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Old 11-09-2008, 06:29 AM   #1
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stick framing scissor trusses


I have approved building plans that I've drawn myself for an addition on my house. The roof pitch is 6.5 over 12 using 2x6 rafters on 16 inch centers. The lower chord is two thirds of the upper chord = 4.375 over 12 using 2x4. I didn't look into scissor trusses, namely because I thought I could do things cheaper myself, but now that I have things erected I'm wondering how "structually sound" it is. One other detail, and I'm not sure this plays a role but just incase, my outside walls on this addition are 10 feet high.

Any shared experience, opinions would be great. Thanks.

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Old 11-10-2008, 07:46 AM   #2
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stick framing scissor trusses


Who approved the building plans? What is the span of the rafters? Are these true scissor trusses or a rafter system and a separate cathedral ceiling joist layout underneath? How do the rafters bear on the top plate? Got any pictures?

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Old 11-10-2008, 03:10 PM   #3
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stick framing scissor trusses


Thanks for answering

The building plans were approved by the local municaplities building permit department.

The span of the rafters is 20 feet.

These are not scissor trusses at all.

I have "normal" 2x6 rafters spanning the 20 foot room.
the lower chord is a seperate 2x4 that rests on the top of the wall up against a rafter. It is nailed to the top of the wall and onto the rafter as far up as possible, until they start to seperate. This bottom chord (2x4) spans across the ceilings centre point and ties into the rafter going down to the opposite outside wall. The oppsoite lower chord 2x4 is attached the same way to the other side of the rafters. I'll try and get a picture if i haven't explained myself clearly enough.
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Old 11-10-2008, 03:21 PM   #4
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stick framing scissor trusses


Sounds like you used 2x4's as collar ties, attaching each end to the rafters. How far are the collar ties spanning? I ask because a 2x4, which is essentially a ceiling joist in this case, won't span all that far.

Whether or not collar ties are adequate to prevent your exterior walls from thrusting outward under snow or wind load is something that only an engineer can tell you.
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Old 11-11-2008, 01:01 PM   #5
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stick framing scissor trusses


I understand what you have built. You have no collar tie at all, so all of the vertical load is being converted into lateral thrust against the top of the walls. You really need an engineer to look at the design.
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Old 11-11-2008, 06:36 PM   #6
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stick framing scissor trusses


Your opinion leads me to question why my drawings would have been approved by the building permit department. Did my drawings not pass through an engineers hands already? I saw this design concept in a framing book and basically copied it to print for my permit. I fully expected some feed back from the building department but they approved all my drawings on my first attempt. I suppose having someone take a second look now would be better than rebuilding down the road. Thanks for your time
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Old 11-11-2008, 10:01 PM   #7
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stick framing scissor trusses


Sometimes they just give them a quick look and stamp approved. I have seen some really bad plans approved, it was so bad that if implemented as drawn the whole place would be on the ground in 10 years.

Most places don't have engineers that look at every set of plans that come through.
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Old 11-12-2008, 06:09 AM   #8
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stick framing scissor trusses


Could be that the inspector assumed you were using engineered trusses. The fix for your situation could be as simple as adding a collar tie between your 2x4s and plated to tie the rafters together. Then through bolted to lock all of the components together.
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Old 11-12-2008, 01:45 PM   #9
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stick framing scissor trusses


So you have a 1.5" gap between each ceiling joist pair? If I'm envisioning this correctly, here's a suggestion, the other guys will chime in:

Install perlings in between the rakes of the rafters and the ceiling joists, located such that you can install a collar tie between each set of ceiling joists (in the gap), either at the apex or down a little, so you have a flat spot at the top of the cathedral. The collar ties will coincide with the rafters if they're in the gap of the ceiling joists, so they'll have to be anchored to the perlings very well. Connecting the ceiling joists to the collar ties, which are connected to the perlings would (in my opinion) form the equivalent of a truss setting on top of two scissor trusses, which should be much stronger than what you've got and you wouldn't have to tear anything down.

Just a thought...

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