Lag Screw: Wednesday's Word(s) Of The Week

Lag screws are standardized, large diameter, partially threaded screws commonly used in wood construction. Because dimensions are set by an ANSI standard, lag screws are readily available from multiple manufacturers with hot dip galvanized finish at relatively low cost. Design values and dimensions are available directly in the National Design Specification for Wood Construction.

Lag screws do require special pre-drilling, equal to the unthreaded body diameter and a fraction of the threaded diameter for the unthreaded and threaded lengths of the screw, respectively. This pre-drilling is typically done during shop fabrication, or clearly described if done in the field.

The unthreaded portion of lag screws can be "full-body diameter" (equal the lag size), or "reduced body diameter." This has an impact on the screw shear capacity and drilling requirements. Unalam only supplies full-body diameter lags on our projects. Screws come in ½" increments up to 6" long, 1" increments up to 12" long, and 2" increments beyond 12". Standard thread length is half the fastener length + ½", up to 6" of thread total.

You may have heard the term "lag bolt" before. This is sometimes used to describe a lag screw because the diameters and hex heads of lag screws are similar to bolts. To avoid any confusion, the term lag bolt should be avoided.