On Friday, October 19, 2012 1:26:36 AM UTC+1, John Rumm wrote:
On 18/10/2012 23:56, meow2222 wrote:
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.

(the limit is somewhat higher than 3mm in 10')

However, its not just about safety, but also producing an adequate
quality of building, where the floors don't bounce up and down, and all
the joints crack in the plaster because there is too much movement.

No, its not. Millions of old houses have timber a fraction the size of the current requirements and don't suffer any of those problems.


There are other requirements that are also taken into account with the
sizes commonly used, such as minimizing twisting of joists (although
strapping will still be required over 2.5m), and providing lateral
restraint of walls where required.

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.

I have yet to be in a house where the floor deflects 6" while walking
across a room. Personally I would take that as an indication one should
get out fast!

obviously that's not relevant

However it is true that what was acceptable in the past would no longer
meet current standards. That is partly a reflection on more stringent
requirements for air tightness of properties,

I've never known a Victorian ceiling/loft floor be draughty

and also changes in
materials. Lime mortar, and soft bricks will accept more movement than
modern materials for example.

Slight movement of loft floor does not move the brickwork. Modern PB is much more tolerant of movement than lath & plaster.


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.

I was responding to your comments about the 2x4 which can deflect 6"
safely rather than the OPs storage floor. A 6" deflection may not cause
it to fail, but it would be excessive by any measure, and would not be
suitable even for loft storage.

Obviously a floor structure that deflects 6" in use has never been proposed. My point was that deflection levels encountered in real life floors are a fraction of failure limits

The OP can use lighter timbers than those that would be required to meet
the standard of a floor if he wants, although if there was a suggestion
that at some point in the future it was upgraded to a habitable room it
would be sensible to build it to the required standards now obviously.

Standards change, so its not obvious, its just an option

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

3.1mm would be fine even for a floor (the limit on that length would be
around 8.4mm IIRC).


NT