I'm going to be moving into a new house that has a finished garage with a high ceiling. I want to build a loft in the garage for storage. There are plenty of online span calculators that should help me figure out the joist dimensions and spacing, but I'm not sure how to support the whole thing.
I'd like to avoid opening the walls, unless there's a big benefit to doing that. The loft will be in the center of the garage, touching two walls on opposite sides (17.5 feet apart). I'm thinking of using 4×4 posts along the walls to support ledger boards, with no posts in the middle of the garage.
I don't know how many posts to use, what size ledger boards to use, or the size and type of fasteners to use. Is there an easy way to calculate all that?
What resources can you suggest that would help me learn how to design the loft correctly? Are there any free or inexpensive design tools I could use to draw up plans for getting a permit?
Best Answer
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.