Lag Bolt (Lag Screw): What It Is and How to Use It

A lag bolt—often called a lag screw—is a heavy-duty wood fastener with a hexagonal head and deep, coarse threads. Unlike standard screws driven with a screwdriver, lag bolts are turned with a wrench or socket. They’re used where strong, secure connections are needed, such as attaching ledger boards, mounting heavy hardware to studs or posts, or fastening wood to masonry via anchors. Because of their size and holding power, lag bolts require a pilot hole to reduce splitting and ensure proper grip.

Where You’ll Use Them

Lag bolts show up in many home projects that need high pull-out and shear strength:

• Decks and outdoor structures: Securing beams, posts, and rail hardware; attaching pergola brackets; connecting playset components
• Wall-mounted loads: Mounting barn door tracks, heavy shelving ledger boards, or large TVs to studs (with proper sizing and embedment)
• Framing repairs: Sistering joists, fastening ledger boards (per code schedules), or reinforcing beams
• Gates and fences: Attaching strap hinges and hardware to posts
• Masonry attachments (with anchors): Fastening sill plates or posts to concrete using lag shield anchors

Types and Variations

While the basic idea is consistent—hex head, wood threads—lag bolts vary in a few useful ways:

• Head style: Most are hex head. Older or rustic versions may be square head. Some have a built-in flange under the head for better load distribution.
• Material and coating:
  • Zinc-plated steel: For dry, indoor use
  • Hot-dip galvanized: For outdoor use, pressure-treated lumber, and damp locations
  • Stainless steel (304/316): For coastal or highly corrosive environments
• Diameter and length: Common diameters range from 1/4 in. to 1/2 in., with lengths from 1 in. to 10+ in. Larger diameters and longer embedment increase capacity.
• Thread length: Many are partially threaded (smooth shank near the head), which can help draw the joint tight. Fully threaded options exist in some sizes.
• Point style: Most have a gimlet point to help start in wood, but you still need a pilot hole.

Related fasteners include engineered "structural screws" (like ledger screws), which offer similar or higher performance with easier installation. They’re not the same as lag bolts but often compete for the same tasks.

How to Choose the Right Lag Bolt

• Size it for the load: Heavier loads and structural connections call for larger diameters (3/8 in. or 1/2 in.) and deeper embedment.
• Length and embedment: Aim to embed at least 6–10 times the bolt’s diameter into solid wood (and not just sheathing). For example, a 3/8 in. lag should ideally embed 2.25–3.75 in. into the main member. Longer is often better, within reason.
• Use the right coating: For pressure-treated lumber, choose hot-dip galvanized or stainless. Zinc-plated fasteners can corrode quickly in treated wood.
• Add washers: A flat washer under the head spreads load and prevents the head from crushing into the wood. For softer wood or larger loads, use a larger-diameter washer.
• Match to the substrate: For concrete or brick, pair the lag bolt with the correct lag shield or masonry anchor sized to the bolt.

Installation: Step-by-Step

1. Plan the layout

   • Keep lag bolts away from board ends and edges to reduce splitting. A practical rule is at least 4 times the bolt diameter from board ends and 2–3 times from edges. Space multiple lags 4–6 diameters apart.

2. Mark and clamp

   • Dry-fit parts, mark holes, and clamp the work to prevent movement. Verify that you’re hitting solid framing, not just sheathing or drywall.

3. Drill a clearance hole in the first piece

   • Through the piece you’re pulling tight (the "fixture"), drill a hole equal to the lag bolt’s shank diameter. This lets the bolt pass through freely and pull the joint together.

4. Drill a pilot hole in the main member

   • In the receiving member (stud, beam, or post), drill a pilot hole smaller than the shank so the threads bite. As a rule of thumb:
     • Softwood: pilot 65–75% of shank diameter
     • Hardwood: pilot 75–90% of shank diameter
   • Example: For a 3/8 in. lag bolt, a 1/4 in. to 5/16 in. pilot often works. Depth should be at least as long as the planned embedment.

5. Optional: Lubricate threads

   • A dab of beeswax, bar soap, or construction lubricant reduces driving torque and heat, especially in hardwoods.

6. Add a washer and drive the bolt

   • Place a flat washer under the head. Use a six-point socket and ratchet or an impact driver with a proper socket. Drive until snug and the washer seats firmly, but stop before crushing wood fibers.

7. For masonry with lag shields

   • Drill the correct-size hole for the shield in concrete or brick, blow out dust, tap in the shield, then drive the lag bolt through your fixture into the shield. Follow the anchor manufacturer’s size and depth specs.

8. Final checks

   • Recheck alignment, verify that the bolt head and washer sit flat, and confirm that the joint is tight without overtightening.

Common Mistakes to Avoid

• Skipping the pilot hole: This can split wood, burn out tools, and reduce holding power.
• Using the wrong coating: Zinc-plated lags in pressure-treated lumber can corrode and fail. Use hot-dip galvanized or stainless outdoors.
• Overtightening: Crushing fibers under the head weakens the joint and can strip threads in the wood.
• Undersizing: Too small or too short won’t handle the load. Increase diameter or embedment, or use more fasteners per engineering or code guidance.
• Poor edge and end distances: Placing lags too close to ends or edges invites splitting and weak connections.
• No washer: The head can sink into wood and lose clamp force.
• Rounding the head: An open-end wrench can slip. Use a six-point socket sized correctly.
• Dirty masonry holes: Dust left in a hole can prevent lag shields from gripping. Clean out debris before installing anchors.

Related Terms

• Lag shield anchor: A sleeve-style masonry anchor used with lag bolts
• Structural screw (ledger screw): Engineered screw alternative to lag bolts
• Carriage bolt: Smooth, round-head bolt used with a nut and washer
• Hex bolt: Machine bolt used with a nut; unlike lag bolts, it doesn’t cut threads in wood
• Coach screw: Another name for lag screw (common outside North America)
• Pilot hole: Pre-drilled hole to guide and ease installation
• Washer: Flat ring under the head or nut that spreads load

Real-World Examples

• Mounting a barn door track: Use 5/16 in. or 3/8 in. lag bolts with washers through the track and a wood ledger, driven into wall studs. Pre-drill clearance holes in the track and ledger, and pilot holes into studs.
• Attaching a pergola bracket: Choose hot-dip galvanized or stainless lags with large washers. Drill clearance holes in the bracket and pilots in the post or beam, and keep adequate edge distance to prevent splits.
• Securing a post base to concrete: Install lag shield anchors in the slab, then fasten the base with matching lag bolts. Verify hole size and depth per the anchor’s instructions and clean out drilling dust.
• Deck ledger attachment: Many builders now favor code-listed structural screws or through-bolts, but lag bolts are still used where allowed. Follow the spacing schedule in your local code or manufacturer’s literature and match the coating to the lumber.

With the right size, coating, pilot hole, and technique, lag bolts provide strong, reliable connections for heavy-duty wood and masonry-backed projects around the house.