Lag screws are large wood screws. Square-headed and hex-headed lag screws are covered by ASME B18.2.1 standards, and the head is typically an external hex. A typical lag screw can range in diameter from 1/4 to 1 1/4 in (6.35 to 31.75 mm), and lengths from 1/4 to 6 in (6.35 to 152.40 mm) or longer, with the coarse threads of a wood-screw.
The materials are usually carbon steel substrate with a coating of zinc galvanization, but for superior corrosion resistance, Stainless Steel is the preferred option. This is because the coating of a galvanized fastener is usually compromised during installation, exposing the carbon steel core to corrosive elements. Even the best galvanized fastener suffers severe corrosion after exposure to harsh conditions.
Lag screws are used to lag together lumber framing, to lag machinery feet to wood floors, and for other heavy carpentry applications. The attributive modifier lag came from an early principal use of such fasteners: the fastening of lags such as barrel staves and other similar parts.
These fasteners are "screws" according to the Machinery's Handbook criteria, and the obsolescent term "lag bolt" has been replaced by "lag screw" in the Handbook. However, to many tradesmen, they are "bolts", because they are large, with hex or square heads.