Lag Bolts (Coach Bolts)

Introduction to Lag Bolts (Coach Bolts)

In the fastener industry, the terms "lag bolts" (more accurately lag screws) and "coach bolts" (commonly known as carriage bolts) describe heavy-duty fasteners primarily used in timber construction. Though often conflated, they serve distinct engineering purposes. Coach bolts feature a domed head with a square anti-rotation neck and rely on an opposing nut to distribute structural tension in through-hole connections. Conversely, lag bolts feature a heavy hex head and coarse, deep threads with a gimlet tip. They transfer loads entirely through thread engagement, designed to maximize pull-out resistance when driven directly into pilot holes in blind applications.

Materials and Finishes

Because these heavy-duty fasteners are frequently exposed to outdoor environments and the moisture inherent in wood framing, they are constructed from resilient materials:

• Carbon Steel: The standard workhorse material, providing excellent sheer strength for heavy timber framing.
• Stainless Steel: Offers premium longevity and corrosion resistance, preventing wood discoloration in high-moisture outdoor applications.

To combat environmental degradation, they are supplied with robust finishes:

• Bright Zinc Plated and Yellow Zinc for standard indoor or mild outdoor protection.
• Hot-Dip Galvanized for superior, thick-coated corrosion resistance in demanding exterior structural applications.
• Passivated finishes specifically applied to stainless steel hardware.

Technical Specifications and Standards

Lag and coach bolts are engineered for heavy load-bearing performance.

• Head and Neck Styles: Coach bolts utilize a smooth domed head with a Square Neck to prevent rotation during nut tightening. Lag bolts utilize a hex head for high-torque wrench installation.
• Thread Types: Lag bolts feature coarse, deep, tapered wood screw threads with a pointed gimlet tip. Coach bolts utilize standard machine threads.
• Grades: Available in strength classes such as 4.6, 4.8, 8.8, A2-70, and A4-70.
• Standards: These fasteners conform to major international standards. Coach bolts generally align with DIN 603 and ISO 8677. Lag bolts are typically governed by DIN 571. Additional applicable specifications include ASTM F593, ASTM A449, and ASTM A307.

Common Applications and Industries

The robust nature of these fasteners makes them essential for high-stress wood applications:

• Heavy Timber Framing: Critical for joining large beams, rafters, and posts in residential and commercial construction.
• Outdoor Infrastructure: Used extensively in bridge construction, decking, pergolas, and retaining walls where galvanized finishes prevent weathering.
• Wood-to-Concrete Anchoring: Lag bolts are frequently paired with masonry shields to anchor heavy timber or metal plates directly to concrete or masonry.

Conclusion

Whether utilizing the high pull-out resistance of a threaded lag bolt or the clean, anti-rotation clamping force of a coach bolt, these fasteners are indispensable for heavy carpentry and construction. By offering robust material strength and thick protective finishes, lag and coach bolts ensure secure, long-lasting structural integrity for demanding timber and masonry applications.