joist size...


Postby Paul Jennings » Wed Nov 12, 2008 7:14 pm

I've been rebuilding a stone house in France and have just reached the roof - but I need to decide on the right joist size for the attic floor. The span between wall plates is 5.28 metres so what size joists do I need and at what span? It's a long span and I've almost come to the conclusion that what I need is a spine beam running the length of the house - but as that's 8.5m we'd be talking a monster timber and I dragged enough of those from the woods for the 1st floor! Anyone help?
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Postby ollyburton » Thu Nov 13, 2008 8:32 pm

hello there i take it steel is out the question as 1 steel h beam in 2no 3mtr sections bolted together running along center then 2.5mtr ish 6x2 would do you .if not put 4 no 9x3 @5.28 bridging wall plates (heavy though) one either gable then 2 splitting difference then you would be able to use 7x2 joists running parallel to wall plates with a span of 3 mtr ish. you can even get away with 2 no 9x3 and chase joist ends into gables
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Last edited by ollyburton on Thu Feb 03, 2011 5:51 pm, edited 1 time in total.
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Postby Perry525 » Tue Nov 18, 2008 8:05 pm

The rule of thumb has always been half the feet in distance as inches plus two inches ie; 12 feet = 6 inches plus 2 inches = 8 inches, therefore 8 x 2.
Modern oriental strand manufactured joists are much stronger, why not visit one of their sites, they are also lighter.
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Postby alankfc02 » Sun Nov 30, 2008 8:34 pm

you would be better using a engineered I joist for this sort of clear span. probably better to be 300mm deep at 600cc. I would recommend Finnforest FJI joists
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Postby AJH162 » Wed Dec 17, 2008 7:04 pm

Just come across your post, if you are still stuck this may help. As you have figured out the 5.28m span is too great for timber according to the TRADA span tables. The usual way of tackling this situation is to span across with a central steel so breaking the 8.5 into two 4.25m long bays. You then have a similar steel at each end of the bays, now run 50mm x 225mm C24 grade floor joists between them with the ends notched into the webs and solid blocked between. the joists will obviously be parallel to the wall plates.
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Postby TheDoctor5 » Tue Jan 20, 2009 11:53 am

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