Framing small flat top mansard?

[Grandad1900]

Where can I find typical roof framing details for a flat-top mansard-type roof on a single-storey extension?
The single story extension projects 2.8m from a two-storey main house wall. This 9.6m x 2.8m extension has a tiled 40° pitched roof on three sides of the box, rising for a height of about 1m to terminate in a flat roof which runs across to meet the main wall just below the bedroom window sills.
I'm particularly interested in the roof truss carpentry (it’s a cut roof) for this warm roof. The insulation is above the horizontal timber roof joists and plywood that supports the roofing felt, with more insulation above the sloping rafters and below the tiles. I'm draining the flat roof at either end of the 9.6m through a gap in the ridge tiles. I could span the 2.8m with a ceiling joist, with a prop above it to support the top end of the sloping rafter where it meets the end of the horizontal roof joist – see sketch (the other end of this roof joist ends at a hanger at the main wall). Or maybe I should use a braced truss to span in the 9.6m direction onto internal walls that cut the max span to 3.8m, and use this to support the roof joists and hang the ceiling joists from the underside?
I’m also having trouble finding ridge tile details at the tiled-pitch to flat-felt-roof junction.
Any pointers towards a source of suitable details would be much appreciated.
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[noseall]

That drawing image is minute, can you make it bigger?

However there seems to be plenty of detail in there and shows the basics of the roof layout.

Have you any experience in cut roof building?

 

[Grandad1900]

Had trouble enlarging the diagram – hope one of these links works better. I’m not a carpenter, I plan to subcontract the work, but I wanted a practical arrangement and spec to get quotes against since it also affects other aspects of the extension.

 

[noseall]

What I'm not getting is why the need for a warm roof layout at all when all you need is insulation at ceiling level and a good venting system.

This would bring the finished height of the roof tiles lower down in respect to the flattie allowing the flat roof water to spill down.

 

[Grandad1900]

The extension roof runs past part of four upper rooms which suffer heat loss from their lower part because the main house walls (1930’s) are solid 9” brick. The idea was to create a warm space next to these main walls to help reduce the heat loss from these rooms, and at the central section the ceiling board follows the rafter slope. Ceiling insulation and, say, some vertical insulation against the main wall outer face could achieve similar results, so thanks noseall for your suggestion, I'll revisit the details from this viewpoint.
However I want to keep the ridge tiles fairly high because there are 2 suntubes thro’ the roof and the ridge tiles in the 9.6m direction help to hide the top of these from the kerbside view (a point raised during planning permission). I intended the longitudinal binders, supported on the props, to give stability to the roof joists along the 9.6m length. At their ends the upper binders terminate at a junction with a sloping rafter, and hip rafter down to the wall corners, and meet heightened room walls near their centre. A prop would stand on each ceiling rafter but the roof rafters would span from main wall to the upper binder, and not necessarily match the spacing of the ceiling joists. I don’t remember seeing details like this around much, so hence my concern?