Scissor Truss Question


Larry_Sweeney
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Truthfully I don't use trusses very often (stick built "old timer") and just thought I could find a quick answer from someone on the forum. I have a customer that wants me to design a pavilion that will have a 4.5/12 roof pitch with a 33' span. The customer wants a cathedral ceiling. I thought it would be much less expensive to use a scissor truss  with a less pitch on the interior. Is there a "rule of thumb" as to how much less of a pitch has to be on the ceiling as compared to the actual roof pitch? I wasn't sure if it would depend on the span or a "rule of thumb" and yes, I do know someone is going to tell me to call an engineer or the truss company. Thanks in advance for your input.

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I was thinking the 2" difference was something I would work with for starters also. I had a call into a truss company from last week and they just got back to me while I was writing this reply. I was told that with a 4.5 pitch on the exterior and a 33' span I could do a max. of 2.75 pitch on the interior using 2x6 chords @ 24" centers. Thanks for everyone's advice. They also told me the 2" "rule of thumb" is a good thing to use.

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Larry

 

Is your pavilion a gable roof, or are you looking to make it a hip roof?  I agree that the rule of thumb for top cord pitch to bottom cord pitch has been 1/2.

 

The reason I ask about the roof itself is that you will probably end up with one direction of the hip, if that is in fact what you have, being a girder truss with the other trusses bearing on it.  If this is the case then I am not sure that the rule of thumb would apply in this situation.  Just what you wanted to hear I bet.  There's always something!

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Another factor to consider for the inside slope is the type of attic insulation. Up here in Canada, our Code limits the slope to 4.5/12 for mineral fibre or cellulose fibre loose-fill insulation, and 2.5/12 for other types of loose-fill. You guys must have something similar in your Codes. If you're planning on using batts or spray-foam, then this isn't as issue, but I thought I'd mention it.

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