roof framing question


On the current series of This Old House, they're (among other things) putting a two-story, one-room-wide-ish addition on a house. The addition has a gambrel roof, at right angles to the main roof.
For the ridge on the addition, they're using a double 2x12. This seems kind of massive, to me. For instance, our (old) barn (not large, but way bigger than their addition), also has a gambrel roof, and just a 1-x-whatever for the ridge board.
I'd be interested in any thoughts on this.
Thanks, George
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Two parallel boards are much less likely to warp.
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I would guess that your barn didn't have a cathedral ceiling, and that their addition did. Their ridge is a structural ridge, holding up the roof, and needs to be pretty big, especially if a snow load is involved. A double 2x12 is on the small side. We have a 20' long addition on the back of our house that has a double 14" Microlam as the ridge.
JK
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On Thu, 12 Nov 2009 06:45:39 -0800 (PST), Big_Jake

OK, thanks. Just to clarify: with collar ties (and suitably-sized rafters), the ridge load would be much lower? When you say the ridge is holding up the roof, does that mean their rafters are in tension?
Thanks again, George
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wrote:

This should answer the question for you with pictures and words:
http://books.google.com/books?id åS9qrSNxx0C&pg=PA130&lpg=PA130&dq=structural+ridge+board&source=bl&ots=yQT1cJGUgL&sig=OKGCFBETPWzwOX0yM5WkaeXmxRo&hl=en&ei=urb9SueKBYz9nQeq6YyZCw&sa=X&oi=book_result&ct=result&resnum=1&ved AgQ6AEwAA#v=onepage&q=structural%20ridge%20board&fúlse
Colbyt
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The ridge load is really nothing. It is being held up by the rafters. And you are correct that a structural ridge puts the rafters are in tension (I think).
I am not a structural engineer, but I did point out a math error to the one that did the calcs for my addition, causing him to change the footing diameter from 6" to 18"!
JK
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Consider a frame consisting of two walls studs and two rafters, with hinges at all the connections. That configuration is not stable: it can freely deform via a mechanism in which the ridge falls and the tops of the walls spread apart. The two usual ways to deal with this are either to provide a tension element (ceiling joist or collar tie) to prevent the walls from spreading, or to provide a structural ridge element which is properly supported.
I think it is typical to do just one or the other, so if you are using collar ties, there would be no ridge load at all. But you could design a hybrid solution that uses both collar ties and a structural ridge. In that case, yes, the collar ties would reduce the load on the ridge.

Keep in mind that the gravity load of the roof provides a distributed axial force on the rafter, so whether the rafter is in compression or tension can change along its length. When using a structural ridge, the reaction from the ridge puts the top end of the rafter in tensions, but the other loads are compressive, so the bottom end of the rafter is in compression.
Cheers, Wayne
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If that's a structural ridge, a double 2x12 seems kind of small to me. There may have been a steel plate sandwiched in between, or the 2xs were actually LVLs.
A structural ridge, such as the ridge over a cathedral ceiling, requires a much bigger design load than a standard ridge. The ceiling joists in a standard ceiling situation tie the rafters together and keep them from spreading. This can also be accomplished by collar ties at a higher level, but may require larger rafters and/or stronger connections to the rafters.
There frequently isn't a ridge board in timber frame construction, and the ridge board in a stick built house is mainly there to assist in building the roof. In most typical stick-framed roof situations you could omit the ridge board entirely and it wouldn't appreciably affect the roof strength. A 2x4 truss roof doesn't typically have a ridge board either.
It's all about the loads.
R
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They used two LVLs for the ridge board. Much stronger then 2x12. There is a lot of weight that this ridge will carry unlike your barn.
Chris
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