On Thursday, October 18, 2012 1:11:45 PM UTC+1, (unknown) wrote:
On Wednesday, October 17, 2012 9:07:08 PM UTC+1, GMM wrote:

On Wednesday, October 17, 2012 8:56:57 PM UTC+1, (unknown) wrote:



Just going by the standard joist span tables for moderately loaded floors. Of courser a lighter joist would be cheaper/easier/more desirable. On the other hand, I'd rather not have the whole thing collapse!



This is a very common misunderstanding. BR requirements are based on sound transmission requirements, producing sizes far in excess of those required for safety. If the table says 2x8 you could use 2x4, fill the loft to the roof, and not be at the remotest risk. A 10' 2x4 can deflect at least 6" safely, a full loft on 10' 2x4s provides a fraction of its safe load limit.



Re noise transmission, the gap between the 2 joist layers has more effect than 2x8s. And I don't expect anyone lives in your loft anyway.





NT

Well, you're right: Nobody does live in my loft ...at least not that I'm dare of...though you never know
who might sneak in there (according to the Daily Mail at least....(!) I'm anticipating that this floor will
ultimately be loaded with all the junk you normally find in lofts - broken toys, old masters and all that.

On the other hand, I'm not sure it would feel too good to walk on an unsupported 14' span of 2 x 4
and if it deflects 6', then I would have to mount them higher than 2 x 8s so they didn't bang on the
top of the ceiling.

I have been working from the BR tables as a) they would seem to give the best outcome and b) I
couldn't find anything else that gave useful information.

I shall take another look and see if I can find any useful info on this. Perhaps 2 x 6s would do the job
reasonably, although there is an attraction in 2 x 8s in that a future (currently completely off the radar)
loft conversion would be possible without having to take it all down and start again.

Of course, there is an argument that smaller timbers in themselves reduce the total dead weight of the
floor a bit but I should think this is a pretty minor effect as is the impact on overall cost.
Probably the most compelling argument (for me) could be the fact that I have to lug them up 3 floors
to get them in, but that only needs to be done once.