A lag screw is a large, heavy-duty wood screw with a square or six-sided (hexagonal) head. It is used because of its strength. A lag screw's ability to resist "pull out" or withdrawal far exceeds that of a common screw or nail.
Nails and Screws
Nails slide between wood grain, and builders depend upon the grain's compressive force to resist the nails coming loose when pressure is placed upon them. As time passes, wood grain gradually loses its strength, and with enough outward pressure nails in the grain can slide out. Often that happens with no warning. All wood screws bite into the wood. forcing a bond between the screws and wood. With their out-sized, deep threads, however, lag screws create nearly a permanent bond.
A lag screw can have as much as a nine times greater bonding factor to wood than a regular nail. If a lag screw with a 5/16-inch shaft is installed correctly, it can withstand pull-out, or withdrawal, of up to 212 pounds in close-grain hem, fir and redwood, and up to 266 pounds in larch Douglas fir, according to information from the American Wood Council. A lag screw's pull-out capacity is strongest with southern pine, at 307 pounds. Each additional inch of screw thread adds the same amount of strength to the bond.
A lag screw must be seated properly to maximize its potential strength. The screw is only as strong as the wood in which it is placed.
Lag screws cannot be on the ends or sides of a piece of wood. Instead, they must be at least one-third of the way toward the middle of a wood piece. A lag screw's head and washer must be screwed flush to the surface but not overtightened; overtightening breaks the wood fibers underneath. If a spacer board is used with a lag screw, then the lag screw must be long enough to pass through the spacer and into underlying support wood.