I recently used a proprietary system supplied by the contractor on an extension.
Its basically a fibreglass roof membrane with a special paint finish on Superply laid to falls on a steel and timber composite roof with Kinspan HD insulation between and plasterboard faced HD insulation below the joists.
The important thing is to ensure the 50mm air gap is included to allow air to circulate per TGD Part F of the Building Regulations, Diagram 6 Page 11.
http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,1647,en.pdf
I was a little paranoid about this suggestion at first and I quizzed him about possible problems with damage or cracking. He pointed out that its a fibreglass roof - the paint finish can be prepared and painted over while the membrane itself is repairable like boats or surfboards are repairable and of acceptable appearance.
I then asked the age old question of the 15-20 year lifespan and renewal problems with removal and replacement. He smiled and pointed out that its a fibreglass roof of 3mm thickness.
The longest he had installed was 14 years old and still going strong, but he was of the opinion that if it needed renewal it was only 3mm thick and quite light so the simplest thing to do would be to clean, key, prepare and install over the existing roof.
We also saw two examples of this kind of roof done by the builder with rooflight lantern lights installed and they looked very well I have to say. He was also the lowest tender by a mile.
I can post contact details if you want them or you can PM me.
HTH
ONQ
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