme circumstances, reduce the life expectancy of the roof covering.

A more common problem is where clumps of moss detach from the roof covering and block gutters and rainwater outlets. The spillage of rainwater from blocked gutters may soak the external walls of your property and cause dampness within your property.

A tell-tale sign of long-term soaking of your walls is the presence of green algae on the external surfaces of walls. We recommend that the moss is carefully cleaned from the roof using a suitable, regulated biocide and a wire brush, or low-pressure wash. Under no circumstances should the roof be cleaned by jet washing or high pressure washing as this can damage the tiles / slates and also force water up between the tiles into the roof space.

If you are concerned about the condition of your roof covering, contact Woodward Chartered Surveyors in Herts, Middlesex and London to arrange for an inspection.

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