Lag Screw Installation

I figure some doper is handy with woodworking so I’m posting this here. I am going to install a lag screw into the wall of my garage to hold one end of a security bar. The bar will cross a seldom used door and be connected on the other side by a sliding bolt. The lag screw will be used to allow the security bar to rotate out of the way when the door does not need to be opened. I’ve never used large lag screws before so I’m wondering if I need to partially drill out the 2x4 it will be installed in. I figure since the screw is of a hefty size, it may possibly cause damage to the 2x4 and render it useless. So if I am going to install a lag screw of say 1/2x8", do I need to drill a slightly smaller hole where the screw will be installed to avoid fracturing the 2x4?

Short answer, yes.

Drill a hole the size of the shank, not the threads, of the lag screw (hold a bit up next to the screw and if you can just barely see the shank, it should be just about right). To ensure that you won’t damage the 2x4 that you are going to install it in, try it on a scrap piece of wood. Also, try to hit as close to the center of the 2x4 as you can to lessen the chances of splitting. Don’t drive the lag in, turn it in as you would any other screw. Finally, drill your pilot hole and squeeze some glue into the hole before turning the lag in.

Yes, pilot drill per octothorpe’s suggestion, but:

  1. A 1/2" lag screw in a 2x4 is going to take out abaout a third of the 2x4 - this will not be a corcern unless the structure is weak, or you really need a really strong install - in which case, sand the 2x4 and a scrap of 2x4, and so their sides are both smooth, clean, etc., and glue (carpenter’s glue or construction adhesive) the scrap to the 2x4 - clamp, let cure. now drill the pilot hole between the two glued pieces.

  2. Aside from epoxies, I know of no glue which will bond a lag screw to wood.

I bet those expanding polyurethane glues would do it pretty well. Bt it would be a pity to tear the structure apart to remove the screw someday…

What everyone else said.

And also consider having the new board hinge on the doorknob side of the door and lock on the hinge side. This will give you the strength of the lag screw to offset the relatively weak doorknob and deadbolt on that side of the door.

Also, consider using brackets on either side of the door to hold a board laid across them to block the door. This is stronger design as you can use 2 lag screws for each bracket for a total of 4 instead of one lag screw and an easily defeated sliding bar (unless the sliding bar is on the hinge side). This design is also safer if that door is an escape route in case of fire, people can figure out that the board is just lifted up to remove instead of having to negotiate several deadbolts and latches.

I wouldn’t drill the pilot hole so big. In most cases you should be able to drive a lag screw with no pilot at all. Drilling the shank diameter will just make the bite of the threads weaker. I’d drill a bit smaller than the shank by maybe 1/6"

The 2x4 this will be installed in is going to be in a wall behind sheetrock, not much I can do in the way of preparation.
Zabrain, I considered that but wasn’t able to find a decent bracket. I’ll search harder.
Thanks everyone.

another tip: put some soap on the lag screw first and it will really go in smoothly

This is generally regarded as a bad idea. No cite, but supposedly the soap can contribute to the fastner rusting. Use wax instead.

I would also drill a hole the size of the shank. As octothorpe outlined.

A note of clarification may be in order judging from the comments from Extraneous:

The glue is not meant to bond to the screw, it is to fill any voids and lock the threads to the wood. Secondarily, it will help seal and bond any cracks that develop from the install.

Again, do not drive the lag screw in, it will tear the fibers of the wood and, with a pilot hole smaller than the shank, will split the 2x4, vastly weakening the install.

Putting lags into the edge of 2X stock can weaken the lumber, but these are going into a doubled stud. I don’t think that the lag will cause the structure to fail.

Since the lag is going into the edge of the wood, I would drill to the shank diameter to keep the stud from splitting. I haven’t needed to drill when going into the side of yellow pine lumber, but that doesn’t apply here…

Don’t forget to put a washer on each side of the security bar. Lubricate the washers, security bar, and lag screw with wax or soap, then tighten the lag screw.
The washers will prevent wear and tear on the wood when as and if you use the door.

“Beware the Cog”

As cornflakes noted, a door is normally framed with two studs on each side – you would want to lag into the one furthest from the door, since the inner one is a jack stud and only holds up the door header. Also, this might be an application where bigger isn’t necessarily better – a 1/2" x 8" lag bolt seems pretty excessive unless you are bothered with folks crashing cars into your door.

A 3/8" x 2 1/2" lag run into a 1/4" pilot hole leaves a bit more wood to hold the bolt, reduces the chance of splitting, is less likely to run through the other side of the wall, and will hold back a charging bull. In this application, since the door might need to be used in a hurry, I’d keep the throw-bolt on the knob side, but use the heavier hardware there rather than on the hinge side.