This is a section through our proposed rear single-storey mid-terrace full-width extension, with parapets each side. We're hoping to do it as a green roof but we'd like to fall back on something cheaper if push comes to shove. We want a flat ceiling.

Originally I was thinking (inspiration from Viking House details):
1. Green roof (mm ?)
2. Roofkrete
3. EPS (150 mm)
4. Plywood deck
5. Pitched rafters + mineral wool (150 mm, although engineer has said 175 mm so 175 mm now).
6. Plasterboard.

Problem is this gives us a sloping ceiling, so as you can see in the section I added a suspended ceiling. But is this really the best approach? Plus would it be easier to build with 175 mm joists laid flat + firing strips?

The flat ceiling is a continuation of the existing ceiling line, so it looks like it was always meant to be. Don't like the idea of a sloping ceiling going up to nowhere, bad feng shui or something. Just somewhere for cooking oils and cobwebs to collect (we're also going to quite a bit of effort to keep the beam holding up the existing external wall above this ceiling line too.) So no technical reason, purely down to spacial quality and aesthetics really.

Any recommendations on waterproofing membrane? The only fail-safe waterproof membrane I know of is cold-applied liquid resin, e.g. Kemperol.

With regard to insulation I was going by these details:
http://www.viking-house.co.uk/gallery/pbb18b.jpg
I assumed the idea of mineral between rafters is that you can get a snugger fit than with EPS?

Hoping to use these Skyvision Ecoline rooflights:
http://www.vitral.co.uk/uploads/attachments/vitral_skyvision_brochure.pdf

The parapets are there because these were originally going to be newly raised party walls. They're party walls now so technically we could change to a standard verge detail. I've hung onto them because it just seemed a nicer way to deal with boundary walls to a full-width extension. You're probably right though, they're probably more hassle than they're worth.

I would slope from the top of the window in the roof to the beam for maximum light in the old part of the room. rest sloping with the pitch,