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First time posting here, as I haven't been able to find this specific question answered anywhere.

I'm a novice and halfway through a garage storage project, and just discovered that I have double-layer drywall, 1.25 inches of gypsum from surface-to-stud. The wall-mounted shelf units I designed (now on their 3rd redesign) are 48" wide and heavy, built mostly with 2" x 10"s. I want them to be solid enough to support a few hundred lbs each. The shelves are structurally solid, but I'm concerned about mounting them.

I planned to use 3/8" x 4" lag bolts to mount the cleats through the drywall into the stud. 4" bolt - 1.5" cleat - 0.5" (assumed)drywall would leave 2" threaded into the stud. I'm concerned now that with the lag bolts basically unsupported through 1.25" of drywall between the cleat and the stud, the bolts may fail under enough weight. I've researched shear-strength for lag bolts, but does that strength rating take into account an unsupported section in the middle of the bolt?

The wall is framed with standard 16" spacing between studs, and the house was built in 2006. The highest shelf will be 70" off the ground, and the garage ceiling is 14' high. I'm hesitant to use lag bolts any thicker than 3/8" for the sake of not chewing through a third of the width of the studs.

  • I am not sure how your cleats fit in your design, but would it make sense to remove the drywall around the cleats and attach them directly to the studs? They'd only stand 1/4" proud above the surface of the drywall. – batsplatsterson Jan 7 '16 at 13:38
  • Actually scratch that, my previous commend is probably a bad idea, the double layer is probably there because it's a fire wall separating the garage from the living space. – batsplatsterson Jan 7 '16 at 13:45
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When the two members are not touching, the bolt starts to act as a "cantilevered beam" (as you have alluded to) and you hit a point where it is not the shear value that governs, it is the moment (bending). You get additional support by the cleat trying to go down and into the wall which puts withdrawel load on the fastener effectively adding more support using the tensile strength of the fastener.

As a note standard zinc hardware store bolts are ungraded steel usually with no actual values assigned to them.

First, if you have the time, you might try to buy some graded bolts online or from a local supplier; not necessarily a high grade, just graded at all so them meet a minimum requirement for strength. Or use structural screws such as Simpson SDS screws.

Secondly, if you have effectively 1-1/4" of cantilever, I would recommend slightly more embedment into the stud. If you hold things flush to the wall with only a 1-1/4" cantilever, I would up the embedment to a minimum of 2-1/2" or more to ensure the wood stud does not crush at bolt under load due to the multiplying effect of the cantilever. Visualize a 12" long bolt embeded 1" in to a 2x4, the wood crushes on the 2x4, the hole enlarges, and the bolt comes out; the bolt does not necessarily even bend before failing. An increased embedment also increases the amount of withdrawal load the fastener can handle when it slides down the wall (if the bolt does bend) and tries to pull out.

Another option is to simply get more fasteners per stud. You can do this by upping the size of the cleat to a 2x6 and use two or three bolts. Or you can add a vertical member below the cleat at each stud and effectively add a bolt every 4" or so in the vertical member with predrilled holes plus the bolt(s) into the cleat. Key is more embedment and/or more bolts to handle the cantilever.

Or, just build them free standing.

  • Damon, thanks for your insight. My "cleat" for each shelf is a 2x10, 48" long crossing 3 beams. I planned to fasten the cleat to each beam with two lag bolts/washers for a total of six 3/8" lags per shelf unit. I intended to use a 1x4 furring strip nailed flush to the wall directly above each cleat as a backboard the shelf. The 1x4 extends 3/4" over the cleat, with the shelf supported by the remaining 3/4" and then 10" end brackets made of 2x10, cut 45* & fastened to the cleat by driving 4 #12 screws through the back (also 2 staggered metal brackets. In this model, shelf/item weight rests... – Joe C Jan 9 '16 at 5:48
  • ...partly on the cleat itself but mostly on the brackets screwed into the cleat, which (I think) increases the cantilevered force on the lag bolts due to the forward moment/leverage on the cleat as the shelf/item weight basically pulls the cleat from the wall. Given your suggestion of larger cleats with more bolts, do you think that six 3/8" zinc lags, 5" long to embed 2-1/4" into the studs, would support considerable weight pressing/pulling on the cleat? Or 6" lags to embed 3-1/4"? Assuming this doesn't compromise the integrity of a 3-1/2" stud, are six cantilevered 3/8 lags adequate? Nine? – Joe C Jan 9 '16 at 5:50
  • Sounds like you have something very specific that you have thought through that I am having a hard time visualizing in its entirety 100%. Mostly you just have to identify what is pulling out, and what is sliding down and then make sure to use proper brackets, screws, lags, nails, etc for the forces at hand. – Damon Jan 9 '16 at 15:55
  • (3) lags embeded 3-1/4" would be as good as (2) embeded 2-1/4" for withdrawel, but (3) lags would be stronger than (2) for whatever downward force their is, although (2) 3/8" lags is very strong for shelving even ungraded. (6) per shelf would probably be more than enough. It sounds like you have brackets unmentioned in your original question that will add support so I think you are hitting the point where you just build it with what you feel is strong enough and if something begins to fail (which it probably will not based on your description), you reinforce the failure point at that time. – Damon Jan 9 '16 at 16:02
  • Thanks for your advice. This is my first project of this type. – Joe C Jan 11 '16 at 8:24
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You will find that 2" into the stud will be plenty. One simple rule of thumb I use is, if I add a washer to the lag and pilot the hole into the stud properly, which should be a 1/4" dia., and do not drill it in the full 2" into the stud, drill only 1 1/2" into the stud, say about 2 3/4" deep total. When you assemble the bracket you plan to use (a 2X4) and draw the lag up to the face of the 2x you should be able to compress the washer into the face of the 2x support with out stripping as long as it is not located at a knot. Place the first and last lag about 2-3" from the ends and add enough in the center to have a spacing about 10-12" apart.

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