8x4 = 32 sg ft
now measure the ceiling length x breadth multiply together that will give you ceiling square feet. Divide the board size (32) into the ceiling size that gives the MINIUM number of sheets as for wastage, cant help there.
I'd be looking at the joist spacing prior to deciding on board size, in particular, looking at using 900mm x 1800mm boards (450mm joist spacing). If they do happen to be spaced in the realms of 400mm c/c then look at using 900mm x 2400mm. Much easier to fix than those big fellas.
Wastage will depend upon board use efficiency and how you stagger the joints. I actually plan out the board fix and work out the wastage based upon how I will cut and use each board.
Work out the exact board area then add an extra board. One extra is usually enough for average sized ceilings.
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