Lag screws are one of the most capable fasteners in construction, but they are also one of the most frequently misused — and the consequences of getting the installation wrong can range from a weakened joint to a structural failure that takes months or years to become apparent. Understanding where things go wrong is just as important as knowing what a lag screw is supposed to do.
Pre-Drilling Is Not Optional
The single most common mistake made with lag screws is driving them without the correct pilot hole. Because of their large diameter and coarse threading, lag screws generate enormous torque as they bite into timber. Skipping the pilot hole or using one that is too small causes the wood fibres around the shank to split rather than compress, which actually reduces holding strength rather than increasing it. The pilot hole diameter should match the shank — not the threads — and should extend to the full depth of the screw.
A second hole, slightly wider, is needed for the smooth shank section near the head. Without this clearance hole, the screw cannot pull the two pieces of timber tight against each other, leaving a gap in the joint that undermines the entire connection.
Torque, Angle, and Spacing All Matter
Driving a lag screw at even a slight angle off perpendicular changes the load path through the joint and can dramatically reduce withdrawal resistance. In structural applications this is not a minor detail. Similarly, installing lag screws too close together or too close to the edge of the timber introduces splitting risk, particularly in seasoned or dried lumber that has less flexibility than green wood. Following those specifications is not over-caution — it is the baseline for a connection that will actually perform as intended over the life of the structure.
