Hi Michael 600mm centres would be too far apart for a flat roof and the problem with 450mm centres is the ply decking would have to be cut to fall on the joists. It's better to set the joists at 406mm (16") centres for strength and to avoid having to cut the ply. When designing a flat roof you need to take into account heavy snow fall will increase the loading on the joists. 13 joists would take you to 4880mm,so 14 if you need it to be 5m. Hope that helps.Ed.
HI Micheal. Beams can be set between 450 and 600mm centres. the more the merrier but it increases labour. The bigger the centers, the greater the risk of sagging between the joists. If the 5 mtr is an accurate measurement, use 10 to keep things simple. But, if you are using 8x4 deck boards then setting them to allow the boards to fall half and half on the beam will make life even easier. Hope this helps Matt - A A Roofing, Widnes
Hi. Too my knowledge there isn't a set distance between beams but I would suggest 600mm between each across the shortest length. So a quick calculation would give 8 beams. Many thanks Dwayne.
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