Quote Originally Posted by splott parker View Post
Don’t forget to check finished floor level to u/s of existing joists. You’re going to need insulation on the existing concrete floor, 100mm at least (although some authorities regs are now 150mm, ridiculous depth really), u/f heating snake clipped to the insulation. 50mm sand/cement screed or liquid screed over the lot. That’s at least 150mm higher than your existing slab. I presume you’ve got bare joists at the moment so insulation (100mm at least) should be fitted between them, this would create a ‘cold’ roof. Given that it’s a flat roof and not pitched you can’t get above the joists to insulate over the top so you should really be using an insulated plasterboard on the ceiling to complete uninterrupted ceiling insulation, this would most probably result in your ceiling finish being 50mm below u/s of existing joists. Doing it this way, which is normal practice now, would result in your storey height being 200mm (8 inches)less than it is now.
There are options, the iso insulation that comes in blanket form might be allowed, reducing loss of height, although the floor screed will have to be a minimum of 75 mm up to 100 mm with chicken wire over the insulation. Madness!