On 18/10/2012 13:11, 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
transmission (while there are building regs on sound transmission, they
don't figure at all in structural calculations of joist sizes at all as
far as I am aware)

(* Where acceptable is usually specified as a function of the joist
length, or an absolute number of mm if over a certain length. The
calculations will also check that the beam is not likely to fail in
shear at the ends, or bending in the middle)

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. 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!


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.

Loft floors (once strengthened) are actually very good for noise
insulation since they typically have two separate sets of joists
carrying the floor and ceiling loads.



--
Cheers,

John.

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