Perhaps I should add further, we have had a 'specialist' GRP roofing company in already, and they made the suggestion that 'all' the outbuilding roof's would benefit by their provided services. Apparently they give three coats for added future protection, this is based on any pre-remedial work required.
Their work comes with a manufacturer's 25 year warranty, but like oresome may have suggested, we are a little concerned about this guarantee?.
The individual outbuildings consist of either flat roof with OSB and two layers of heat sealed felt, with top layer of heat sealed mineralised felt. The other outbuildings consist of pitched roofs with either corrugated steel, corrugated bitumen sheets or 'asbestos' type corrugated sheets.
The OSB/Felted flat roof and one of the corrugated bitumen sheet roof's had a recent incident, when early one morning we had intruder's who 'went through' part of the roof's, hence the question on using GRP?.