How to Use Lag Screws in Metal

Lag screws are common fasteners

Lag screws are specially designed screws that are used for the purpose of joining wood to wood or metal to wood. The distinction between a lag screw and a lag bolt is whether the device creates its own threads as it is tightened. Lag screws, by definition, create their own threads into the wood in which they are attached. To use a lag screw in metal, the lag screw must be inserted through a hole in the metal component, then fastened to a piece of wood beneath the metal -- thus affixing the two pieces together.

Things You'll Need

• Tape measure
• Electric drill
• Eye protection
• Drill bits (for both metal and wood drilling)
• Hammer
• Nail punch
• Hex head socket or Crescent wrench

Instructions

1. Determine the correct size and quantity of lag screws that will be needed for your project. Size is determined by measuring the combined thickness of the two components to be joined together using a tape measure. The lag screw should never be allowed to fully penetrate both materials, so make sure it is a bit shorter in length than the combined width of the materials being joined.

2. Estimate the thickness of the lag screw (the width of the shank) so that it will be sufficiently strong to accomplish the task desired of it. Too thin a shank, and the screw may snap under stress, too thick a shank and the screw will take up extra room that may hinder other things attached to the wood or metal. There is no set rule for determining the correct size shank to use, however, some jurisdictions do set minimum allowable shank sizes to be used in residential and commercial construction.

3. Use a properly sized washer with the lag screw you have selected to use, and allow at least one per lag screw used. Washers are required to dissipating the downward force under the head of the lag screw. Wider washers will spread the downward force across a wider surface area preventing damage to the finish of the material being fastened. The washer must be wider than the head of the lag screw and the washer hole must be only slightly larger than the shank of the screw to derive the most benefit washer use.

4. Make sure the metal component is placed securely and firmly against the wood that it will be screwed to. Using a nail punch and hammer, tap firmly where you wish to place the lag screw so that you form a small dent on the metal surface.

5. Use an electric drill with a metal cutting drill bit attached to drill a small hole through the metal surface. Replace the drill bit with a slightly larger one and enlarge the hole. Repeat this process until the hole is slightly larger than the shank of the lag screw. Next, replace the the drill bit with a wood cutting bit that is half the diameter of the shank of the lag screw. Drill a pilot hole in the wood where the metal hole is located. Only drill to a depth of 3/4 of the length of the lag screw.

6. Insert a flat washer onto the lag screw. Thread the screw through the metal hole and into the wood. Hand tighten it as much as possible. Using a hex head socket or crescent wrench, tighten the lag screw by turning it in a clockwise direction. Align the materials being fastened before you tighten the lag screw all the way.

References

• Heavy Connections: Lag Screws
• Logology Fasteners: Fasteners Turn Logs Into Walls
• Montgomery County Department of Permitting: Deck Details