Just bought a house in Ohio and the rafter ties in the garage are in pretty bad shape. Looks like the previous owner store a lot of heavy items on them and they are sagging and bowed. The garage is 25x40 with 6/12 roof pitch. The existing ties are spaced 4’ on center and are 2x6’s spliced in the middle. I would like to run a solid 25’ piece without a splice. What is the correct sizing to run a rafter tie at this 25’ length?
The PURPOSE of rafter ties is to keep the roof joists from spreading apart and consequently pushing the top of the walls apart.
SIZING the rafter ties is based on the load on the roof pushing down and the pitch of the roof. (The steeper the pitch of the roof, then the less “pushing out” on the walls.)
Assuming a snow load of 30 lbs. per square foot plus a dead load (roofing, sheathing, rafters, etc.) of 10 psf, you’d have a reaction of about 250 lbs. per linear foot. If you are installing the ties at 4’ o.c., then you have a thrust of about 1,000 lbs. at each tie. (One-half at each end.)
I’d fasten the tie to the rafter with 5-6 16d nails at each end. I’d also fasten them to the top plate with a Simpson clip like A35.