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Lag screws — also known as lag bolts — are heavy-duty fasteners designed for wood-to-wood and wood-to-metal connections that demand high load-bearing strength. Unlike standard wood screws, lag screws feature a coarse thread, a hex or square head, and are driven with a wrench or socket rather than a screwdriver. They are widely used in construction, decking, framing, and outdoor structural applications.
Lag screws are large, externally threaded fasteners with a pointed tip and a hex or square drive head. They are engineered to create strong, lasting joints in heavy timber and structural wood assemblies. The coarse threads grip wood fibers tightly, providing exceptional pullout resistance and shear strength.
⚠️ Key Difference: Lag screws do not require a nut — they thread directly into the base material. A pre-drilled pilot hole is typically required to prevent wood splitting and ensure proper installation.
| Diameter | Thread Type | Head Style | Drive Type | Common Lengths | Material |
|---|---|---|---|---|---|
| 1/4" (6mm) | Coarse Thread | Hex Head | Wrench / Socket | 1" – 4" | Carbon Steel / SS |
| 5/16" (8mm) | Coarse Thread | Hex Head | Wrench / Socket | 1.5" – 5" | Carbon Steel / SS |
| 3/8" (10mm) | Coarse Thread | Hex Head | Wrench / Socket | 2" – 6" | Galvanized / SS |
| 1/2" (12mm) | Coarse Thread | Hex Head | Wrench / Socket | 2.5" – 6" | Galvanized / SS |
| 5/8" (16mm) | Coarse Thread | Hex Head | Wrench / Socket | 3" – 8" | Hot-Dip Galvanized |
🔹 Always pre-drill a pilot hole sized at approximately 70% of the lag screw's shank diameter to prevent wood splitting.
🔹 For treated lumber or outdoor applications, use hot-dip galvanized or stainless steel lag screws to resist corrosion from chemicals in pressure-treated wood.
🔹 Do not over-torque — driving the head flush or slightly below the surface is sufficient. Over-tightening can strip threads or crush wood fibers.
🔹 Use a washer under the head when connecting softer woods to distribute load and prevent the head from pulling through the material.
