Structural Screws vs. Lag Screws - Definition
Structural screws and lag screws are two commonly used screw types in the construction field. This article explains the design, production, costs, and other considerations to make when choosing to use structural or lag screws.
Lag screws, also known as lag bolts, are similar to wood screws. They have a larger diameter and length, however, and often have coarse threads. Lag screw heads are most often hexagonal, which is why they are also called lag bolts. They must usually be installed with a wrench, not a screwdriver. The screws are most often used to secure heavy pieces of wood to each other or to other construction materials, such as concrete.
When using a lag screw with concrete or masonry walls, a lag is often used. A lag is an insert for the screw that is made out of a soft metal alloy. As the screw threads into the lag, the lag expands outward somewhat and deforms around the screw threads. This creates a watertight seal around the screw, preventing corrosion and increasing strength.
Image: Lag Bolts - Flickr - James Lee
Structural screws are a relatively new type of fastener. They are often used in place of lag screws because they are much more convenient to work with. The increased convenience comes from the design of structural screws, which eliminates the need for pilot hole drilling and lags. When using with metal, though, the material should still be pre-drilled to avoid damage to the structure's coating. Structural screws are often stronger than lag screws and create a more resilient connection between materials.