I’ve used 9.5mm sterling board for the roof, its flexible enough to bend into the dip of the roof.
Measure from front to back and add on a few millimetres because when the board is screwed down in the curve it will pull the board in a little, I added only needed to add on 5mm for mine.
Measure the highest end and cut out a squared piece, then offer it up on the roof and mark and cut to length.
