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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 4x4 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?

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    The "big advantage" of opening the walls is that you don't need 4x4 posts taking up space in the garage against the walls, because there is very likely plenty of support for your ledgers in the existing wall framing. Drywall repair (which is all you avoid) is trivial in the scope of a project like this. – Ecnerwal Sep 21 '20 at 22:16
  • That sounds like a good reason. Would I just open the wall immediately behind the ledgers, and use 1/2" lag bolts to attach the ledges to the studs? – mrog Sep 21 '20 at 22:20
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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.

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  • What are your thoughts on opening the walls? Is it okay to have drywall sandwiched between the ledgers and the studs? – mrog Sep 22 '20 at 21:36
  • @mrog If either wall is adjacent to the house, then the drywall is required to stay. However, you MUST drill the lag bolts exactly in the center of the stud...take your time and do it right. Also, insure the length of the bolts have 2 1/2” penetration. – Lee Sam Sep 22 '20 at 22:27
  • I'm trying to understand the math for the lag screws. How is the 660 plf figure derived? – mrog Sep 23 '20 at 16:47
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    @mrog My book shows 1/2” bolts in “shear” between two boards for loads perpendicular to grain is about 880 lbs. , depending on the species and grade of the lumber. Therefore, with studs at 16” on center, that resistance is lowered from 880 to 660 plf. Then multiply by 2 for two bolts and divide by 2 for fasteners at 32” on center = 660 plf. BTW, keep the bolts a minimum of 2” from the edges of the ledger and don’t split the studs or you’ll have no value...you may want to predrill a pilot hole. – Lee Sam Sep 23 '20 at 18:20
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    @mrog Hmmm...I think we’re close. I’m saying install two 1/2” bolts at 32” on center. The bolts can be staggered with 1 bolt every 16” on center....and yes, the bolts need 2 1/2” penetration into the studs. (That means the bolt needs to be 2.5” long for penetration + 1/2” for the drywall + 1 1/2” for the thickness of the ledger = 4.5” total length. – Lee Sam Sep 24 '20 at 0:07

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