What Is a Lag Bolt?

When it comes to securing heavy building materials and structures, the right bolt or screw is key. One of the most reliable options when it comes to these heavy-duty fasteners is a lag bolt—also known as a lag screw—which helps hold everything from wooden beams to outdoor decks firmly in place. But what exactly is a lag bolt, and how do you use it?

Lag screws are used to attach steel shapes or equipment to wood structures. Their design is based on the same principles as traditional anchors, which include the placement of the bolt perpendicular to the grain. In these types of connections, the lag screw’s threading creates friction between the wood and the metal, which helps prevent loosening over time.

Unlike other fasteners, a lag screw has a hexagonal or square head that allows it to be driven by a wrench or ratchet. The bolts also have a shank and threading that allow them to be tightened. The shank is tapered to make it easier to drive into the wood, and the threading creates a connection between the two materials that can be tightened.

While lag screws can be effective for connecting timber members, they are not always suitable for shear applications. In fact, if lag screws are screwed in without predrilling a hole or driven with a hammer, they can crack the wood [1]. This type of damage weakens the clamp force generated by the lag screw and may lead to poor adhesion between the members. Construction managers need to be especially careful when using lag screws to join wood. Lag Bolt