Best way to cut roof insulation around these little dormers...?

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We're having a new roof on our renovation.

building control are onboard etc. Replacement roof will have a u value of 0.16.

I'm using superfoil sf40BB which is insulation foil and breather membrane in one.

this should be a simple job but for my little dormers.

Ive laid a couple of sheets on the roof with clamps for now, while I decide whether its best to cut or fold the bottom sheet where it goes up the little dormers...

any advice please....?

looks like a straight cut in the membrane at the top of each dormer then put a fill piece over the top of each dormer ridge.

Many thanks

Dan
 

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Is there a manufacturer's guide for cutting and sealing? I would suspect that it needs to be cut centrally to the valley, and then both pieces sealed with tape on both sides of the insulation. Alternatively cut a little longer than the centre line and lap the insulation on a central batten.

Running insulation in this way over the valley means that the valley boards themselves need to be raised (replicated) above the insulation.
 
cut tight at the top no gaps, tape all cut edges.tape all joins then a strip of underlay to weather the ridge as you suggest. avoid double thickness of the insulation content it will cause slight humps in the battens.
A problem if you are having a slate roof
 
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