Joists/rafters - 450 or 600 centres

I am at the stage of wanting to buy the timber to frame my ‘granny flat inside the shed’. I’ve looked at span tables & am having trouble deciding whether to use 450mm or 600mm spacing for the joists/rafters. These will form the ceiling for the dwelling & provide a mezzanine floor for general storage. They will be covered with gyprock underneath & yellow tongue type floor sheeting above. I expect that a mezzanine floor capable of supporting the same as a normal domestic floor would suffice.

Rafters/joists will be MGP10 240 x 45 spanning (single span) 3900mm (including the width of the supporting walls - 90mm x 2).

According to the span table http://ift.tt/1O0WNd5 max span for this size timber using 600mm spacing is 4300mm with a floor loading of 40kg/M2 & max span @450mm spacing is 4800mm. (Local supplier does not carry 240x35).

What I am unsure of is what sort of floor loading the gyprock & yellow tongue will give. I donÂ’t have any doubt that 600mm spacing will hold things up, but what IÂ’m not sure about is whether it will give sufficient rigidity to prevent the plaster underneath cracking when the mezzanine floor is being walked on.
I will go with 450mm spacing if I need to (even though I already have a number of packs of insulation batts intended for 600mm centres) but doing so will cost an additional $450 so I would prefer not to be ‘over engineering just to be safe’.

Advice welcome ........... please.

Thanks,
Cuppa.


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