firrings - need to run on joists?

[pilsbury]

My flat roof joists will run left to right. If I wanted the slope of the roof to run exactly this way, the firrings would sit nicely on each joist. If I wanted the slope to run at 45 degrees to the joists, can I fix the firrings accordingly or is this a recepie for sagging as the firrings will only be fixed to joists every 400mm?

 

[noseall]

My flat roof joists will run left to right. If I wanted the slope of the roof to run exactly this way, the firrings would sit nicely on each joist. If I wanted the slope to run at 45 degrees to the joists, can I fix the firrings accordingly or is this a recepie for sagging as the firrings will only be fixed to joists every 400mm?

Yes fine but - you will only be able to fix your sheet material in pre-determined positions, i.e. where the furring and the joist overlap. The furring will be too weak to fix to in any other position.

So yes - you will need to add noggins.