Joists for 5.65m span

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Hi,

Starting to build a man cave in the garden tomorrow...
Could you please help me verify joists of 47x220 C24 are ok for a span of 5.65m?
As far as I've found, some tables say it's ok and some say it's not.
I'm also doing 2 skylights of 90x120 and doubling up the joists around them, so I guess I doubling all joists will get me in the allowed range?

Thanks in advance!
Yakov
 
Thanks Blup. Spacing is 40cm. Your link provides spans for C16 floor joists. Mine are C24 flat roof joists with minimal dead load - 3 layer felt, 18mm or 25mm ply, 100mm celotex, and plasterboard.
I found 5.66 here -> https://www.brookridgetimber.co.uk/wp-content/uploads/2017/10/SPANTABLES.pdf
And 5.386 here -> https://www.timberbeamcalculator.co.uk/span-table/flat-roof-joists?class=C24&slope=&load=1, but the load setting was 1kN/m2, which I believe is wrong. When selecting 0.75kN, the section under 0.5kN says 5.705!
Still a difference, but a smaller one, and both tables agree 5.65m is ok :D
Hooray!
 
Just to justify load <0.5kN/m²:
- 3 Layer Felt 0.07 kN/m²
- Boarding 0.15 kN/m²
- Firings 0.05 kN/m²
- Insulation 0.01 kN/m²
- 12mm Plasterboard + Skim 0.2 kN/m²
Total: 0.48kN/m²
 
And snow.

Looks like the joists will be over spanned even at 400 centres. You could try any work these out, as the tables do have a certain allowance, or you could like you say double up the joists, but it would be guessing.

You may be better off with engineered I-joists.
 
Hmmm... didnt allow for snow, you're right. Will ralk to my builder about it. Thanks ^woody^!
 
Hmmm... didnt allow for snow, you're right. Will ralk to my builder about it. Thanks ^woody^!
 
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