New roof without touching existing ceiling

We currently have a bungalow and intend to convert it into a dorma.

We need to increase the pitch of the roof to accommodate this and this means new roof trusses - however we don't want to touch the downstairs ceilings - which leaves us with a dilema. How do we seat the new roof trusses on the wall plate?

We have seen conversions done by Moduloft - and they appear to take the roof off and drop the new modules pre-built above the existing ceiling of a property, without actually affecting the existing timbers - so they must build up the wall plate somehow.

Does anyone know how this is done?

I have seen some references to something called a 'pole plate' and from what I have read (although information is very scarce) - it appears to sit on top of the ceiling joists - forming what is essentially a new wall plate. Is this correct?

I have mocked up how I think this would look (existing wall plate is blue, new 'pole plate'? is red and the orange blocks are essentially to inflill between the existing ceiling joists and help transfer the load from the pole plate to the wall plate and take some of the burden off the ceiling timbers). The new trusses would then be seated on this pole plate and there would therefore be a gap between the bottom of the first floor joists and the top of the existing ceiling timbers.

I've heard of the system, but can't comment on it, but what you call a pole plate can be created with a 3x4 beam pushed down at the angle between the joist and the rafters and nailed in place, and then joist hangers can be attached so that the joists hang between, or alongside the existing ceiling joists. But this method does depend on the span across the ceiling. Is there a central wall that the joists can rest upon.

I wouldn't need to push the 'pole plate' between the rafters and the joists - as the existing roof along with the rafters are being removed.

These section diagrams may better illustrate what I was getting at. The first diagram is the existing roof structure, the second on is how I would envisage it looking (not to scale). The new 'pole plate' (red) would be inserted over the existing ceiling joists after the old roof has been removed - then the new trusses (green) would sit on top.

Hi Moony, what was the outcome of your project? Did you build it like that? I am facing a similar challenge and would be thankful for your advise. Many thanks!