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Re: Valley Rafters

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Hips and valleys don't work by the numbers. Any engineer who has set pencil to paper will verify this. What you (and practically all) engineers miss is the secondary effects of the shear diaphragm and the hoop stresses in walls and top plate. The problem is that the actual stresses are a function of more than the structural members, and include GWB, moment capacity of nailed joints, etc. If you were paid $2M to design a house, and it was to be fabricated from MSR lumber, with a 3 view plus isometric connection detail of every component, and QA verification of every location (with all mis-fired nails resulting in removing all affected pieces and replaced), you could figure it out. That's how spaceflight hardware is designed and built (but without the wood).

Simple 2x hip and valley rafters are not structural - they're filler wood, just as most ridges are non-structural. You could replace that valley with the rafters butted together and blocking between and get nearly the same effect. How? Mentally eliminate the hip and trace the load paths that result.

1. For that valley to fall, the rafters must rotate. For the rafter to rotate, it has to pull away from the ridge. On top the rafter is sheathing which is tied to the ridge - a tension tie. On the bottom of the rafter there is direct contact with the ridge - a compression connection. That couple will resist the rotation. 

2. If the valley is just a compression member, and not structural, then statics will show a thrust vector. How is that resisted? Well, the only way that end can move is to slide in a vertical plane, along the valley. Except that it's (a) nailed to the valley, so slip has to overcome ~3 12d nails of shear and (b) it's tied to the rest of the roof with sheathing, and has to overcome that shear as well, or drag the whole house with it.

3. Your ceiling diaphragm (aka drywall) will also prevent a valley or hip from slipping outward. It's not much, but it's enough in most cases. For hips, there's also the tension tie (hoop stress) which will be generated in the top plate. Sure, it's not perfect, and we've all seen houses which bow outward when they have inadequate ties, along with the requisite ridge sag.

The question is, are you being required to justify it on paper, and are you getting paid enough for the calculations and details to ensure these secondary methods will work. In my experience, it's cheaper to spec a double LVL and be done with it. Nobody will cheer you for spending $1500 to prove a 2x12 will work.  I've done it both ways, but the "it all hangs together" only flies when I'm doing the whole framing job and it's a design-build job with a good crew. Remember that you may have counted that diaphragm shear for lateral, so in high load cases it may be double-counted if you anticipate it holding the roof up too.  In the worst of (liability) cases, the chances of a failure occurring prior to the statute of repose is very small - but that's a cop out, in my opinion.

BTW - anyone who ways that it has never failed really means they've never been sued over it. Failures occur all the time, and I see them dozens of times every year in my small town. They're not "building collapsing and people dying" failures, but they are costing clients 5 figures in repair costs so that they can sell their house. Most of them take 15-30 years to get bad enough that someone realizes there is a problem. By then the designer and/or builder is long gone.

ENGRLAINES(--nospam--at) wrote:
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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