Membrane above celotex insulation?

[andypitts]

Am planning to insulate between the sloping roof rafters in the attic of a large 1860's stone built semi.
As it happens, the next door semi is having their roof slates replaced at the moment. Like mine, these are the original thick old Welsh slates - and there is no felt beneath them at all ie they rest directly on the wood sarking boards with the attic space underneath this. So the builders are laying down a breathable waterproof membrane on the sarking first to then lay the new slates on top of.
I can't afford a new slate roof myself but am worried that old cracked slates plus no underlying felt = leaks into the attic space.
Should I pin some membrane pieces up to the underside of the sarking boards first before fitting the Celotex insulation panels in place?
Maybe I can use the same membrane next doors builders are using - says M7550-1B on it?
Cheers

[stoneyboy]

andypitts,
Replace the broken slates and fix the insulation under the rafters so that there is a ventilated gap between the boarding and the insulation.
end

[bd3cc]

Think you could be wasting your time. Sorry!

[andypitts]

thanks but not not sure in what respect I might be wasting my time?
cheers

[Perry525]

Its hard to understand where you are going with this?
Lets start by saying that "old Welsh slates" may be old but, they are good for four hundred years (if they are Welsh slates.)
So, why would anyone wish to disturb them?
Have the nails gone?
Do the slates rattle in the wind?
One type of repair is to slide a piece of aluminium under the slate, cut to size it will be invisible.
I have seen roofs where they have been glued together.
Your slates seem to have worked well for over a hundred years, without a membrane...........