chappers
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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
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