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How to Install a Lag Bolt (Lag Screw)

Do you often twist the head off lag bolts during installation? Do you suspect low quality lag bolts? Well, you are not alone. I have found this problem to be somewhat common among do-it-yourselfers. However, the problem is not likely related to low quality bolts. It is more likely a result of improper installation techniques. This article will describe the proper ways for installing a lag bolt, or lag screw, in an effort to avoid installation errors and improve the safety and soundness of the wood connection.

Lag bolts are technically called Standard Hex Lag Screws and are referred to as lag screws throughout the wood design code. Lag screws for structural wood connections must meet the requirements of ANSI/ASME Standard B18.2.1. The design strength and installation of standard lag screws is outlined in the National Design Specification for Wood Construction (NDS), which is the model building code for wood construction. The NDS code is published by the American Forrest & Paper Association (AF&PA) through their wood products division, known as the American Wood Council (AWC).

According to the NDS, lead holes (aka pilot holes) shall be bored to specific dimensions to avoid splitting of the wood during installation. These dimensions also help to prevent twisting the head off lag screws during installation. There are two separate lead holes required when installing lag screws, one for the threaded portion of the screw and one for the smooth shank portion of the screw. The lead hole for the shank is called the clearance hole.

Twisting the head off a lag screw is generally caused by a lack of clearance hole, as the lag screw will bind up if only a single lead hole is used for the threaded portion of the screw. As the shank of the lag screw engages the smaller lead hole, it binds to the wood, thereby preventing further movement. Ultimately, the torsional shear strength of the lag screw is exceeded while tightening and the head of the screw twists off.

The following text briefly describes the basic dimensional requirements for each lead hole as indicated in the 2005 NDS code. It is not all inclusive. Always refer to the latest locally adopted NDS code (http://www.awc.org/standards/nds.php) for current requirements, as well as additional information governing the design and installation of standard lag screws.

1) The clearance hole for the shank shall have the same diameter as the shank and the same depth of penetration as the length of the unthreaded shank.
2) The lead hole for the threaded portion of the screw shall have a diameter equal to the following:

a) 65% to 85% of the shank diameter for wood with a specific gravity (G) > 0.6.
b) 60% to 75% of the shank diameter for wood with 0.5 < G ≤ 0.6.
c) 40% to 70% of the shank diameter for wood with G ≤ 0.5.

3) The threaded portion of the lag screw shall be inserted in its lead hole by turning with a wrench, not by driving with a hammer.
4) Soap or other lubricant can be used on the lag screw or in the lead holes to facilitate insertion and prevent damage to the lag screw.

kaiser_blog 1 pic_lag screw
Figure 1: Standard Hex Lag Screw & Lead Holes Diagram

The NDS states that lead holes or clearance holes are not required for lag screws 3/8″ in diameter and smaller. However, I would always recommend using lead holes that follow the guidelines presented in the code, regardless of how small the lag screw diameter is.

Following these guidelines will allow you to confidently install lag screws and provide a safe and sound wood connection. In doing so, you will avoid the hassles, extra cost, and down time associated with split lumber or shearing off the head of a lag screw.

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