Hex Lag Screws
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.
Materials & Finishes
- 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.
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)
Hex Lag Screws FAQs
See the accordion below for pilot-hole guidance, masonry attachment, stainless vs. galvanized choices, and more.
Disclaimer: This information is general guidance. Follow product datasheets, local codes, and engineered designs.
Yes. Pre-drilling reduces splitting and allows full thread engagement. Size the pilot close to the screw’s minor (root) diameter; in hardwoods, go slightly larger.
As a rule of thumb, use a pilot near the root diameter for softwoods and a bit larger for hardwoods. Always confirm with the product or code table for your application.
Yes—washers spread the load and protect the surface. Match the washer material/finish (e.g., HDG with HDG, stainless with stainless).
Not directly. Use a compatible masonry anchor or shield sized for the lag screw, or choose a purpose-built masonry fastener.
Lag screws cut their own threads into wood and are not used with a nut. Hex bolts are machine bolts used with a nut or tapped hole.
304 (18-8) works for most outdoor projects. Choose 316 near saltwater, de-icing salts, or chemicals for better corrosion resistance.
HDG has a thick zinc layer for outdoor/structural exposure; zinc plated is thin and best for dry interiors.
Yes, with the correct hex socket. Keep the driver aligned and avoid over-torque that can crush wood fibers or strip threads.
Select a length that provides adequate embedment into the main member after passing through the attached material. Follow code or engineering guidance for your joint.
The pilot may have been too small or too close to the edge/end grain. Increase pilot size for dense species and maintain proper edge/end distances.
Use only where permitted by code or design. Some structural joints require specific fasteners (e.g., structural screws or bolts) and inspected installation methods.
If threads and head are undamaged and the hole remains sound, many non-critical applications allow reuse. For safety-critical or code-governed work, use new fasteners as directed.