On Thu, 27 Sep 2007 04:45:59 -0700 Martin Bonner wrote :
Does anybody (eg Tony) know if Superbeam will model such a system?

I'm still working on my shed, and Superbeam tells me that the 2x3
rafters at 2' centres aren't good enough to hold a heavy green roof.

Without checking out the calcs (am drowning under work ATM), most
timber calculations result in the member being sized to keep the
deflection within limits, and the standard limit (0.003 x span) is
really to do with preventing damage to plaster finishes and stopping
floors feeling uncomfortably springy (whilst being safe). If the
calculation shows the applied bending stress as being less than the
permissible then all should be OK.

--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk