On 23/10/2012 01:43, John Rumm wrote:


Probably not as "industry standard" or trustworthy, but there
are a number of smart phone apps about that will do calcs at least on a
single beam.
Might be worth trying one of those if you have platform for that.


It's interesting that there seem to be more apps available than
web-based calculators. Unfortunately most of these seem to be in
American and I'm not sure how C16 compares with their specs but, putting
a trial dimension of 4m in, using Redwood or Southern Pine (which seem
to come out the same and probably equate broadly) gives a deflection of
69mm for a 6x2 for a light floor loading, which seems an awful lot. For
some reason, the one I was using seems not to accept 2" beams after the
first use so I can't go back and check but the deflection for 8x2 was
about 20-something mm so about a 2" difference.
So to keep the floor clear of the ceiling, either size would have to be
mounted at the same height (roughly), which is interesting. So it might
well be best to go for the bigger size and do it 'properly' after all.
I suspect this is a worst case as adding boards and struts would stiffen
the whole thing. On the other hand, there is bound to be a bit of
gradual sag over time, as you saw, even in an unloaded joist, which
might offset this.
I suppose an alternative (to use narrower timbers) would be to change
the joist pattern but the only way I can see to do this would be to put
something very substantial across the middle of the 6m dimension (or
split it into 3) and run the joists at right angles to this. This would
make most joists around 3m, where 6x2 would probably be fine. I'm not
sure this would be wise though as it would increase the loading on the
walls to a great extent where this beam was mounted and would generate
problems with actually getting everything into place.
Thinking back to my last place, it's certainly true that we had a loft
which I floored using chipboard panels onto 4x2 ceiling joists with no
problems. I think the differences there were a) the spans were much
smaller and b) I didn't give it any thought at all!