Long story short: I am working on a 1960's house with no rafter ties. The rafters themselves are now attached with screws and strong-ties (previously just double nailed!) but I don't have faith in the tensile strength of this arrangement.
I want to add rafter ties. I could just bolt on timbers (as this will be sufficient to offset the rafter thrust) but this approach will not prevent the walls from shifting while the timber settles/dries.
I want to use threaded rod instead so that I can manually adjust the tension as needed.
I can figure the actual rafter thrust, but the cost difference between smaller rods and larger ones isn't that much (though the entire arrangement is a bit expensive).
As I know that douglas fir is acceptable for rafter ties here, I looked up the longitudinal tensile strength of the wood (~1400 psi). I intend to use 5/8 threaded rod because the tensile strength of the turnbuckle in that size is 2200 psi. I could even save money and go with 1/2" rod (1500 psi) and still be ahead.
My rod strength will be higher than the failure point of timber, which is otherwise acceptable for this task.
Is there something wrong in my reasoning here? Is there a reason I can't / shouldn't use threaded rod for rafter ties?