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I recently had a contractor to help me install a TV mount. The mount is fixed to the wall and studs behind it using four 5/16x2-3/4 lag bolts. Later I found out you are supposed to drill pilot holes before putting in lag bolts, but the contractor did not do this. How likely are the 2x4 studs behind the wall damaged?

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  • Consequence - is a buddy watches you do this and in his head is thinking "what kind of idiot would work that hard on knocking those things in when they could have drill a hole first?" If you don't have a buddy around and unlimited time, very few consequences.
    – DMoore
    Apr 19, 2021 at 20:00
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    Old type lag bolts really benefited from predrilling the holes, but some more modern structural screws are self drilling. What kind did you have? Apr 20, 2021 at 2:03
  • @Jim - Probably the old fashioned ones. They came with the TV mount. This is the mount we have and there's a drawing of the lag bolt on page 9.
    – Mike
    Apr 20, 2021 at 7:24
  • If the lag bots shown in the installation you referenced are the ones the contractor installed, I'm surprised he was even able to get them into the stub without pilot holes.
    – SteveSh
    Apr 20, 2021 at 14:07
  • @SteveSh - He did drill holes with a smaller screw before installing the lag bolts as part of locating the stud (not sure how deep). Maybe that helped a bit?
    – Mike
    Apr 20, 2021 at 17:21

5 Answers 5

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Pilot holes are done for two reasons. One is to help prevent the stud from splitting and the second is to make the lag bolt or screw easier to install. The ease of installation is important because the head of a lag bolt can shear off when excessive torque is applied.
As long as your lag bolts are firmly holding the TV in place with no wobble in the mounting bracket you should be fine.

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    Yes. And it's easier to direct a longer bolt, e.g. perpendicular, esp. around (hidden) knots.
    – P2000
    Apr 19, 2021 at 16:51
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    I drill because I've learned the hard way that getting them home without drilling is 10 times more work than drilling, if I can get them there at all. I've never damaged a stud this way but I've spent half an hour trying to back out a lag bolt that I couldn't advance any more and i've broken heads off. Same with drilling too small. I only skip drilling for high quality screws that I bought myself into framing lumber, max #10 max 2 inches.
    – jay613
    Apr 19, 2021 at 19:49
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    I think the age of the stud will factor into this as well. New studs are pretty spongy. In older framing, it's a lot harder and more brittle. You would likely snap the heads off before the stud cracks though.
    – JimmyJames
    Apr 19, 2021 at 21:12
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Pilot holes reduce the risk of splitting the stud. If the lag bolt is holding, it probably didn't split the stud.

Note that a socket wrench can easily apply enough torque to strip the threads in soft wood, so if you want to test if the bolts are secure, don't over torque.

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There's more to this than splits and installation torque. Final holding power is at stake.

If a lag screw (not "bolt") is run in without a pilot hole, the wood (unless it's very soft) tends to split locally around the screw shank. This may not result in a full split of the lumber, but it effectively reduces the grab of the threads on two opposing sides of the screw.

Here's a simplified diagram of what happens:

                   |||| <-- wood grain
                    /\ <-- gapped area
                   /--\
                  (    ) <-- screw shaft
                   \--/
                    \/ <-- gapped area
                   ||||

The wedge-shaped gaps above and below the screw shaft in this diagram provide little or no hold--the threads are barely or not at all in connection with the wood. The screw will pull out with less force than it would from a proper pilot hole.

It's worth mentioning that lag screws rarely pull out of modern SPF lumber, but if you're trying to snug hardware against a wall with force, for example, this can be an issue. Your gizmo probably won't fall off the wall, but the screw may slip more than you'd like during final tightening. In extreme load scenarios like extendable TV mounts and floating desktops, pullout strength is critical, however, but screws are usually sized with enough overhead capacity that it's not a concern.

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  • yes, "screw", fixed my old bad habit
    – P2000
    Apr 20, 2021 at 17:43
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Even if the studs are damaged due to shoddy work, it doesn't necessarily mean there's a problem with those studs or their ability to carry a load. Studs can take a lot of drilling & notching (or chipping) before they fail their structural purpose.

The bigger problem could be that the stud is not adequately able to hold the screw: little can be assured about the withdrawal force of the screw if the wood is split.

Also, we don't know if these large diameter screws are perfectly centred into the stud, or whether they partially missed the stud. Stud finders can be off. This contributes to the uncertainty of the holding power.

So, the stud may be able to carry the load but not hold the screw.

Vibrations from movement or seismic activity matter too. If the mount has a retractable arm, and the TV is relatively large, it may be best to take your losses and have the work redone. The mounting plate will likely cover the old screw holes.

Ideally you'd mount to different studs, or mount a few inches below (not above) the existing location. This allows the bottom screws to transfer their load into pristine studs, downward.

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  • Would it be fine if we remove the bolts, patch the holes with wood filler, then drill pilot holes at the same locations and put everything back?
    – Mike
    Apr 20, 2021 at 17:36
  • @Mike, not for strength. You need to drill into fresh wood. There are some circumstances where filling a hole and re-screwing into it is fine: light loads and pristine wood around it. This is likely not your case.
    – P2000
    Apr 20, 2021 at 17:41
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Thanks for your question. Yes, it is generally better to pre-drill holes big enough so that only the threads of a lag screw grab to help prevent splitting the studs.

However, I've seen people install punching bags with screws that had larger diameter than the ones in your kit, and just as long, without pre-drilling, and the bags are holding fine with 120+ pound kids swinging from them daily for over 10 years. Some of the screws didn't go all of the way in. The studs were split by the screws and it was no problem.

Moral of the story, while pre-drilling is good practice, even if the wood split the TV will probably hold just fine. I wouldn't worry about it.

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    Consider that anecdotal evidence is not any sort of guarantee. If you predrill lags and only one in a million fails, but if you don't and one in a thousand fails then you've increased the risk of failure by a thousand times, even if 999 out of a thousand done this way end up not failing. I've made these numbers up, but certainly the real world numbers will do something like this.
    – J...
    Apr 20, 2021 at 11:40
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    When I installed my upper kitchen cabinet for glassware, the stud cracked, and I was not going to leave it at that. My helper said it would be ok, but when I (jokingly) asked whether they would pay for the damages to the cabinet, granite and flooring should it ever fail, we decided otherwise. Circumstances matter.
    – P2000
    Apr 20, 2021 at 14:46
  • @J... "I've made these numbers up, but certainly the real world numbers will do something like this." Are you referring to the tendency to reduce strength due to the split compared to the non-split at some unspecified difference in probability, or are you referring to your million to thousand ratio?
    – DCommK
    Apr 20, 2021 at 21:02
  • @DCommK Yes - the million to thousand ratio. I don't know what the stats would actually be, but yes, I'm talking about the quasi-random reduction in holding strength that is gambled away by not pre-drilling. In some cases it won't matter, in other cases it will, but in all cases it won't be as strong as a pre-drilled lag.
    – J...
    Apr 21, 2021 at 0:32

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