Scissor Truss Question

[Larry_Sweeney]

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.

[Bill_Emery]

The rule that I use is 2 pitches less than the upper truss.  This can vary with the pitch, span, and if you have a raised heel; but it's a starting point.

[DRAWZILLA]

I try to keep a  2" difference

Outside 5:12

Inside 3"12

 

But I am no expert either.

[JLDrafting]

The rule of thumb I was given is 1/2 of the top chord. Of course the truss manufacturer can engineer other options possibly.

[Larry_Sweeney]

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.

[Alaskan_Son]

We have used a lot of engineered scissor trusses over the years and the interior pitch has always been half of the exterior pitch. We also typically have a lot of snow load to account for though too.

[rlackore]

If you have the room to provide a large heel height, you could probably achieve a true cathedral with sloping parallel chord trusses.

[RodCole]

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!

[Larry_Sweeney]

It's gable at both ends and I'm not able to add a raised heel due to other factors. I had gone over all this with the truss manufacturer and they said the 2.75 pitch was workable for the span I noted.

[LevisL]

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.