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Old 01-04-2010, 12:06 PM   #1
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Scissor truss

I have a 24x 26 garage that i am finishing the upstairs. there are 4 foot side walls on the front and back of the building. The height from floor to peak is 16', and 12' from top plate to peak. (4' walls) the rafter span is 12', and i have used 2x10 for the rafters and 2x6 walls rafters and walls 16" on center. I have installed rafter ties on every rafter of my vaulted ceiling at a height of 8', floor to rafter ties, (4' top plate to ceiling) to prevent wall spreading.

Are the 2x10 rafters strong enough to have the rafter ties 1/3 of the way up with a 70 psf ground snow load? I'm sure it won't collapse but I want it to code.

If it is not strong enough, can a scissor truss be retrofitted to the existing rafters to eleviate some of the load?


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Old 01-04-2010, 05:54 PM   #2
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Are you up on Canada (70psf snow load)? Around here its 40. But anyway, is the 4' wall part of the main sidewalls, or is it a separate wall build on top of you second floor system? I was having some difficulty deciphering your description, are you building a vaulted ceiling inside your rafters using 2 x 6's? If so, are you asking about putting collar ties above the vaulted ceiling? Some pics would help.
The 2 x 10's sound beefy enough at your 12/12 pitch. My concern would be the 4' wall, if it is built on top of the floor system, is going to want to act like a hinge. I don't think the vaulted ceiling would keep the rafters from wanting to spread the walls. You would almost be better off if the rafters went right to your outside wall plate of your main walls and not have the 4' wall there. Then at least they could be nailed in alongside the floor joists.
You may want to check with your local building department to be sure. You have a few variables there worth checking up on.
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Old 01-04-2010, 07:28 PM   #3
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Side Walls

Yes the 4 foot walls are built on top of the second floor. I do have collar ties above the rafter ties, and the vaulted ceiling gives the room an 8 foot height. I believe from everything I have read, on this forum as well as other books (and the link I attached). That as long as the rafter ties are in the bottom third of the rafter it will resist the lateral forces, preventing the side wall from acting like a hinge. (I would hope!) I live in Maine, and I don't know if the pitch of the roof would offset the snow ratinf or not.

Last edited by timsgills; 01-04-2010 at 07:32 PM. Reason: adding link
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Old 01-04-2010, 09:48 PM   #4
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I agree with Mike, check with your local B.D. They know the requirements and when a roof needs to be engineered or not. Here is an example: Once you reach a certain distance off the top plate with the ties and the short hinge walls, things get complicated quickly. Especially if you are drywalling the ceiling:

Then other things come into play- nailing the connections more than minimum code:
A structural engineer does this for a living, and the safety liability would be his, not yours. (Especially if a Homeowners Claim was filed). Think--- paper trail. Permits, engineering, etc. for your minimum safety.

Be safe, Gary
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