When a Roof has a layer of insulation and often a plywood layer on top with the felt on top of that it is nessacerry to use threaded bar to fit through the roof and reach the original roof deck as shown in the picture, we do not supply the threaded bar or the timber to line the inside of the cut hole, but these can be easily purchased from any DIY store to suit your particuar construction.
When a Roof has internal cladding this will offten not be strong enough to fix through and it is recomended that the original roof deck is used to fix the window too. In order to get the collar through the cut hole of the cladding it will be nessacerry to cut the collar in two pieces and reassemble once fitted. Remove any insullation material to allow easy access. Offer the collar up to the window base to ensure any cut lines do not clash with a fixing Hole.