In a flat roof you have rafters, say, 50x170. On top of this goes the deck, osb or ply, 2.4 x 1.2m max size. Where 2 sheets on of a roof larger than 2.4x1.2 you need a noggin, between the rafters, to support the edges of the osb/ply. What size wood does the noggin have to be?
Say noggin is...