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Re: Valley Rafters

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Charles, have your looked into the requirements of conventional construction which might apply to your benefit.  It depends on the slope of the roof.  As others on the group have indicated, depending on the slope, the rafter might behave like a truss or a beam.  The following section from the IBC will help you codify the theory.

"2308.10 Roof and ceiling framing. The framing details required in this section apply to roofs having a minimum slope of three units vertical in 12 units horizontal (25-percent slope) or greater. Where the roof slope is less than three units vertical in 12 units horizontal (25-percent slope), members supporting rafters and ceiling joists such as ridge board, hips and valleys shall be designed as beams."

Ashwin Ranga Swamy, P.E.
Los Angeles, California

Sent: Saturday, February 07, 2009 7:23 PM
Subject: Valley Rafters

An architect client asked me to answer a building official's request to verify the strength of a valley rafter (or beam) in a wood roof. The architect specified a valley rafter 2" deeper than the rafters (as usual), but this time he is being challenged to prove it. I analyzed it as a simple beam taking half the tributary load from the jack rafters framing into it and it is in fact way off! The architect maintains that he and everyone he knows have always spec'd 2" deeper for decades with no failures or even challenges. He even showed me text books and other reference books with this recommendation. Am I missing something?
Charles O. Laines, S.E.

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