On 22/05/10 15:31, Roger Mills wrote:

Can't give any absolute answers, but the main problem with excessive
deflection is that it cracks the ceilings below. In your situation, most
of the deflection will come from the boards themselves without bending
the joists very much - so may not matter, apart from feeling a bit springy.

Indeed. The joists are OK. Interestingly, the sagulator gives a much
larger deflection estimate for even a 4x2 ceiling rafter than I see in
reality. I can stand in the middle of one of these without any
discernable movement (and there's no floor screwed to the top yet) -
though there is are some "virtual noggins" by way of cross beams across
the top that are fixed through,

I would seriously suggest using T&G though - which will spread the load
between boards and considerably reduce the deflection due to point loads
- and will probably reduce the joist deflections as well. OK, they're
then harder to lift - but you should be able to anticipate which ones
you're most likely to need to lift (areas with plumbing and electrical
connections underneath) and make little "trap doors" (with the bit under
the groove removed, and screwed down rather than nailed) at the time of
laying.

I agree - but I have made it my primary design goal that there will be
no T&G as the boards I may potentially want to lift are all of them

I've built this place to support easy upgrading of comms circuits
(networking, TV etc) - every power socket is partnered by at least a
single backbox with dry conduit, so it's fully expected I'll want to
lift many of the boards perhaps ones every 2-3 years. I'm designing that
into the way the boards are set for lenght and the staggering of the
centre cuts so that I don't end up with trying to lift long boards with
both ends trapped under skirtings.

--
Tim Watts

Hung parliament? Rather have a hanged parliament.