Hi there, I've started building a flat roofed timber framed garden office. It measures 3.6m x 3.6m and it's max height (without applying for planning permission) is 2.5m. I was hoping to use lengths of 4 x 2 spaced at 400mm for the roof, clad with ply before the roof covering goes on but I'm starting to realise that 4 x 2 over 3.6 isn't nearly strong enough, my problem is by using 6 x 2's I'm really compromising the height of the ceiling inside. The roof will have 60mm firings Does anyone have any suggestions? Can i use more 4x2 as sister joists? Or 5 x 2 perhaps? Will 5 x 3 help? Or, is there little option but to use 6 x 2? Thanks in advance for your advice, Cheers!