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My neighbor did the same thing you are proposing and after a heavy snow some of the rafters broke between the collar ties and the wall. The collar ties limit the ability of the rafter to flex and move the load to the point between the tie and the wall.
When you put in a insulated, finished ceiling most codes want a 10 lb sq. ft rated truss. You will be adding quite a load that the original structure was not designed for so you may actually want to have it looked at by a engineer or code enforcement official before going to far. I went through that with a pole barn that I had built. The trusses were specd out at 2 lb per sq ft for a storage structure, and I had to upgrade the trusses to a 10 lb per sq ft lower cord so it could be finished later. .
First off I'm NO Engineer, but if you are concerned about the rafter deflecting and pushing out on the top of the wall below the collar tie, could you add another 2X ?? from the top of the wall out to the end of the new collar tie across to the other rafter in effect making a modified scissors truss ??
Your garage describes ours which is now a horse barn. I do not think they had a measuring device that was on beyween them spaci g was all over the place.
In reality you are dealing with existing 2 x 4 trusses that are being modified to support a differently distributed load. It seems to me like it would pay to get someone to do some calculations to specify the hardware and verify the loading on the existing 2x4's will comply to current codes for that type of structure before you start buying materials.
On your drawing envision the snow covering the roof from the gussets up. That load is trying to push the walls apart, the collar ties are holding them in place but a huge strain has been added to the rafters from the collar ties now with no ability to flex. Your original collar ties are only to keep the walls from spreading apart. Your idea of making the rafters into trusses will work but you should try to get some engineering assistance
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