What Are Hex Lag Screws?
Hex lag screws—often called “lag bolts”—are heavy-duty wood fasteners with a hex head and deep, coarse threads. They are driven with a wrench or socket and are used to fasten wood to wood, or wood to masonry when paired with the appropriate anchor. Unlike machine bolts, lag screws thread directly into the base material and are not used with a nut.
What Are Hex Lag Screws Used For?
- Decking, ledger boards, and structural wood framing (per code/engineering).
- Mounting posts, brackets, pergolas, and heavy hardware to wood.
- Securing equipment, fixtures, and plates into wood members.
- Wood-to-masonry attachment when used with a compatible masonry anchor or shield.
Benefits of a Hex Lag Screw
- High holding power: deep threads bite into wood for strong, long-term connections.
- Positive driving: hex head allows high torque with a wrench or impact driver.
- Versatile options: multiple diameters, lengths, and corrosion-resistant materials.
- Clean installation: wide selection of finishes for indoor or outdoor projects.
Which materials are lag bolts made of?
- 304 / 18-8 Stainless: general corrosion resistance for most outdoor uses.
- 316 Stainless: upgraded resistance for coastal/marine or chemical exposure.
- Hot-Dip Galvanized (HDG): heavy zinc coating for exterior and structural applications; pair with HDG washers.
- Zinc Plated Steel: economical for dry interior environments.
- Silicon Bronze: corrosion resistance and warm appearance for marine/architectural work.
Is it called a lag bolt or lag screw?
Lags are often called "lag bolts" and "lag screws" and the terms are used interchangeably. The correct term according to ANSI and ASTM standards (and most engineering specifications) is “lag screws.”
They are technically screws, not bolts, because:
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They are driven with a wrench or hex head, but they do not use a nut.
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They form their own threads in wood or lag shields (unlike bolts, which are used with a nut or tapped hole).
Sizes & Selection
Lag screws are specified by diameter × length (e.g., 3/8" × 4"). Common diameters range from 1/4" to 1/2" (larger sizes are available). Length is measured from under the head to the tip. Choose a diameter and embedment appropriate to the load, wood species, and code requirements.
Installation Basics
- Pre-drill: Drill a pilot hole sized near the screw’s minor (root) diameter; in harder woods, increase the pilot slightly. Drill a clearance hole through any fixture being attached.
- Use washers: Place a flat washer under the head to distribute load and protect the surface.
- Drive: Use a wrench or impact driver; tighten snugly without crushing the wood fibers. Avoid over-torque.
- Masonry: When fastening to concrete, brick, or block, use a compatible masonry anchor or shield sized for the lag screw.
Tip: For stainless-to-stainless assemblies, a small amount of anti-seize can help reduce thread galling.
Related Hardware
- Flat and fender washers (HDG or stainless to match)
- Masonry anchors/shields for wood-to-masonry installations
- Structural screws or bolts (alternative solutions when required by code)