Coach screw
Coach screw: heavy-duty hex-head timber fastener driven with a wrench. Needs a pilot hole. Standard alternative is a structural screw. AS 1720.1.
Ask Chalkline about this →A coach screw is a heavy-duty hex-head fastener with a coarse self-tapping thread, driven with a spanner or socket into timber without requiring a nut. It is sometimes called a lag screw or lag bolt, though it should not be confused with a coach bolt (a through-bolt product with a nut). Sizes run M6 to M16 in Australian residential construction; each size requires a pilot hole to the full penetration depth to avoid splitting (M10 in softwood: 6mm pilot, M10 in hardwood: 7mm pilot).
Coach screws are designed to AS 1720.1-2010 for lateral (shear) loads treated as bolts, and for withdrawal (axial) loads per Table 4.13 of the same standard. End grain withdrawal is capped at 60% of side-grain capacity. They suit single-sided access applications (ledger boards, post brackets, bearer-to-stump) where a through-bolt would require access from both sides.
Also known as: lag screw, lag bolt (informal).
Category: fixings / structural fasteners.
The main limitation of coach screws compared to modern structural screws is the mandatory pilot hole, which slows installation. In most applications structural screws now offer equivalent or higher capacity without pre-drilling.
Related
- Structural screws for timber construction
- Screws and fasteners for residential construction: overview
- Bolts (M-series metric) for residential construction
See also
Last updated: 2026-05-10. Verified: 2026-05-10. Quarterly review for AS 1720.1 currency.