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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?

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I think that would work but I would use some big washers (possibly a pyramid stack) on both sides of that threaded rod to spread out the compression force. –  Dan D. Feb 24 '13 at 4:55
    
I was thinking 2" square washers. –  Matthew Feb 24 '13 at 5:01
    
How do you propose to attach the rod? Lateral load connectors might be useful : strongtie.com/products/connectors/DTT2.asp –  HerrBag Feb 25 '13 at 19:55
    
@HerrBag I was looking for brackets like that, but that pinch the timber and bolt through. –  Matthew Feb 25 '13 at 21:07
    
You could still use those and just send a couple bolts, nuts and washers through (be sure to use hot dipped galv to match simpson). Nail the rest with the strongtie nails –  HerrBag Feb 25 '13 at 21:13
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1 Answer

Corrosion would be one concern. Another would be carefully and slowly tightening the turnbuckles in a sufficient and even manner. Your rod could well deform the wall/roof if adjustments are done too quickly. Small adjustments over months. Perhaps a laser and mirror could give clues.

I used SS cable with stainless fittings for my leaning garage and used 2 levels to put a leaning wall straight.

Many issues regarding fixing leaning, sagging buildings are here

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