Lag bolt

A lag bolt (also called a lag screw) is a heavy-duty fastener with a hex head, coarse threads, and a pointed tip, designed to be driven into wood with a wrench or socket. It combines the clamping strength of a bolt with the wood-gripping ability of a screw.

How it works

A lag bolt is driven into a pre-drilled pilot hole in wood. The coarse, deep threads bite into the wood fibres, creating a strong mechanical bond. The hex head allows high torque to be applied with a wrench, and the washer distributes the clamping force. Unlike a standard screw, a lag bolt can be several inches long and is designed for heavy structural loads.

When to use them

• Deck construction: Attaching ledger boards to house framing.
• Heavy shelving: Securing brackets to wall studs.
• Timber framing: Joining large wooden members.
• Structural repairs: Reinforcing joists, beams, and headers.

Why a pilot hole is essential

Driving a lag bolt without a pilot hole risks splitting the wood, especially near edges or ends. The pilot hole should be slightly smaller than the bolt’s root diameter (the solid core, not the thread diameter). This removes enough wood to prevent splitting while leaving enough material for the threads to grip.

Lag bolt vs. carriage bolt vs. machine screw

A lag bolt has wood screw threads and is driven into wood. A carriage bolt has a smooth shank and a square neck that bites into wood — it is secured with a nut on the other side. A machine screw has fine threads and is designed for metal or tapped holes. Each serves a different application.