How to Install Lag Bolts (Screws)

How to Install Lag Bolts (Screws)

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For those of you who don’t know what lag bolts are, these are kind of like a combination between a normal bolt that uses a nut and a normal screw. In terms of the look of it, it looks like a very large and fat screw that has a hex-shaped head.

In other words, it’s almost like a large bolt with a thread on it for screwing. Now you might be wondering what these fasteners are used for and how you would install one. Well, this is exactly what we are here to teach you today.

When Should You Use Lag Bolts?

Yes, there are many different types of bolts and screws out there, which means that you might be confused in terms of when a lag bolt is used. Well, a lag bolt is used when you need to connect pieces of wood to very large and heavy materials that need to bear intense loads and weight.

So, for instance, if you are building a roof out of solid timber and you’re connecting the roof joists to each other, a lag bolt makes for the perfect candidate, as the threading allows it to take hold easily, the nut on the end keeps everything tight and secure, and the bolt or screw is thick and durable enough to handle extreme loads and pressures.

Generally speaking, lag bolts are used to connect lumber and other heavy materials that are required to bear large loads. On the other hand, you wouldn’t use them to connect a pair of 2x4s that do not need to bear any load.

How to Install Lag Bolts

This might all sound quite complicated, but the reality is that installing lag bolts is actually very simple. It’s really just a big screw with a head of a bolt.

Step 1: Get the Lag Bolts

The first thing that you need to do is to get your lag bolts. In terms of choosing the size, the rule of thumb is that the lag bolt should be 1/4-inch shorter than the accumulated width of the two pieces being fastened. Moreover, for most general-purpose projects, lag bolts that are 5/16-inch in diameter should be more than enough. However, there are also much larger lag bolts out there used for super heavy duty construction purposes, ones as wide as 1-1/4-inch in diameter.

You may want to do some research for the specific task that you are looking to accomplish, just to make sure that you choose the best size of lag bolt for the job at hand.

Lag Bolts

Step 2: Line Up the Materials and Mark

Now, just line up the two materials that you want to fasten together with the lag bolt and make a mark on the spot where the lag bolt is to be inserted. If you want to be extra careful, measure both of the items being fastened, make individual markings where the lag bolt will be inserted, and then make sure that they line up.

An old rule of thumb in construction is to measure twice and cut once, something we recommend adhering to here.

Step 3: Drill a Pilot Hole

One thing that you need to be aware of here is that lag bolts have hex heads, which means that you cannot just stick them inside of a drill like you would with a screw. In other words, you can’t just attach the lag bolt to the drill and then blast it into the wood. You need to first drill a pilot hole.

So, the size of the bit you used to drill the pilot hole that directly depends on the size of the lag bolt. To make things easier for you here we have included a few examples that illustrate exactly what the thread diameter of the pilot hole needs to be in accordance with the size of the lag bolt:

  • 3/8-inch bolt diameter: 15/64-inch pilot hole
  • 1/2-inch bolt diameter: 5/16-inch pilot hole
  • 5/8-inch bolt diameter: 13/32-inch pilot hole
  • 1-inch bolt diameter: 23/32-inch pilot hole
  • 1-1/4-inch bolt diameter: 15/16-inch pilot hole

Once you have selected the proper drill bit, use your power drill to make a pilot hole through both of the materials being fastened. While the hole should be deep enough to be able to accommodate the bolt, make sure not to drill through completely or to make the hole too deep.

Step 4: Insert the Lag Bolt

Using your fingers, insert the lag bolt into the pilot hole that was drilled in the previous step. Although you can use a ratchet or hex wrench to insert it, at this point, you should be able to do it by hand. All you need to do at this point is make sure the lag bolt is inserted straight and that it is holding steady in the wood, ready to be tightened.

Typically, you will also want to put a washer between the screw head and the wood.

Lag Bolt with a Washer

Step 5: Tighten

For this, you will need an impact driver or an impact wrench with the right bit. The key is to tighten the lag bolt enough to be holding the two pieces of wood tightly but not to overtighten them.

While it might seem like the more you tighten the two pieces the better, if you overtighten, you will put stress on the wood and potentially damage it. As such, you should stop the moment the washer stops moving.

Nuts Driver Bits

Mistakes to Avoid, Tips & Tricks

With all the above in mind, here are a few more tips on how to make the process smoother:

  • Always follow proper safety procedures when working with power tools.
  • When using drills, wear eye protection.
  • Don’t make the mistake of thinking that the pilot hole needs to be the same size as the lag bolt (it needs to be a bit smaller than the bolt). If the pilot hole is the same size as the bolt, then the threading becomes useless.


There you have it people, everything you need to know about how to use a lag bolt. As you can see, it’s actually quite easy!