Evening all,

The room is an extension, single story with a flat roof (yes I know!)
approx size is 6.6m * 5.3m (with a corner cut out) the joists run across
the short edge (there is a RSJ from the corner of the cut out to the
short wall to which the joists 'join'). There was no insulation at all
hence it was so hot/cold, I've almost put that right.

The question si as the joists are 15" centres - not a divisor of standard
board sizes, which way to run the boards and should I/ do I need to nog
the joins/edges of the boards?

My feeling is I need to put quite a bit of wood in there to support board
ends/edges to prevent sagging.

As a follow up question 9.5 or 12.5mm board?

TIA

Have a good week-end

Nick