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WFCM and hip and valley beam sizes

Sifu

Sifu

SAWHORSE
Joined
Sep 3, 2011
Messages
2,830
Wood Frame Construction Manual Table 3.28-Hip and valley beam sizes (prescriptive design) gives spans and sizes for certain beam sizes. The only single ply member it lists is a 2x6. Anybody know why? Are there no values for single ply 2x10 or 2x12? There are values for those members in the ridge beam table, 3.29A.
 
My understanding is the table does not provide you with values for individual members, it provides minimum sized framing members for the hip/valley area based upon the roof live load/ground snow load and the roof dead load. For example for a hip that is 17' long, with a ground snow load of 30 psf and a roof dead load of 10 psf you would go to the left side of the table and find your 17' span and go across until you find the column for 30 psf ground snow load and then under it the 10 psf roof dead load. intersect that column with the row from the 17' span and you get a two ply 2x12. Hope that helps.
 
It helps a little, I guess I have been looking at it from the wrong direction but the information (or lack of it) remains the same. I guess the real issue for me is the type of framing I see here everyday. Very common to see single ply 2x10 or 2x12 hips and valleys. It would be helpful to know the limitations of those members. The table in question gives us at least an indication of the limits for some members but not all. I guess I'll continue to treat the 2x10's like 2x6's.
 
GBrackins said:
remember, just because it's always been done a certain way doesn't make it the correct way.
Click to expand...
Couldn't agree more. IMO the code is weak when it come to prescriptive stick frame roof systems. It only tells us to support hips and valleys at the top and bottom. It also tells us we can use a single 2x member. I try to improve the framing practices of these roofs but have little prescriptive ammunition to go by. Thats why I turn to the WFCM, it has a little more to go on. I don't have the manual with me but I am guessing now since you reversed the way the table is read that the reason the larger single ply members aren't listed is that the tributary areas would exceed those listed in the table. I'll stick to the table. Thanks for enlightening on how to read that table.
 
FYI, "The Journal of Light Construction" (May 1994) has a well written and easily understood article called "Straight Talk about Hip and Valley Rafters" that explains total roof load and the tributary loads found on Hips and Valleys. This article makes to easy to see if hip and valley rafters are compliant. Well worth the time!! You should be able to retrieve it from their web site for under $5.00.
 
Thanks, When I get time I'll check that out. I have the JLC archives, maybe its in there.
 
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