Is this a good idea for a flat roof,please help?

Hi all,

I've just joined this site after reading all the good info thats given. I have a question. Im building a flat roof over my bathroom. The walls are Victorian style, not double skinned, and has 2 small double glazed windows. The ceiling is 2.7M high and will have a light/fan built in over the shower venting through the wall and downlights will be installed. Im building a flat roof but unsure of the best way to do this. Heres what Im planning to do, starting from the inside out:-

Ive used 2 x 4 joists on top of each other with the top joist lifted at house end to create the slope.

12.5 plasterboard attached to joists, then 50mm polystyrene slab on top. Then a layer of silver backed 12.5 plaster board screwed to the top of the first joist, silverside aimed down. Resting on this is B and Qs fiberglass alternative filler and on top of that some 150mm space blanket. The roof will be covered by 20mm chipboard and flame felt underlay and mineral felt top coat applied.

Questions are 1. Does this sound like a suitable construction?
2. Will heat be kept in?
3. Do I need more ventaliation?
4. Will condensation be a problem?
5. Can I make this cheaper?

Any advice will be of great benefit.
Sorry this is so long for my first post.
Thanks in advance,

Lee

If you have any air voids whatsoever then there is a strong likelihood you will have condensation.

Go for a 'warm deck' option as it is soooo much simpler.

noseall said:

If you have any air voids whatsoever then there is a strong likelihood you will have condensation.

Go for a 'warm deck' option as it is soooo much simpler.

Click to expand...

Thanks for a quick reply.

I looked into the warm deck idea but was told by my roofer that you need special boards that would cost me in the region of £200 to felt to and a breathable membrane to attach to the roof base? Ive already bought the chipboard and felt. Is it really simplier? it seems more expensive and time consuming from what I've read. I also found that I would have to order the special materials in as they arn't used often, is this the case?

Thanks again for the reply.

Lee

All you need to buy is 97mm insulated ply deck from kingspan and some long screws. To satisfy current reg's you are also required to fit an extra 25mm between joists fitted tight to the underside of the deck.