Insulating a concrete flat roof

[richg]

Hi, our house was built in the 50's, it's brick with a single story rear extension with a concrete roof. This houses the bathroom, toilet and boiler with small hall and back door. In the winter it is much colder in this area, and with heating there is a significant build up of condensation on the outer walls, windows and the wood panelled ceiling. During the summer it is much hotter in this room. From what I can tell the walls are the same as the rest of the house, and not just a single brick skin. The roof on the other hand is just a single slab of concrete (prob about 5" thick). Internally there isn't enough headroom to insulate the roof. Does anyone have any experience of insulating the roof from the outside (warm deck roofing) From what I gather it's just a case of applying a water proofing layer - bitumen and then lay some kingspan / celotex panels. Apply a second coat of water proofer - bitumen and then cover with some ballast of somekind. Is what I'm suggesting OK, are there any better suggestions and will it make any difference to the temperature and condensation problems in the extension.
All help and suggestions will be gratefully received. Cheers, Richard

[welsh brickie]

fix 2x2 treated batten 50mm insulation,Marine ply.And get a fibreglass company to cover it

[richg]

cheers, I'll have a look at the fibreglassing option.