Need help designing a garage loft
It kinda depends on what you’re going to store up there. If it’s Christmas decorations and your empty suitcases, then that’s pretty minor load (maybe 10 lbs. per square foot). However, if you’re a machine shop salesman and you’re storing large steel samples, then that’s significant (maybe 100 lbs. per square foot).
Let’s assume something in the middle like 50 psf. (You can spread your stuff out to double check my guess. Residential construction requires 40 psf, but if you look around you’ll see empty spaces...your loft probably will be piled sky high.)
Therefore, at 50 psf there is about 5 psf for Dead Load (the weight of the lumber, etc.), which leaves about 45 psf.
Depending on the species and grade of your joists, 2x10’s can span 17’-6” and support 50 psf when installed at 24” on center or about 66 psf at 16” on center spanning 17’-6”.
1/2” plywood can span 16” or 24”, but will sag if heavy things are installed on joists 24” on center...it won’t fail, but will look unsightly after awhile if that’s important. (You can always increase the plywood to 5/8” if you’re concerned.)
I like the ledger idea because it keeps all the posts out of the garage. The load on the ledger is 50 psf x 17.5’ / 2 = 425 lbs. per foot.
I’d use 2 - 1/2” wood lags at 32” on center into a 2x10 ledger, which equals 660 plf. (I’m assuming your studs are 16” on center.)
The load is now transferred down to the footing. Adding 425 plf to your footing is not much. However, if you live where it’s swampy, or you have other loads on those footings, you may want to consult a structural engineer or architect.