size of timber floor joists


Postby katiesdad » Tue Feb 10, 2009 6:54 pm

I need some advise on the size of floor joists to be used in a loft conversion. The longest span needed is 4.130m. From the building regs, table A1, i only need 195 x 50mm at 400mm centres, but I want to put 1.2m high stud wall supporting the slate roof at midpoint. (removing the purlin). This means that the size/spacing requires upgrading. I have some timber floor joists that are 250 X 50mm that i want to use. Can anybody help with the calculations and the new spacings (i am ignoring the original ceiling joists) Its 30 years since i did this at collage, and the brain's going !!!
Many thanks.
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Postby thedoctor » Tue Feb 10, 2009 10:12 pm

See our project on loft conversions. You will need Building Regulation approval for a conversion and you will need to "proove" that your planned method will be structurally sound. There are many regs involved with a loft conversion and you will need to comply with every one or your house may not be insured,
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Postby katiesdad » Wed Feb 11, 2009 12:18 pm

Thanks, the page on loft conversions is helpful, but the calcs to 'prove' the floor joists are the sticking point.
thanks.
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Postby fireman sam » Thu Feb 12, 2009 3:29 pm

I have used a demo program called superbeam4 for all my calcs. Download demo version free at http://www.sda.co.uk/sbwdemo.htm. It does all the calculations you want, just have to format the results yourself, but at least it gives you good figures. Building control have been more than happy with the results. You will need to know what loads you are applying to the beam.
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Postby katiesdad » Fri Feb 13, 2009 11:07 am

thanks fireman sam, i will try this out and let you know.
Cheers katiesdad
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Postby katiesdad » Sat Feb 14, 2009 10:16 am

I can calculate the dead loads ok, but 2000kg/sqm (2kN/sqm) for live floor load seems a bit high. Also what live load should i use for a 42 degree pitch roof in London, even recently the snow hasn't been 2 feet thick!!
many thanks
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Postby katiesdad » Sat Feb 21, 2009 7:03 am

It would seem that at over 430 viewings there is a lot of interest in this subject. If someone knows what the live loads might be, or can assist in any other way, please reply- its really easy.
Many thanks.
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Postby fireman sam » Sat Feb 21, 2009 2:17 pm

Generally, as per timber tables, floor live loadings are taken as 1.5kN/m2. I don't know if it was a typo, but 2kN/m2 is approx 200kg/m2 not 2000! There are 9.8N per kg. As for roof loads, I used 0.9kN/m2 dead load and 0.75kN/m2 Live load (in plan) for a 45degree pitched clay tiled roof in the south east uk. These loads will need to be calculated over the joist spacing and added as a point load to the joist where the stud wall sits in addition to the 0.5kN/m2 dead load and1.5kN/ms for the floor load as a UDL.
You might also find some help with roof loads on the roofing websites such as Marley, redland etc in the technical data sections
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Postby katiesdad » Sun Feb 22, 2009 3:45 pm

Thanks Fireman Sam
I think that info has made sense of it now.
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Postby katiesdad » Thu Jul 29, 2010 6:38 pm

Well how about that, a simple question, and it's had over 10,000 hits!!!!
Thanks again Fireman Sam for your help.
Cheers !!!
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Postby happyhacker » Mon Oct 11, 2010 10:11 am

Pity that "sda.co.uk/sbwdemo.htm" does not seem to be available now. Anyone know why?
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Postby katiesdad » Sat Oct 30, 2010 12:18 pm

try Googling "superbeam.co.uk/sbwdemo.htm" as the demo version has been updated.
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Postby katerossmail » Fri Feb 04, 2011 6:49 pm

Everything is ok, but 2000kg/sqm for live floor load is a bit high.
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