Can I use lag bolts when mounting heavy shelf units through 2 layer drywall?

Question

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.

Answer 1

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.

Answer 2

I'm necro posting this only because it's a top result in Google, but contains an important misunderstanding about how connectors such as screws and bolts are designed to work. There is some important information missing.

Imagine a 2x10 laying on the ground. Beneath the 2x10 is some drywall. Six large men, each 200lbs, are standing on the 2x10. Your task is to push the 2x10 width-wise across the floor, with 1,200 pounds pressing it down into the drywall.

The six large men are the lag screws. Torquing the bolts down presses the wood against the drywall. That's what primarily prevents it from the sliding down the wall. Not the bending strength of the fastener, but friction between the wood and the wall, caused by the fasteners pressing the wood into the wall.

Returning to the 2x10 on the floor image, consider if there were 8 layers of drywall on the floor, then the wood and 1200 pounds pressing the wood down. It's easy to see that the drywall thickness doesn't matter. What matters is the pressure of the fastener pressing the wood against the wall.

Answer 3

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.