roof joist spacing

huangyhg

roof joist spacing
folks,
are there any limits for the maximum spacing of roof open web joists?
structurally, i don't really think so.  not at least due to strength or stiffness concerns of the joists proper... there are practical limits based on the roofing materials.  also, the standard dimensions of anything that uses nice even spacings to acheive 1200 and 2400 edge distances on the cladding/roofing is best.
there are specialty products, etc, available that permit other spacings, however i only ever have non-1200 or 2400 multiples on jobs when the client/architect has specified that such products will definitely be used.
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
thats a fairly open ended question...several limits come to mind:  capability of deck to span between joists, how big/how much load your joist can support, and then if you have factory mutual requirements you are limited to their requirements as well.  typically about 5'-6" o.c. is an economical arrangement for general roof loads.  
i have found that joist spacing is limited either by how far the deck can span or how much load the joist can take.  for example, no k joist can support more than 550 plf.
daveatkins
agree w/ willisv and you wouldn't want 12'' deep joists at say 8'-0'' apart.  the bracing would not be effective.
deck requirements are probably the most limiting factor
deck diaphragm values can also be a consideration since they are a function of joist spacing.  
the reason i asked is because i have a condition where i have joists spanning about 40' and they are supporting 2" of concrete on the roof in addition to come high superimposed loads. the deck is a 2" deck.
i am not limited by depth and wanted to know if there were any reasons why i could not use a 24" or 30" deep joist at 7'-6" or 8' on center?
thanks for your inputs.
in that situation i might consider using composite joists to help with the high superimposed loads - i've done that in the past with a similiar situation.  space them at the max span the deck can handle.   
do we really need to point out that the spacing of joists is governed by the capacity of the joist?  i couldn't imagine that this was what the op was asking...
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
ys: lol, no i was not asking if the maximum is based on the capacity of the joist. i think i have my answer.
joist spacing is limited by deck and joist capacity, roofing limits (practical limitations so the deflection will not damage the membrane), underwriter's requirements and ul assemblies.