On Thursday, October 18, 2012 1:49:59 PM UTC+1, John Rumm wrote:
On 18/10/2012 13:11, meow2222 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,

They are based on acceptable* levels of deflection rather than sound

Whether that's true depends on how you define acceptable. Limiting deflection to 3mm over 10' has absolutely nothing to do with safety or what I would call acceptable deflection limits.


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.

As you highlight, the purpose of the deflection limits is not because
that is where a joist will fail catastrophically, but that is where any
more movement would become unacceptable.

Well, not in any sense of acceptable that I would buy into. Lots of old houses have ceiling deflection levels numerous times as big as BR allows, and I don't see any problems or complaints resulting.

L&P ceilings can move a fair
amount, but don't expect one that is supported by a floor structure that
deflects 5 inches every time someone walks over it to last long!

Timber that light isn't being discussed, and the OP's new joists won't be supporting a ceiling.


If 3.1mm deflection would upset the op, go with the tables. I personally wouldnt have any problem with half an inch on a heavily loaded section


NT