A lag screw is a heavy-duty fastener with a hex head used to secure wood or other materials in structural applications.

A lag screw, also known as a lag bolt, is a large, heavy-duty fastener designed for high-strength connections, primarily in wood construction. It features a hex head and coarse threads that provide strong holding power in wood, engineered lumber, and similar materials.

In construction, lag screws are commonly used to attach ledgers, beams, posts, decks, and heavy hardware such as brackets or machinery mounts. They are typically driven with a wrench or socket and often require a pre-drilled pilot hole to prevent splitting and ensure proper thread engagement.

Proper installation of lag screws is essential for structural performance and safety. Factors such as screw diameter, length, embedment depth, spacing, and edge distance must meet engineering and code requirements. In exterior or moisture-exposed applications, corrosion-resistant coatings or stainless steel lag screws are used to ensure long-term durability.

Despite being commonly called “lag bolts,” lag screws aren’t actually bolts because they don’t use a nut—they thread directly into the material.

• Lag Bolt
• Self-Tapping Screw