If your roof tiles are actually the very light ones (0.1kN sq m?) why use a dead load of 1.0?
Even with normal concrete tiles, your dead load would be around 0.85, so you could reduce your dead load to - say - 0.6, and that would still be conservative.
Then your loading would be 1.35 kN sq m.
Total load 12 x 1.35 = 16.2kN.
For a deflection of no more than 4mm, you would need a beam with an 'I' of
0.6 Imposed roof load is in clause 4.3.1 of BS6399-3:1988.
0.75 Imposed roof load comes from clause 4.3.2 which can be used as an optional alternative for small buildings, and means that the more complicated snow load calcs do not need to be carried out.
I've done the snow load calcs a few times and unless you are very high up and with a roof shape that leads to snow drifting, the calculated load never approaches 0.6kN/m2, so generally I don't see a problem with using 0.6. Although 0.75 does give an extra bit of sleep at night factor
that's very true,
and thanks to all you guys for helping me,
clearly the SE i used went way over the top on his vaules
so the beams were too much!!!!
anyway all sorted and thanks again
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