Lag screws are bolts with sharp points and coarse threads designed to penetrate and grip wood fibre as shown in Figure 5.13 below. They are used to anchor metal, or wood, to wood in areas inaccessible to the placement of a nut for a throughbolt, or where an especially long bolt would be needed to penetrate a joint fully.

Although lag screws do have some unique applications, throughbolts are considered to be a more positive means of connection since they are less dependent on workmanship for reliable installation.

The resistance of a lag screw generally increases with the length of the embedded thread portion. However, it is also affected by other considerations such as side plate thickness.

As with other types of metal fasteners, sufficient end and edge distance must be provided to prevent splitting and to provide sufficient area for shear and bearing resistance in accordance with engineering design codes.

Stock sizes of lag screws range from 25 to 400mm (1" to 16") in length and 6 to 25mm (1/4" to 1") in diameter.

Washers

The same requirements for washers apply to those used lag screws as to those used with bolts.

Installation

Two hole diameters are used to prepare a member to accept a lag screw, drill bits of two dimensions must be used. The smaller diameter hole is drilled to accept the threaded portion of the lag screw, and a larger diameter hole is drilled to accomodate the shank portion. 

Figure 5.13: Lag Screws for Wood Construction

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