lag screw
A large metal wood screw with a head that takes a wrench.
lag screw: the wrench-driven fastener for heavy timber
A lag screw is a large fastener that drives into wood with a wrench or socket, not a screwdriver. It has a square or hexagonal head, a threaded shank that tapers toward a sharp point, and typically measures 1/4 inch to 1 inch in diameter and 1 to 16 inches long. The wrench head allows you to apply high torque without stripping the driving tool, which makes lag screws suited to fastening heavy loads in timber framing, equipment mounting, and structural repairs.
Lag screws come in two main head styles: square head (the traditional pattern) and hex head (more common in modern supply). Material is usually steel, galvanized for outdoor and damp conditions. Shank diameters follow standard sizes (5/16 inch, 3/8 inch, 1/2 inch are common), and the threaded portion occupies roughly the bottom half to two-thirds of the screw length. The unthreaded shank just below the head acts as a bolt, pulling wood fibers together without splitting as the threads bite in.
Installation requires drilling a pilot hole to the root diameter of the threads; skipping this step in dense or dry wood invites splitting or shearing off the screw. A socket wrench or open-end wrench sized to the head is necessary, and high torque during insertion can bend or snap the fastener if wood is too hard or the hole too small. Once seated, a lag screw does not back out easily under vibration, making it reliable for machinery bases, deck ledgers, and timber-to-timber joints in industrial settings.
The term lag likely derives from an old meaning: to fall behind or move slowly. Some sources suggest it once referred to a wooden peg driven into stone, later applied to the slower insertion rate of this larger fastener compared to common nails. The wrench-head design distinguishes lag screws from wood screws (which take screwdrivers) and from carriage bolts (which have a round head and a square neck underneath). In modern timber framing and equipment installation, lag screws remain the go-to fastener when structural strength and resistance to withdrawal matter.