Lag Screws: What They Are and How to Use Them

What is a Lag Screw?

A lag screw – also referred to as a lag bolt or coach screw – is an extremely durable, high-strength type of fastener. In fact, lag screws are among the toughest types of fasteners. Most often made from steel and stainless steel, Bossard offers lag screws that are zinc-coated or hot dip galvanized. Both types of coatings provide corrosion protection.

Lag screws are only available with hex heads, which improve grip and provide excellent torque transfer when tightened with a ratcheting socket or wrench. Lag screws are much larger than typical wood, self-drilling, or sheet metal screws – at a minimum, one-inch long, and ¼-inch thick.

What are Lag Screws Used for?

These extremely sturdy fasteners are usually used to connect heavy lumber or other heavy materials that must bear an intense load. However, they can also be used on composite materials, stone, concrete, and certain types of metal. A common outdoor application has lag screws being used for composite decks.

So, what other differences are there between a typical wood screw and a lag screw? And more importantly, how are lag screws used?

Lag screws also use a nut to add extra strength and security to help hold things together. Used for intense load applications, lag screws can support a much heavier load than the average sheet metal or wood screw. Another difference is that lag screws are only available with hex heads, while more common wood and sheet metal screws are available in multiple head types.

How to Install Lag Screws

With the sole option of a hex head, installation of the screw will require a nut driver or ratchet. The right driver bit and a power drill can be used to finish the tightening of the lag screw. The hex-head lag screw is designed that way because hex-headed fasteners are made to hold up in applications where a lot of torque is necessary. Lag screws have sharp lead thread points and are available in steel/zinc, stainless steel, and steel/hot dip galvanized.

Here are the steps involved in using lag screws to support heavy loads for large pieces of wood, lumber, or composite wood materials:

1. Select Your Lag Screw: Your lag screw length should be ¼ to ½ inch shorter than the total width of the two materials you are joining together.
2. Align the Wood or Composite Wood Substitutes: Once you’ve selected the ideal length, you’ll need to align the material with a clamp. If needed, use two clamps to ensure that the two pieces of lumber or composite are properly aligned and tight.
3. Drill Pilot Hole: Once everything is secure, drill a pilot hole using a bit with a slightly smaller diameter than the lag screw that will be used. When drilling the hole, make sure to drill through the area where the screw will eventually be.
4. Drill Countersink (Optional): This fourth step is optional. While the hex head of the lag screw helps improve torque transfer, its head is unable to lie flush against the wood. This isn’t a problem if you’re installing lag screws on posts. However, it can be unsightly in other applications. If you want the lag screw to be flush with the wood, then you’ll need to drill a countersink. This is especially beneficial if you decide to cover the countersink hole with some wood filler to hide the lag screw.
5. Apply Lag Screw and Cover (Filler or Wood Plugs): A ratchet socket is the best tool for applying a lag bolt. Its long handle is a more ergonomic solution to applying the torque to the required bolt. If you’ve drilled a countersink, then you’ll simply torque the lag screw down past the countersink. If you want to hide the lag screw, then you can create some wood plugs to fill the countersink hole or use a wood filler or ready patch.

Final Notes on Lag Screw Installation

The right driver bit and a power drill can be used to finish tightening the lag screw if you choose not to use a wrench or ratchet socket. The hex-head lag screw is designed that way because hex-headed fasteners are made to hold up in applications where a lot of torque is necessary.