Lag bolts, also known as lag screws, are large wood screws with hex heads that are ideal for projects like installing a new flat-screen TV in your home. While using heavy-duty lag bolts to hang your TV might be overkill, there are some things you really don't want to drop, and that's what we love about lag bolts.
Generally speaking, lag screws are used to semi-permanently attach something to wood after pilot holes have been pre-drilled in the fixed wood beam. It's easy to drive lag bolts with an impact driver or wrench, but you need to pay attention to the final torque of the bolt, as it's not uncommon for the head to pop out due to over-tightening the bolt, which can quickly ruin your equipment.
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Lagoon bolts have a sharp point and coarse threads to provide maximum hold in wood, but must be pre-drilled due to their larger diameter. Extra care must be taken to ensure that the pilot hole is centered on the stud and that the predrilled holes are drilled as deep as the entire screw will ultimately be.
What are square head bolts used for:
roof beam
garage pillars
TV wall mounted
heavy construction
How much weight can a lag bolt bear*
1/4" lag bolt - 2.0" embedment depth, 272 lbs. pure
5/16" lag bolt - 2.5" embedment depth, 368 lbs. pure
3/8" lag bolt - 3.0" embedment depth, 432 lbs. pure
1/2" lag bolt - 3.0" embedment depth, 624 lbs. pure
The threads must be embedded in the side threads and must be located in the middle third of the structural member for maximum strength.
For 5/16" lag bolts, the pullout force ranges from 205 - 307 pounds per inch of thread depth, depending on the wood selected.
The threads must be embedded in the side threads and must be located in the middle third of the structural member for maximum strength.
How to measure lag bolts:
The length is the length from the bearing surface under the head to the very end of the screw
Diameter is the main thread diameter of the screw or the width of the unthreaded shoulder
tensile strength:
Steel Minimum 60,000 psi
Stainless Steel Minimum 100,000 psi (18-8 Stainless Steel)
How to remove a lag bolt with a broken head:
Hammer the center punch in the middle of the broken bolt so that the drill bit has a starting point.
Drill a pilot hole and work slowly using a special extraction drill bit, then reverse the drill bit into the pilot hole. Before attempting to drill the hole, it may be helpful to tap the drill bit containing the extraction bit into the pilot hole with a light hammer. If all goes well, continue reversing until the bolt is removed, or you can use a vise to clamp the bolt to completely remove it.
*How much a bolt can hold depends on many factors, including but not limited to wood type, thread depth, and load angle.