Lag Bolt Pullout Resistance

jbnimbleOctober 26, 2006

Need data on force needed to pull out lag bolts from ordinary studs.

Considering 1/4, 5/16, and 3/8 bolts, with 2 or 2 1/2 inches in the studs.

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Google found a useful link.


Here is a link that might be useful: lag bolt loading specifications

    Bookmark   October 27, 2006 at 5:33AM
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Thanks for that link. Two to three-hundred lbs per inch of depth is pretty darn strong!

    Bookmark   October 27, 2006 at 6:22AM
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The bottom line is how many studs, and how well installed are they? It would be better to bolt to the rim joist. Is the ledger going to support a deck, a pergola...or a kitchen counter?


    Bookmark   October 27, 2006 at 6:53AM
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The actual usable rating is from 1/8 to 1/10 of the ultimate strength.
This makes up for variations in the wood and less than optimal installation.
You also need to consider the stud flexing and what it will transfer the load to.

    Bookmark   October 27, 2006 at 7:30PM
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in my experience, lags are not strong enough to hold railings in wood (if railings is what we are talking about). The force exerted by a 42" railing is very significant, as if you had a 42" crowbar, and will pull lags right out of all but the densest wood.

    Bookmark   November 1, 2006 at 6:17AM
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I've demolished enough railings to agree with you there, it should be bolted, if that's what the app is. On the other hand, if it's a face plate for a deck being screwed through the outer sheathing to the outside wall studs, then I guess your only choice is lag screws (I haven't been back in N.America for long after an extended stay overseas, so I'm assuming a lag screw is a hex-headed self-tapping wood bolt/screw, just larger and heavier than what we'd just refer to as a screw?)

    Bookmark   November 1, 2006 at 6:25PM
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Yes, "lag bolt" and "lag screw" mean the same thing, which is simply a giant hex-head screw designed for penetrating wood.

They have an identity crisis because they have the leading edge of a screw and the trailing edge of most common bolts (See Wikipedia). But you raise a good point that some machine screws (notably in computers) also have hex heads, and they're not lag screws. Someone has probably defined a more precise distinction (e.g., anything above a #14).

If they were truly a bolt, they would not be capable of self-tapping (they can, though for larger diameters a pilot hole of the commonly recommended size, drilled into a center punch, is well-advised), and they would be capable of receiving nuts (they are not).

As for railings being easily destroyed, I can imagine. Fatigue failure (shake-shake-shake repetitive motion) on wood is a pretty low bar. One thing you can do is to screw (lots of screws) or liquid-nails a steel plate to the wood on both sides. Drill a hole the size of your bolt through the whole sandwich. Put a spring washer on your bolt, put the bolt through, and hit it with a second spring washer on the end. Top with a nice big nut (dome-shaped for safety, hex doughnut for easier installation).

Curious to see what you'd all say about warped warehouse beams ... normal or not?

    Bookmark   August 5, 2010 at 9:46AM
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