Lag Bolt
Bolts · lag screw · coach screw
Definition: A lag bolt (lag screw) is a heavy-duty fastener with coarse wood threads and a hex head, driven directly into wood with a wrench — no nut required.
Anatomy
■ Head■ Shank■ Threads■ Point
Governing standards: ASME B18.2.1
Common Sizes
| Inch | Metric | Head width | Head height |
|---|---|---|---|
| 1/4 x 2 | M6 x 50 | 7/16 | 5/32 |
| 3/8 x 3 | M10 x 75 | 9/16 | 15/64 |
| 1/2 x 4 | M12 x 100 | 3/4 | 5/16 |
Representative sizes. Full dimensional data lives in the standards listed below.
Grades & Strength
| Grade | Material | Tensile (psi) | Marking |
|---|---|---|---|
| Standard | low carbon steel | 60,000 | None |
| High-strength | medium carbon steel, Q&T | 120,000 | 3 radial lines |
Pilot Hole Sizes
| Screw size | Softwood pilot | Hardwood pilot |
|---|---|---|
| 1/4 | 3/16 | 13/64 |
| 3/8 | 5/16 | 11/32 |
| 1/2 | 13/32 | 7/16 |
Drill the pilot hole to roughly the screw root diameter. Always test on scrap.
Materials
- Zinc-plated steel
- Hot-dip galvanized steel
- Stainless steel
- Silicon bronze
Pros & Cons
Pros
- Excellent pull-out resistance in wood
- No nut or access to the back side needed
- Removable and reusable in sound wood
Cons
- Pre-drilling is essential to avoid splitting wood
- Over-tightening strips the wood or snaps the bolt
- Rusts outdoors unless galvanized or stainless
Typical Uses
- Heavy wood framing and ledger boards
- Deck posts and beams
- Lagging for retaining walls
- Hanging heavy objects on studs
Alternatives & Related
- Structural Screw — Modern structural screws often match or beat lag strength with no pre-drilling
- Carriage Bolt — Through-bolt with a nut when you can reach both sides
Where to Buy
Affiliate links — we may earn a commission at no cost to you.
FAQ
What size pilot hole for a lag bolt?
Roughly the diameter of the unthreaded shank for the top section, and the minor (root) diameter of the threads for the threaded section. For softwood use about 75% of the bolt diameter; hardwood about 90%. Always test on scrap first.
Do lag bolts need washers?
Yes — a flat washer under the head protects the wood surface and spreads the clamping load.