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Wood roof joist/rafter spacing

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Bill:
 
Great question.  Structurally, plywood diaphragms work well with16" or 24" supports.
 
To me, the answer is not structural.  For roofing material (built-up or asphalt shingles) supported by plywood over joists spaced at  24", the plywood, at the unsupported joints can be a source of roof leaks.  Should a foot step on one side of the plywood joint,  deflection across the joint may cause a tear in the roofing.  We have mitigated this by using plyclips at 12" o/c when we specify joists at 24" o/c.  This allows the plywood to deflect uniformly regardless of where a person may step.  Another solution may be to use T & G plywood over joists at 24" o/c, or a blocked diaphragm.
 
With Joy and Hope,
Bob Freeman, Architect
Structural Designer
Integrated Design Services, Inc.
(714) 368-5080