The same procedure applies whether you are putting the flat roof on a new building or extension, or replacing an old flat roof that has deteriorated.
Flat roofs are not completely flat, but have a very gentle slope to direct the rain water towards the drainage system, normally guttering. The joists are laid lengthways across the roof opening and furrings, which supply the required gradient, are laid across the joists at right angles.
Decking, or roof boards, are then laid on top of the structure, usually with insulation material inserted beneath. The decking or boards should be nailed or screwed to the furring.
The final part is the material which is laid on top, at the “sharp end”, keeping out the rainwater and protecting the whole building, or extension, from the elements. Until recent times this would invariably be roofing felt or mastic asphalt. This was messy, had a life span of no more than ten or fifteen years, and notorious for springing leaks well before that period had elapsed.
This material has now largely been supplanted by a single sheet of custom made rubber, cut to measure, such as Firestone’s EPDM product. This is extremely tough, has an extremely long life of upwards of 30 years, and easily resists everything the British climate can throw at it. Fitting it, however, is usually left to a professional roofer, and two or three people are required in the task.
You may wish to avoid not only wet weather when fitting this kind of material, but hot weather as well, as the rubber heats up very quickly and can become difficult to handle.
Carefully measure the flat roof area and then order a sheet of EPDM large enough to comfortably cover the whole roof and extend outwards several inches in all directions. Allow for any surrounding ridge in making sure the rubber can extend to underneath the coping used to fit around the edge and secure it.
Where the flat roof meets a pitched roof, ensure the rubber sheeting runs partially upwards and underneath the lower line of tiles or slates so as to prevent any possible rain penetration. Use heavy duty glue to ensure a permanent fix of the rubber sheeting to the roofing boards, and glue right up to the edge in order to make sure there is no kink that could hinder the drainage of rainwater away.
Any flashing that may be necessary where the two roofs meet should be kept as flat as possible and laid first, so the rubber sheet lies above it and on a downward slope where the edge is, again, to prevent any possible escape of rainwater to beneath the roof surface.
That’s how to install flat roofing in a professional way. If you’re thinking of doing all the work yourself then think carefully about finding a reliable roofer to do it for you.