A lag screw is a piece of hardware use to connect wood members together, and a lag screw provide strength to a connection through it’s specific physical design. A lag screw consist of a hex head, an unthreaded shank, and a threaded portion. You use a wrench to turn the hex head, and the unthreaded shank pass through the first wood member, and the threaded portion anchors in the second wood member.
While many people focuses on the threaded portion of the lag screw, the unthreaded shank is important because the unthreaded shank dictates the bending yield strength of the connection. The bending yield strength is the ability of the lag screw to resist bending when a lateral load are applied to the connection. The strength of a lag screw connection depend on the orientation of the wood grain, because wood is an inconsistent material.
How to Use and Install Lag Screws
If you drive a lag screw parallel to the grain, the wood fibers provides high resistance, but if you drive a lag screw against the grain, the wood fibers provide less resistance. Therefore, the orientation of the grain affect the capacity of the lag screw connection. Additionally, the species of wood affect the capacity of the lag screw connection because different wood species has different densities.
Dense hardwoods like Red Oak offer high resistance to pressure, but softwoods like Spruce Pine Fir have lower density and are weaker. When you use a softwood, the wood fibers may compress and fail before the lag screw reach its limit, so you must consider the density of the wood species. The number of wood members in a connection affect the total strength of the connection.
A single shear connection occur when a lag screw pass through one side member and into a main beam, but a double shear connection occur when a central beam is sandwiched between two side member. A double shear connection is stronger than a single shear connection because a double shear connection provide two separate plane to resist the load. Using a double shear connection allow you to increase the capacity of the connection without using larger hardware.
However, larger hardware can increase the risk of splitting the wood, so you must choose the correct size for the lag screw. The diameter of a lag screw affect both the strength of the connection and the ease of installation. A 1/2 inch lag screw provide more lateral capacity than a 3/8 inch lag screw, but a 1/2 inch lag screw also require more careful installation.
If you drive a large diameter lag screw too close to the edge of a board, the lag screw will act as a wedge and split the wood. To prevent splitting, you must maintain proper edge distance and drill pilot hole. You should drill a pilot hole for the unthreaded shank first, and then you should drill a smaller pilot hole for the threaded portion of the lag screw.
Drilling two separate pilot hole ensures that the lag screw can anchor into the wood without splitting the wood. You must use the correct tool during the installation of a lag screw. You should never use a hammer on a lag screw, because a hammer will crush the thread and the crushed thread will reduce the strength of the connection.
You must use a wrench or an impact driver to turn the hex head of the lag screw. Additionally, you must use a flat washer underneath the hex head of the lag screw. The flat washer is necessary because the flat washer distribute the bearing force and prevents the hex head from pulling through the wood.
If you follow these installation step, the lag screw will provide a stable connection for the structure.
