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What is a Lag Screw?

By Lori Kilchermann
Updated May 17, 2024
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A lag screw is a large, coarse thread screw with a hex head that is used to tighten or remove it. A traditional screw uses a screwdriver to tighten or loosen it, unlike the lag screw, which uses a wrench. Much like a bolt, the lag screw is used in situations where the utmost in strength and clamping force is mandated. Projects such as securing a deck to the wall or foundation of a home typically call for the use of lag screw-mounting construction. Mounting gates, swinging doors and even garage door tracks are typical lag uses.

When using a lag screw to fasten an object such as a wood plank to another wood structure, a pilot hole should be drilled through the two boards first. The pilot hole allows the lag screw to be screwed through the first board and into the second without splitting either of the boards. Attempting to drive the lag screw through the first board without a pilot hole will usually result in a split or cracked board that will require replacement. It is a good practice for individuals to use a flat washer under the head of the lag screw to prevent the head from being sucked into the wood over time.

One of the benefits of using a lag is that it can be tightened far beyond that of a standard screw. By placing a wrench on the hex head of the lag, the screw can have an enormous amount of pressure placed on it without fear of stripping the screw head. The hex head also permits for easy removal even after a great deal of time. The use of a wrench in place of a screwdriver also allows manipulation of the screw in much tighter areas than a screwdriver is able to be used in.

The typical lag bolt or screw is chrome-plated or covered with a bright galvanized finish, which prevents rust and corrosion from weakening or breaking the screw. When used with pressure-treated lumber and a lag-type fastener, the average home deck can survive for years with very little maintenance. The great strength of a lag screw also makes the fastener a wise choice when mounting gate hinges to a wooden post. The weight and movement of a heavy iron gate will not easily pull the lag free. The gate will likely remain tight for years with no need to re-tighten the hinge screws.

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Discussion Comments
By Logicfest — On Apr 16, 2014

@Terrificli -- in some applications, the washer has been taken to the extreme. There are some out there that have "teeth" that clamp into the wood as the lag screw is tightened. That design serves as more assurance the lag screw will remain in place and the connection will stay secure.

By Terrificli — On Apr 15, 2014

It isn't just good practice to use a washer with a lag screw when working with wood -- it's downright essential. That is because so much force can be applied to a lag screw that it can be pulled through the wood if too much force is used.

That washer spreads the pressure out over a larger area and is, therefore, critical to cranking the lag screw up to its maximum pressure -- the whole idea behind using such a fastener in the first place.

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