How to install lag screws

Written by emily patterson
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Lag screws, also called lag bolts, have a hex head and a sharp end with coarse threads. They are the fastener of choice in wood applications. Square-head lag screws are also available. The shape of the head, however, does not change the function of the fastener. Standard metals used to fabricate lag screws are steel with zinc, stainless steel and hot-coated galvanised steel. Standard lag screws range in size from ¼ inch in diameter to 1 inch long and larger.

Skill level:

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Things you need

  • Clamps (optional)
  • Lag screw
  • Washers (optional)
  • Drill
  • Hex-head screwdriver or drill bit the appropriate size for the lag screw
  • Drill bit smaller than the root diameter of the lag screw
  • Safety glasses or goggles

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  1. 1

    Align pieces to be fastened and either hold or clamp into position.

  2. 2

    Use a drill and drill bit to drill a pilot hole through the materials to be fastened where you will install the lag screw.

  3. 3

    Use the appropriate hex-head screwdriver bit to screw the lag bolt through the materials. Use a washer or locknut to distribute the pressure of the screw head to prevent it from digging into the wood and becoming loose over time. If you are using a washer or locknut, put the lag screw through the washer or locknut before installing the lag screw.

Tips and warnings

  • Rub soap on the threads of lag screws, and they will screw in more easily.
  • Always wear safety glasses or goggles when working with power tools.

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