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Question about lag screws into studs

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Forum topic by Travis posted 07-21-2019 06:26 AM 546 views 0 times favorited 20 replies Add to Favorites Watch
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Travis

264 posts in 218 days


07-21-2019 06:26 AM

Hey all,

I’m building the frame for my built-in bench seating in my dinette. I’m using 3/8” x 2.5” lag screws to attach 2×4’s to the studs in the wall. There are some electrical wires running through the studs so I wanted to minimize the chance of me piercing one, hence the 2.5” length.

I’ve never used lag screws or attached 2×4s to my wall before, so I don’t know the cause of my trouble.

I am using a forstner bit to sink the lag screws into the 2×4, and the lag screws clearly enter the stud in the wall. But I can’t pull the board tight to the wall. They stick out about 1/4”. The screws are tight and one board actually split when I tried to keep tightening. The boards feel very sturdy, but I’m puzzled/concerned why they don’t get sucked to the wall. I estimate they are probably about .75” into the studs (2.5” length, about 1” in 2×4 after sinking .5”, .25” between 2×4 and wall since they aren’t pulled flush, and .5” in drywall). That’s probably not as much as they should be, so maybe I need to buy longer lag screws. But it’s really frustrating that they won’t pull the boards tight. Am I misunderstanding something? Shouldn’t they pull flush to the wall?

Any ideas?

Here are several pictures to illustrate:

One wall:

The other wall:

Smaller gap:

The worst gap:

The board split when tightening:

-- The plan is wrong; my finished piece is right.


20 replies so far

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Travis

264 posts in 218 days


#1 posted 07-21-2019 06:54 AM

I’m wondering if it’s because my drill bit is not long enough to drill a pilot hole all the way through. I’ll mount the first screw and then pre-drill the rest with it mounted so that my pilot holes in the board and the wall line up. My drill bit can’t reach through the 2×4 and deep into the stud. Could it be that when the pilot hole ends about .5” into the stud that the lag screw won’t continue to drive through?

-- The plan is wrong; my finished piece is right.

View Kazooman's profile

Kazooman

1333 posts in 2403 days


#2 posted 07-21-2019 11:03 AM

Yep, the threads of the screws are biting into the 2×4. You need a pilot hole all the way through that allows the bolt to just slide on through.

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Fred Hargis

5642 posts in 2944 days


#3 posted 07-21-2019 11:08 AM

I think your problem is that the hole in the 2x stock isn’t large enough. If true, the lag threads are catching the 2x stock and they are tight when the head hits the washer….it won’t turn anymore, and the 2x still isn’t flush to the wall. Now if this is the case, the answer is simply to make the shank hole big enough that the threads don’t catch…if it’s not the case it’s just another time I’m wrong.

-- Our village hasn't lost it's idiot, he was elected to congress.

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controlfreak

106 posts in 52 days


#4 posted 07-21-2019 11:12 AM

The pilot hole in the surface stud needs to be larger or if you get lag screws that are not 100% threaded. Some lag screws will have a smooth range near the head so they don’t bite into the material you are securing to he wall studs.

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Delete

439 posts in 823 days


#5 posted 07-21-2019 12:02 PM

+++ Ditto with everyone. If the lags are 3/8, you need to drill the 2 X 4’s for 3/8” and the counterbore’s the same size as the washers. The lags are tightening up on the 2 X 4’s and splitting them, rather than pulling tight to the wall. You may have to go 1/2” longer on the lags to keep them from pulling out of the wall. Will still be ok with your wiring if it is well centered on your studs.

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Lazyman

3686 posts in 1838 days


#6 posted 07-21-2019 02:38 PM

I would probably start with straighter 2×4’s as well. Hopefully, your wall isn’t that wavy. I would also want a little longer bolts to get more of it into the wood but that may just be me. The wires generally run about the same level as the outlets. I think that code in most places dictates the height they need to run. And since you are bolting a 2×4 there anyway, you could just cut or drill a small hole in the sheetrock next to a couple of studs that will be covered by the 2×4 so you can see where the wire actually is.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

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Travis

264 posts in 218 days


#7 posted 07-21-2019 03:12 PM

Ahhh…. That makes so much sense! I thought I needed the same size pilot hole in both materials. K, I’ll make the pilot hole in the 2×4s large enough for the lag screw to just slide through. I’ll also get longer lag screws.

Thank you!!

-- The plan is wrong; my finished piece is right.

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SMP

1310 posts in 357 days


#8 posted 07-21-2019 03:25 PM

I usually use 3 1/2” bolts, which usually have no threads by the head. Studs where electrical or pipes run should have metallic plates over them. You’ll know right away if you are drilling into them(just use a regular bit, no need to waste a forstner on it). Another trick is once it catches into the stud, slowly back the bolt out while pushing pressure on the stud, once the stud goes flush against the wall, then retighten the bolt. May need to repeat until flush.

View ibewjon's profile

ibewjon

811 posts in 3244 days


#9 posted 07-21-2019 04:08 PM

There is no code as to where the wire is run. Above, below, anyplace is ok. Yes there SHOULD be protection plates over the wire, but don’t bet on it. Get a stud finder that pucks up live wires in the wall.

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Travis

264 posts in 218 days


#10 posted 07-21-2019 04:17 PM



There is no code as to where the wire is run. Above, below, anyplace is ok. Yes there SHOULD be protection plates over the wire, but don t bet on it. Get a stud finder that pucks up live wires in the wall.

- ibewjon

I am fortunate in that I have a picture from when it was under construction. I can see the electrical wiring (though no measurement as to it’s height). I can also see which studs have a metal plate on them. It appears that only studs where the wire is closer to the surface have a plate on them. Several do not.

-- The plan is wrong; my finished piece is right.

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Travis

264 posts in 218 days


#11 posted 07-21-2019 04:18 PM



I usually use 3 1/2” bolts, which usually have no threads by the head. Studs where electrical or pipes run should have metallic plates over them. You’ll know right away if you are drilling into them(just use a regular bit, no need to waste a forstner on it). Another trick is once it catches into the stud, slowly back the bolt out while pushing pressure on the stud, once the stud goes flush against the wall, then retighten the bolt. May need to repeat until flush.

- SMP


Thanks for the tip!

-- The plan is wrong; my finished piece is right.

View LeeRoyMan's profile

LeeRoyMan

211 posts in 178 days


#12 posted 07-21-2019 05:32 PM

Personally this is overkill, nothing wrong with overkill, but you don’t want those 3/8” lags to split the studs.
Your pilot holes will help with that.

In my opinion 1- 1/4” lag on each stud would have been plenty.
If I were doing it, for me, I would have used 2 #10×3 1/4” screws in each stud.

That said, making your holes a little bigger and you will be homeward bound.
Be sure to post your finish project.

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ibewjon

811 posts in 3244 days


#13 posted 07-21-2019 06:09 PM

The plates are only REQUIRED where the wire is 1 1/4 inch from face of stud or less. Written for drywall screws, not 3” lag screws. I would use them everywhere, but conduit is by far the best choice.

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GR8HUNTER

6340 posts in 1164 days


#14 posted 07-21-2019 07:18 PM

it could be possible your pilot hole is to big into studs behind the wall and you are not getting a good bite into it just my 1 cent :<))))))))

-- Tony---- Reinholds,Pa.------ REMEMBER TO ALWAYS HAVE FUN

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MrUnix

7446 posts in 2650 days


#15 posted 07-22-2019 12:04 AM

Lots of good advice, but the simplest way is you either want to make your through holes on the 2x’s bigger so they don’t engage the bolt threads, or get some lag bolts that have a smooth shank where it goes through the 2x’s so the threads only engage the studs in the wall.

Side note: Don’t forget that there is more than electric in them walls! I once put in a garage workbench with 2x’s lagged to the wall, only to find out a year and a half later that I hit the A/C condensate line when drilling for them. Only found out when the condensate line backed up and started leaking inside the wall where the hole was.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

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