When you are performing structural projects such as timber framing or deck building, you must select the correct lag screw for your projects. A lag screw must be long enough to connect two piece of wood securely, and a lag screw must be thick enough to support weight of the load. If a lag screw is too short, the lag screw will not reach deep enough into the base member to create a strong bond.
If a lag screw dont create a strong bond, the structural integrity of the projects will be compromised. When you choose a lag screw, you must consider thickness of the material, the weight of the load, and the amount of the lag screw that will be inside the receiving timber. Many people focuses on the diameter of the lag screw, but the length of the lag screw is also important for stability.
How to Choose the Right Lag Screw
You should follow the two-thirds rule when you are selecting a lag screw. The two-thirds rule state that at least two-thirds of the total length of the lag screw should be buried inside the base member. Because the threads need surface area to resist being pull out, the lag screw must be buried deep enough to prevent the lag screw from slipping out under pressures.
For example, if you are attaching a deck ledger, you must use a lag screw that is long enough to pass through the thick board and still bury several inches into the rim joist. The diameter of the lag screw determine the strength of the fastener, and the diameter of the lag screw must match the requirement of the load. A thin 1/4 inch lag screw is suitable for light tasks like small bracket, but a 1/4 inch lag screw is not strong enough for structural tasks.
You should use a 3/8 inch or an 1/2 inch diameter lag screw for heavy tasks like stair stringers or pergola rafters. Because thicker lag screws provides more strength, thicker lag screws are necessary for heavy lifting, but thicker lag screws also increase the risk of splitting the wood. To prevent the wood from splitting, you must drill pilot holes before you drive the lag screw into the wood.
If you do not drill pilot hole, the lag screw will expand the wood fibers and cause the timber to split. You must drill two different types of pilot holes. First, you must drill a clearance hole through the top board, and the clearance hole must be the same diameter as the shank of the lag screw so the threads do not grab the top board.
Second, you must drill a smaller pilot hole into the base member so the threads can bite into the base member without cracking the grain. The environment where you use the lag screw will affect how long the lag screw last. If you use a zinc-plated lag screw outdoors, the zinc-plated lag screw will rust.
If you are working with pressure-treated wood, you should use a hot-dip galvanized lag screw. If you are building near a coast where salt air is present, you should use a stainless steel lag screw. Stainless steel lag screws are the best choice for salt air because stainless steel lag screws resists corrosion.
When you install the lag screw, you should use an impact driver and a flat washer. An impact driver help the hex head of the lag screw sit flush, and a flat washer prevents the head of the lag screw from sinking into the wood fibers. If the head of the lag screw sink into the wood, the clamping force of the lag screw will decrease.
Additionally, you can rub beeswax on the threads of the lag screw to make the installation smoother. Using beeswax prevent the lag screw from binding or snapping during installation. If you use the correct length, the correct diameter, and the correct pilot holes, the lag screw will provide a secure connection for the structure.
