# Question about lag screws into studs



## TravisN (Jan 15, 2019)

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:


----------



## TravisN (Jan 15, 2019)

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?


----------



## Kazooman (Jan 20, 2013)

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.


----------



## wapakfred (Jul 29, 2011)

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.


----------



## controlfreak (Jun 29, 2019)

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.


----------



## Delete (May 19, 2017)

+++ 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.


----------



## Lazyman (Aug 8, 2014)

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.


----------



## TravisN (Jan 15, 2019)

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!!


----------



## SMP (Aug 29, 2018)

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.


----------



## ibewjon (Oct 2, 2010)

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.


----------



## TravisN (Jan 15, 2019)

> 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.


----------



## TravisN (Jan 15, 2019)

> 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!


----------



## LeeRoyMan (Feb 23, 2019)

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.


----------



## ibewjon (Oct 2, 2010)

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.


----------



## GR8HUNTER (Jun 13, 2016)

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 :<))))))))


----------



## MrUnix (May 18, 2012)

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


----------



## ibewjon (Oct 2, 2010)

Hitting anything hidden in walls is bad, but electricity hurts the most!!


----------



## theart (Nov 18, 2016)

> 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.


This right here. I would also chamfer the wall sides of the pilot holes a little with a coutersink. That way, any wallboard that gets churned up (like in the second to last photo) has somewhere to go.


----------



## WoodenDreams (Aug 23, 2018)

This is overkill, I would have used 3 1/2" deck screws with torx heads. No pilot hole would have been needed, except for the ends of the 2×4.


----------



## therealSteveN (Oct 29, 2016)

Not being there, so I can only guess, but if the Lag isn't going all the way in, AND pulling the board tight behind it I can only think of 3 possibles.

1, You are plain missing center of the stud, and after having gone through the side you don't keep having pull as the lag spins.

2, You are hitting the stud squarely, but the drill you are using doesn't have enough azz to force the bit in quick enough to pull tight? I don't think this is too likely, but a maybe, if 1 and 3 are certain not to be it.

3, You aren't allowing the tool to just slam the lags home. You are pulling back, because you think you will drive through the board. I think this is it, and maybe a little of not enough azz on the drill combined.

I do these all the time, and have never had an issue in literally thousands of lags shot into 2x material, and then into studs hidden in a wall behind drywall.

I use one of these, and yes it a HF special… There is a guy I used to run with, a published woodworker that makes items for museums, Really fine woodworker. He claims to have been a carpenter. Of that I am not sure, he was shocked when I told him what I planned to do. After I did several of them, he was LHAO, then in a bit he was lemme see that thing. I'd created a Monster…...










No pilot hole needed, or helpful. If anything it just means you have to drill the exact same angle twice in a row. Using a socket head to match the bolt. Have someone hold up the 2×4 board, or tack it where you want it with a plain screw or 3. Mark out your studs, Find the center, if you have difficulty doing this exactly stay very close to your ledge board, and make repeated penetrations through the drywall, and feel out the exact edges of the studs. THIS IS IMPORTANT, not hitting and staying in the stud are where a problem can occur. An ice pick is great for the penetrations, and you have some feel with the handle to know you are bumping, or not bumping the stud. You will cover all of that when you hang your item.

You had a real concern, crippling the studs interior in your walls, or penetrating electrical wires, or water pipes. Finding electric is easy, they are the studs where the boxes are hanging on the walls, The lines are usually almost always 16 to 22" off the floor, and then going up to light switches, or other utilities. In most rooms outlets, and lines are going to be really close, you can use the ground, but it sounds like what you are doing is all going to be well above the lines.

If you are not sure, buy that app for cell phones that shows the inside of the wall, they do work.

So in real use, above the outlets, you usually won't find much, unless it is a wall back to a bathroom, or kitchen, then you need to find water lines, same phone app…..

So the important part is hitting center of stud every time, straight in, no angle dangle stuff, and I use 3 1/2" so they do keep driving, and believe me that drill will suck a 2×4 to the wall right now.

Your split board split because you are way too close to the end of that stud. I stay 12" minimum away from ends, and I do not have a splitting problem, unless I miss the center of the ledger board. This is a stupid, I'm not paying any attention at all mistake that I make once in a while in spite of knowing better.

Big bonus tip: To keep from overdriving the lags, which actually you can do, put a big honkin fender washer on every lag, they are twofold help. They keep you from over penetrating, and that keeps the 2x from splitting because of a wedge effect. In soft woods the lags penetrate easily if they have enough power behind them, the split happens when the bigger non threaded head comes blasting into the board.


----------



## kelvancra (May 4, 2010)

Quick note:

To stop the 2x's from cracking, when tightening the lag bolts, clamp them, temporarily.


----------

