“How does one frame a roof structure without collar ties in order to create a vaulted ceiling?”
One doesn’t. Collar ties are generally required by building codes under all circumstances.
“Can an existing roof be modified to accomplish this? “
Not normally and certainly not without an engineer/architect and his supervision.
“Would it be possible to tie all the rafters together and then have exposed collar ties every second or third rafter?”
Again. Not normally because collar ties are usually required by code on each rafter.
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But you do have options:
1)Install roof ‘scissors trusses’ which vault the ceiling without the need for collar ties.
2)Have your existing roof engineered
to see if it can be turned into a quasi-scissors truss.
3) Install collars ties and simply have no exposed beams by drywalling the underside of this ‘flat’ center ceiling
4) Install collar ties and expose all of them on all rafters.
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