I have a flat roof surrounded by parapet walls / house wall. Roofing contractors are in the process of stripping and recovering. In order not to raise the height of the final deck covering too far, Building Control agreed to some insulation between joists ( cold deck ), then board over the joists and further insulation above before the final covering, with a vapour barrier somewhere in the sandwiched construction above the joists ( warm deck ).
Now we find that the joists need replacing, and BC then suggested a vapour barrier between ceiling and underside of joists would be a good idea. There is currently no ventilation of the voids between the joists, so to my mind any moisture in the wood would be trapped between ceiling and decking and this doesn’t sound good.
Anyone come across this before ? All advice welcome !