In windy wet weather my old fashioned  clay pan-tiled sloping roof sometimes leaks and in one room water comes through the ceiling because it was nailed through the roofing felt onto the underside of the battens supporting the tiles, but not by me!

There are several unusual features here; the rafters are 7.5” x 3”, length  9’9”and spaced at 4’. Battens are 2” square. Ceiling height runs from 7’3” down to 4’ (yes, four feet - the building is very old) [In a way I can see why the ceiling panels were fixed to the underside of the battens but whoever did it should have known better]

Because the roofing felt is damaged I intend to replace it with new vapour permeable sheet but re-use the existing tiles. Of course the ceiling will go at that time and I want to replace it with something else at a slightly lower level. I cant afford to lose too much head room between the rafters but would like to introduce about 1” of insulation, and I realise I must allow a little space for the vapour permeable sheet  to sag between the rafters so that any rain blown in through the tiles will drift away from the rafters.

So I hope you might guide me as to what panelling will remain flat when spanning just under 4’ and what insulation could I use? Is there any insulation which has its own surface on one side which could serve as the surface of the ceiling. I am not too fussy as to appearance as the room is variously used a playroom/study.

I realise from general reading that the underside of roofing felt should have a small amount of ventilation to prevent condensation but I am hoping by using vapour permeable sheeting in place of the present impermeable felt that I need not do anything else re condensation. I have never seen any condensation but that may be because it would form between the ceiling and the roof felt which are in contact with each other ie out of sight.

Sorry I couldn’t keep this any shorter, I did try.

with your rafter spacing this one is always going to be a bit wierd your best bet to get round all your problems is to do as you suggest remove the tiles and battens and replace the sarking (i wouldnt bother using expensive breathable membranes just add over fascia vents and some tile vents at high level up the roof, remember to cut the sarking around the vents).
With regards to your insulation to meet buliding regs you need to leave a 50mm air gap between your insulation and sarking, as your rafters are so far apart you may be able to get away with battening out your rafters by 25mm and using 50 mm celotex to insulate between the rafters, but don't quote me on that.
To be sureyou can batten out your rafters by 25mm then use 50mm between the rafters and 35mm under the rafters and the plasterboard over that.
My prefered solution however would be to use tri iso super 9 and batten with 25mm battens at right angles to the rafters, set at 400mm centers down the roof and then plasterboard over them.
Hope this helps

Gareth