Understanding roof spans -conventional coupled roof and roof trusses

Hi everyone,

I have a question for the building experts here, I am trying to get my head around roof framing, and sizing of members. Lets say I have a house that's 9m x 9m (just to keep it simple lol)
And I am trying to size my top plates from as1684 table A23. Lets call it a tiled roof with 450mm stud spacing. Now with a truss roof, would the span be 9m, and with a coupled roof, would the rafter span be half of that, 4.5m? Assuming there are no purlins.
So would that make the top plates for:
truss roof, top plates using MGP 10 2/90x45
Coupled roof, top plates using MGP10 2/90x35

For the truss roof I used the tile roof row, 450mm stud, truss span 9000mm
For the Coupled roof I used Tile roof row, 450mm stud spacing, 6000mm rafter span (closest to my span of 4500mm)

Hope I am on the right track, and would love to hear some expert views.

Many thanks everyone.

Scott.


Read the full thread at RenovateForum.com...

Its an old thread but I just saw it! The rafter span on a pitched roof is the same as for trusses if the roof doesnt have an under-purlin. If it has an under-purlin then as long as the purlin is mid-span you can say that the 9.0m span becomes not 4.5m as for trusses but 2.250m as the span is halved by the under-purlin