4 Tips for Drilling a Lag Screw

A pile of hexagonal lag screws sits on a white background.

A lag screw is the poor man's concrete anchor. They are thicker than ordinary screws, have a hexagonal head, and they are used in a myriad of materials to create structural integrity where it is needed most.

Once one is installed, it takes a brunt of the weight and removes it from the structure itself. You'll often see lag screws on patios, porch swings, and entertainment centers. The following tips will help you so that using a lag screw is not so difficult.

1. Always Use a Pilot Hole

A pilot hole is a hole that is drilled into the material you wish to use a lag screw on. The pilot hole serves as a guide for the screw to prevent it from going off course or splintering the material it is being installed in. If you do not use a pilot hole with a lag screw, you will find that they tend to go in crooked, which can prevent them from doing their job properly.

Drill a hole into the material that is smaller than the diameter of the lag screw. The smaller the pilot hole, the more effective the lag screw will be. You also do not have to go very deep. A good rule is to drill your pilot hole a third of the length of the screw.

2. Measure

When working with any material, you should never use lag screws that are not proportionate. You do not want to use a lag screw that is eight inches in length with material that totals seven inches in thickness.

Always measure the materials that are meant to be joined; the length of the lag screws used should not be more than half the total thickness of this measurement. For example, if your material totals 10 inches thick, then using a lag screw that is five inches long will be more than adequate for your needs.

However, you may use slightly longer screws if the finished project will be subject to more than normal weight or use. The thickness of the lag screws also matters. If they are too thick they can easily splinter your material.

3. Use the Right Tool

Because lag screws are not the same as normal screws, they’re not installed with the exact same tools. The hexagonal head of a lag screw means you will need a special, hexagonal bit in order to install them, tighten them, or remove them. Using pliers or a wrench can get the job done, but it will never be as secure as it would be using a powered device.

4. Punch It In

This doesn't mean use a hammer, but refers to how you actually install the screw. Most other screw types can be installed slowly. A lag screw, however, is best installed in one quick burst. This prevents the screw from wobbling too much and splintering the wood.