Rafter Ties on Vaulted Ceiling
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 ceiling, 4' top plate to ceiling) to prevent wall spreading. My question is,which I have read many places but am not sure if it is relevant to vaulted ceilings, can I double up the rafter ties and bolt them to the rafters then remove 2 of every 3 making them 4 feet on center? Also 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
What I learned.
The rafter ties in the bottom 1/3 of the span are good enough to keep the outer walls from spreading, the next thing you have to look for is the weight of the roof is now all on the lower third of the rafter greatly reducing it's span capability. You also have to look at how the pitch effects the snowload (the steeper the pitch the less of an effect the ground snow load has). Anyway, i found out I am good.
It is not a good practice according to the building inspector to double up the rafter ties on one rafter and skip the next two. He told me I needed it on every rafter.
Lesson learned: If you move the rafter ties above the top plate, things get complicated quickly and an engineer should be consulted. In the future I would use a ridge beam or prefabricated scissor trusses for an elevated ceiling.
Thank you for that follow-up! I'm glad you figured it out with our help. For anyone reading, here is part 1: http://www.diychatroom.com/f19/scissor-truss-61006/
Be safe, Gary
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