residential ceiling fire code plus…

need to sheath a 24″ oc truss ceiling. the final finish will be a 1×4 tg cypress or poplar in a decorative design that might not always fall along the truss line. so my questions are: what size ply would you sheath the ceiling with to allow for t&g attachment not on truss lines…and then would the ply plus the T&G meet fire code.
thanks
Replies
Our Canadian codes allow fire rated ceilings to be finished with materials having a flame spread rating of 1 or less. All wood t and g is rated as 1. The plywood does not effect the fire rating, as horizontal assemblies are rated from the bottom up. Having said all this, you should probably get another take on this from someone familiar with your code.
Do you need to sheath in ply, or would strapping do?