I've got 8x2 roof joists on my garden room, so ripped some in half to create 8x1 backing fascia. I'm using 9mm capping upvc fascia and was wondering what size polytop will be sufficient. Depth of timber backing and upvc is about 32mm. Will 30mm doubled up next to each other hold the upvc fascia. If moving to 40mm, will going through the total depth reduce hold? TIA