Selecting and Using Concrete Lag Bolts

Concrete lag bolts are fasteners with a coarse thread that is typically used to fasten wood to concrete or masonry. They tend to be longer than the average fastener with lengths up to 15 inches. Widths carried by home hardware stores for the do-it-yourselfer are usually 0.25 inches to 0.5 inches. Civil engineering and mining lag bolts are longer. These types of bolts have hexagonal-shaped drive heads. Unlike a screw, a lag bolt doesn't have a pointed tip. Instead, it has a smooth base.

How Concrete Lag Bolts Work

Lag bolts for concrete are used with an anchor or insert called a lag. The lag is an expanding insert with a jacket of hard metal that grips the sides of the drill hole. The center part of the lag is made out of a softer metal like zinc mixed with soft iron or lead. The bolt, which is made out of a harder metal, is screwed into the lag, and the coarse threads deform the lag slightly, creating a water-tight secure fastening that's also anti-corroding.

Bolts Must Be Installed Level

Concrete lag bolts that are holding up the frame of wood walls like those in a basement or garage need to be level to make sure the wood sits straight. They also need to be installed in the center of the wall framing, so getting the correct measurement is crucial. To make sure the bolts are level, check with a small pocket level. The bolts also need to be positioned at a specific distance depending on the width of the walls. Concrete lad bolts are placed 1 3/4 inches from the outside edge of the concrete for a 2-by-4 wall and 2 3/4 inches from the outside edge of the concrete for a 2-by-6 wall.

Lag Bolts Can Be Removed from Concrete

Bolts sticking out of a concrete pad that no longer needs lag bolts to support a wooden frame can pose a safety hazard, making removal necessary. There's no need to hire a professional to do this since an average do-it-yourselfer can take on this task with only a few basic tools. Even though the bolt has basically bored itself through the anchor/lag to create a secure seal, all that's needed to remove the bolt is some muscle power and a high-powered electric impact wrench with a socket size to match the concrete lag bolt head.

It's Easy to Determine the Size Needed

It's important to have the correct size of the concrete lag bolt before you begin your project. Determining the needed size is simple. The lag bolt needs to be long enough to go through the concrete and securely penetrate the wood by at least five inches. The diameter needs to be considered as well when choosing a size. A hardware store's staff can help you figure this out, but usually, a 2-by-4 piece of wood will need a concrete lag bolt with a diameter of half an inch.

Can Be Placed in Set Concrete

The insert for concrete lag bolts is usually placed in the concrete when it's wet. Yet it's possible to place a lag bolt in set concrete and hang something heavy on it, like a big-screen TV mounted on a concrete wall for example, by pre-drilling a hole with a high powered drill and masonry bit.