Lag bolts fasten wood or similar material to concrete or masonry. The bolts are a two-part system that utilizes an anchor and bolt to hold tight in hard surfaces such as concrete, blocks or bricks. Installing lag bolts requires some specialized tools and techniques but falls within the capabilities of most do-it-yourselfers.
Creating the Hole
This is the part that requires some specialized equipment. You need a masonry drill to create a hole in the concrete. A masonry or hammer drill along with the masonry drill bit expedite the hole-drilling process. The hole should have the same diameter as the anchor portion of the lag bolt.
Inserting the Anchor
The anchor is the working part of the lag bolt system. The anchor is a soft metal insert that fits within the hole and includes a threaded interior to accept the bolt. The length of the anchor must be long enough to accept the entire length of the bolt. You may need to use a hammer to tap the anchor into the hole.
The bolt passes through the wood object and then connects to the anchor within the concrete. As the bolt enters the anchor it causes expansion of the soft metal of the anchor. This expansion fits the metal tightly within the hole. creating an adhesion. It is the pressure between the anchor and concrete that holds the lag bolt to the concrete.
Lag Bolt Limits
Different size lag bolts are rated for different load limits. Use the proper bolt for the anticipated load of the fixture. Use anchors of corrosion-resistant materials and galvanized bolts if the fixture will be used in moist environments. The size of the lag bolt refers to the exterior diameter of the bolt and the interior diameter of the anchor.