Joist size and spacings, flex in the roof

Joined
30 Jan 2014
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Location
Cheshire
Country
United Kingdom
We have just put a roof out our small extension, external size is 5x3m with 125 cavity.
Did online calculator to determine the roof components. Came back 7x2 joists spaced at 600mm with 18mm OSB3 board. This has two skylights of 600x1200 and will be covered by an EPDM rubber roof.
Having built the roof, all openings have been doulble braced around the window, however their is a little flex on the sheets when walked on (assume the 600m space and thinner obs sheets). is this normal or do we need to add more joists at 450 wide to reduce any flex out of the roof? Happy as is, being very little weight on top and short span for the joists, from joist hanger to sitting on brickwork roughly 2.6m.
Should we leave as is or add more joists?
 
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All seems within spec, how much is your deflection, allowable is about 8mm with snow loading. 100kk bloke walking about could cause 4mm deflection
 

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