lag bolt

A lag bolt is a heavy-duty fastener halfway between a wood screw and a bolt, designed to anchor structural timber to masonry, steel, or other wood with significant holding power. It has a partially threaded shank, a square or hexagonal head sized for a wrench or socket, and a sharp point that cuts its own path without requiring a pilot hole in softer wood. Typical sizes range from 1/4 inch to 1 inch in diameter and 1 to 12 inches long, with the unthreaded portion (the shank) occupying roughly the first third to half of the fastener.

The square head version, sometimes called a coach bolt in older usage, became standard in timber framing, dam construction, and industrial equipment mounting through the 19th and early 20th centuries. The hexagonal head variant gained favor in the mid-20th century as it paired naturally with metric socket sets and power tools. Both designs allow the fastener to be driven with torque applied from above rather than requiring access from underneath, making it invaluable for affixing machinery beds to concrete floors or securing large timber members in place during assembly.

Lag bolts became largely obsolete after the 1970s, displaced by modern anchors and through-bolts that offer more predictable pullout resistance and easier load calculations. Engineers moved toward expansion anchors in masonry and machine bolts with washers in timber work, both of which allow clearer load documentation. The lag bolt's holding power depends heavily on wood grain direction, moisture content, and installation angle, variables that modern fastening standards prefer to avoid or explicitly control.

Corrosion is a persistent weakness. Galvanized lag bolts, the standard offering, suffer from galvanic attack when driven into pressure-treated timber, and plain steel versions rust rapidly in damp conditions like agricultural buildings or outdoor structures. This failure mode made them unreliable for critical load-bearing work once better alternatives entered the supply chain.

You will encounter lag bolts mainly in existing structures built before 1980. Replacing them typically means installing through-bolts with washers and nuts, or (in masonry) drilling clean holes and using modern epoxy or mechanical anchors that provide certified load ratings. Keep them in mind as a diagnostic clue when assessing a structure's age and original design intent.