On Sunday, October 21, 2012 2:16:39 AM UTC+1, Tony Bryer wrote:
On Sat, 20 Oct 2012 22:33:14 +0100 John Rumm wrote :

Out of interest I had an experiment with superbeam to see what you can

get away with on a 3x2 (well 72x47mm) and a typical floor load

(uniformly distributed 0.8kN/m on each joist). 1.3m seems to be about

the limit - so you could probably still do a landing with it and comply

with modern building regs. (having said that, its generally simpler to

use one depth all over to save having to buy lots of timber sizes)



Our old rule of thumb when I was a BCO which matched the tables pretty

well was that for floor joists double the depth in inches and subtract two

to get the permissible span in feet; flat roof joists, subtract one (2"

joists).



As you say, in most cases practicality requires all joists to be the same

depth (you need tops of joists to be level and want them all to bear on

wall at one level) so except for the largest span they are generally

oversized. There's also more in reserve in that for virtually all joists,

deflection governs the size, not bending stress and few floors are loaded

to BR design loads (1.5kN/m2 30lb/ft2).



--

Tony Bryer, Greentram: 'Software to build on',

Melbourne, Australia www.greentram.com

So for a 14ft span (as here), the 8" joists I originally proposed would be about right?
I wonder if the tables are constructed from that rule of thumb or from a complex calculation that gives the same result?
I guess most of the discussion (now) is about what you might be able to get away with, rather than what should be done, but I'd prefer to over-engineer than under, for the sake of a couple inches.
Of course, I'm equally concerned that they are mounted securely, as Mr R outlined......