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
 
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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
 
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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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