Collar ties

Okay, now that I've learned a lot about my attic, another question came to mind.
On the subject of collar ties, why would they not be put on every rafter? Reading a bit online about this subject I came across the references about how some codes mandate 4 feet between them (approximately every other rafter). Why would having them on every rafter be a bad idea? Seems like putting them on all rafters would do a lot toward strengthening the structure.
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4' is every third rafter. Where they came up with the rule of thumb for a collar tie on every third rafter, I don't know. They are structural - in another post you mentioned that you didn't think they are. They're there to take the spreading force that the inclined rafters would impart to the opposing walls. Hence - ties. There's no problem having them on every other or every rafter pair.
The weird thing is if you engineer the required number of fasteners on a collar tie you'll be surprised at the number of nails you need, and you _never_ see that many nails in old time construction, yet they still seem to do the job.
R
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