DIY Doctor UK DIY Forums

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.

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,

Thanks, the page on loft conversions is helpful, but the calcs to 'prove' the floor joists are the sticking point.
thanks.

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.

thanks fireman sam, i will try this out and let you know.
Cheers katiesdad

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

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.

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

Thanks Fireman Sam
I think that info has made sense of it now.

Well how about that, a simple question, and it's had over 10,000 hits!!!!
Thanks again Fireman Sam for your help.
Cheers !!!

Pity that "sda.co.uk/sbwdemo.htm" does not seem to be available now. Anyone know why?

try Googling "superbeam.co.uk/sbwdemo.htm" as the demo version has been updated.

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