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Wood roof joist/rafter spacing[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: Wood roof joist/rafter spacing
- From: "Robert Freeman" <robert.freeman(--nospam--at)idsse.com>
- Date: Tue, 17 Aug 2004 14:05:53 -0700
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
Integrated Design Services, Inc.
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