second pitched roof? warm roof?

Hi,
I've been lurker here for a long time now, but never got round to signing up, nearly every question i ever had has been here for the searching...

Anyway, a few months back I helped a friend install a 'secondary' flat roof on his kitchen (for the insulation) we used roofmate, all went fine and he is very happy. (just cleaned and laid on top of the old one, then extended the sides etc as per roofmate site.

I was looking at My roof as it's in need of work and a lot of insulation, its mono pitched but I have no loft space, the ceilings are on the bottom of the rafters. and there is only 1 inch of rockwall type insulation under the felt.

So to get the insulation in i can either take down all ceilings or take off the tiles.

I've had a few quotes coming in at silly money (15 grand) it's 'only' single storey, 108sqm of FC slate, no ridge, 21m of lead flashing. 100mm kingspan or equiv, Some have said it's currently asbestos cement slates and some that it's not.

Now the question bit, as the roof is (i think) water tight could i bitumen over the asbestos slate, lay roofmate style insulation over that, then just batten it up and lay new slate on top (make watertight like a flat roof), or even just 'dress it' to look like slate. roof line won't actually change at there is a parapet wall along the back/top and side edges. rafters are 11x4 inches so loading shouldn't be an issue. (and block old soffits etc, maybe even fill the now still air space with loose fill insulation)

I'm sure there are lots of reasons why i can't do the above, i just can't think of them.

Thanks for reading this far,

Dave

Fix celotex or kingspan to the ceiling below and overboard with foil back boards then skim.

Fit some roof vents too.

Thanks for the reply, I looked into that option a while back, the problem is that the current ceiling height at the lowest point is only 1.9m, and the windows actually meet the ceiling, I could sculpt the insulation and taper it at windows and the low point.

Also forgot to say that the roof is water tight now, it's only because I replace slates as they break.... (about 5-6 a year) I'm sure it's the magpies as i've seen them bringing down the odd bit of slate attached to the moss), they are quite brittle and quite a few are 'curling up' at the edges.

Dave,