Coach screws for residential construction

TL;DR

Coach screws (hex-head lag screws) are the workhorse timber-to-timber fastener for decks, pergola frames, post brackets, and structural connections where a bolt-through is impractical. M10 x 75 mm or M12 x 100 mm HDG is the default spec for pergola rafter-to-beam and beam-to-post face fixes. The single biggest site mistake is using electroplated (zinc-plated) coach screws in H2 or H3 treated pine: the copper-based preservatives (ACQ, CCA) attack the thin zinc coating within 2-5 years outdoors. Hot-dip galvanised (HDG) to AS/NZS 1214:2016 is the minimum for any coach screw in treated timber or exposed to weather. Always pre-drill to full penetration depth: a hardwood pilot hole sized for softwood will shear the screw head off before it reaches depth.

What it is

A coach screw is a large-diameter wood-thread fastener with a hexagonal head, driven with a socket wrench or impact driver rather than a screwdriver. The hex head allows high torque to be applied without camming out, making it suitable for structural timber connections that smaller screws cannot achieve.

Also known as: lag screw, lag bolt, hex lag screw. In Australian trade usage, “coach screw” is the standard term; “lag screw” or “lag bolt” is more common in North American literature and is sometimes used in structural engineering drawings.

The governing dimensional standard for coach screws in Australia is AS/NZS 1393:1996 (R2016), Coach screws - Metric series with ISO hexagon heads (verified 2026-05-10). The standard covers ISO preferred series diameters from M6 to M20 with lengths up to 200 mm. Mechanical properties (property class 4.6) are governed by AS 4291.1:2015, Mechanical properties of fasteners made of carbon steel and alloy steel (verified 2026-05-10).

Properties

Common sizes in residential construction

Size	Hex across flats	Common lengths	Typical application
M6	10 mm	30-100 mm	Light fixings, fence rails, light joinery brackets
M8	13 mm	30-100 mm	Decking board to joist (hardwood decking), joist hanger side-fixing, batten to rafter
M10	17 mm	40-150 mm	Pergola rafter to beam, ledger board to framing, deck joist to bearer
M12	19 mm	40-200 mm	Beam to post (face-fix), pergola post bracket, structural timber-to-timber
M16	24 mm	50-200 mm	Heavy structural connections, retaining wall tie-back, engineer-specified

Lengths up to 200 mm are covered under AS/NZS 1393:1996; longer lengths (up to 300 mm) are available from some suppliers but fall outside the standard’s scope.

Property class and strength

Standard construction coach screws are property class 4.6 per AS 4291.1:2015: minimum tensile strength 400 MPa, minimum yield strength 240 MPa (verified 2026-05-10, Standards Australia store). This is adequate for most residential timber connections. High-tensile (8.8) coach screws are available for engineer-specified heavy structural applications; verify the class marking on the head.

Grades and variants

By coating

Coating	Standard	When to use
Bare (uncoated)	None	Internal dry applications only; not suitable for treated timber or external locations
Electroplated zinc	AS 1897	Light internal applications; NOT for external, treated pine, or in-ground
Hot-dip galvanised (HDG)	AS/NZS 1214:2016	Any external application, treated pine (H2 and above), in-ground, masonry contact
Stainless steel 316	-	Coastal locations (within 1 km of surf coast), swimming pool structures, highly corrosive environments

Hot-dip galvanised coating to AS/NZS 1214:2016 is the minimum corrosion protection for any coach screw installed in treated timber, outdoors, or in contact with concrete or masonry (verified 2026-05-10, Standards Australia store).

Treated pine rule: CCA and ACQ copper-based preservatives used in H2 and above treated pine are chemically aggressive to electroplated zinc. Electroplated coach screws in treated pine will corrode through within 2-5 years in outdoor conditions, causing rust staining and structural failure at the fixing. HDG or stainless is non-negotiable for treated timber (verified 2026-05-10, Allfasteners - treated pine screws guide).

Tech-Shield and equivalent proprietary coatings (e.g. ITW Zenith Tech-Shield): Purpose-formulated for treated pine, marketed as an alternative to HDG. Check the manufacturer’s technical data for the specific preservative system (CCA vs ACQ) to confirm compatibility before substituting for HDG.

By shank type

Shank	Description	Application
Full body (standard)	Shank diameter equals thread root diameter	Timber-to-timber connections where maximum thread engagement is needed
Reduced diameter (scant)	Shank smaller than thread diameter	Allows slight movement; used in some engineered timber connection designs

Where to use

Pergola frames: M10 x 75-100 mm HDG to face-fix rafters to beams; M12 x 100 mm HDG to fix beams to posts where a through-bolt cannot be used.
Deck framing: M10 x 75 mm HDG to fix ledger boards to wall framing through the joist-to-bearer connection; M8-M10 HDG for joist hangers with side-fix requirements.
Post base brackets: Coach screws fix the timber post to a bolt-down post bracket (Simpson Strong-Tie ABU, Pryda, MiTek and similar); the bracket specification governs screw size, count, and pattern.
Retaining wall construction: Coach screws fix tie-back rail to retaining posts; use HDG or stainless for any in-ground or ground-contact application.
Structural timber-to-timber (face-fix): Carrying beams face-fixed to posts, ridge beams to gable studs, portal frame connections where through-bolts are impractical.

Where NOT to use

Do not use electroplated coach screws externally or in treated timber. The thin zinc plating (~5-15 microns) is insufficient for outdoor conditions and is attacked by copper preservatives in treated pine. Always specify HDG for external and treated timber.
Do not substitute coach screws for through-bolts where the engineer specifies through-bolts. A through-bolt (bolt + nut + washer) provides clamping force on both faces; a coach screw bears only in thread engagement. They are not structurally equivalent.
Do not use coach screws as concrete anchor bolts. Coach screws are a timber fastener; for concrete anchoring, use purpose-designed mechanical or chemical anchors per AS 5216:2021.
Do not use without pre-drilling in hardwood or dense engineered timber. Forcing a coach screw into underprepared hardwood will shear the head or strip the timber thread. Always pre-drill.
Do not omit washers on structural connections. The hex head bears on the timber face; without a washer, the head embeds into the timber grain under load, reducing clamping force.

Fixing and installation

Pre-drilling

Pre-drilling is mandatory. Coach screws have a wood-thread shank, not a self-drilling point, and will not drive into timber without a pilot hole. Driving without pre-drilling in hardwood will shear the head; in softwood, it will split the timber near the end.

Step sequence:

Drill a clearance hole through the top (receiving) member at the full shank diameter.
Drill a pilot hole into the bottom (fixed) member to the intended penetration depth.
Thread a washer onto the screw, start by hand to confirm alignment, then drive with a socket wrench or impact driver at reduced speed.

Pilot hole diameter by timber type:

Coach screw size	Softwood (radiata pine, treated pine)	Hardwood (merbau, spotted gum, ironbark)
M6	3.5 mm	4.5 mm
M8	5 mm	6 mm
M10	6 mm	7 mm
M12	7 mm	9 mm
M16	10 mm	11 mm

Source: AIMS Industrial coach screw installation guide (verified 2026-05-10, aimsindustrial.com.au).

Drill the pilot hole to the full penetration depth of the coach screw, not just a starter hole. Under-drilling in hardwood is the most common cause of head shear on site. If the driver is straining significantly before the screw reaches depth, stop and re-drill to the hardwood diameter.

Installation tips

Drive at reduced speed with a socket on an impact driver; full-speed driving generates excess heat, which can strip the thread in the timber or damage the galvanised coating at the head.
Tighten until the washer and head bear firmly on the timber face; do not over-torque and crush or split the timber grain.
In post-bracket applications, follow the bracket manufacturer’s installation guide for minimum screw count, pattern, and any requirement for specific structural screws versus coach screws.

Tolerances and acceptance

Dimensional

Item	Tolerance	Source
Head bearing on timber face	Washer and head in full contact; no gap; timber not split or crushed	Site inspection; manufacturer guides
Pilot hole alignment	Screw runs perpendicular to timber face; no visible lean at head	Visual

Workmanship

Per current HIA Guide to Materials and Workmanship. Verified numerical workmanship tolerances for coach screw connections in residential construction pending HIA member access. [HIA-148]

Visual acceptance

Galvanised coating intact at head, shank, and washer; no bare steel at thread cuts or impact marks.
Washer present under head for all structural connections.
No rust staining at the screw head or on the timber surface at the fixing within the defects liability period.
No splitting of timber at the screw entry point (indicates insufficient pilot hole depth or diameter).
No screw head embedded below the washer bearing face (indicates over-torquing).

Working with other trades

Chippy (framing carpenter): Selects coach screw size, coating, and pilot hole diameter for the structural application. Responsible for pre-drilling to the correct diameter and depth, and for confirming bracket manufacturer’s fixing schedule is followed.
Structural engineer: For engineer-specified connections, the structural drawings govern screw size, count, penetration depth, and spacing. Do not substitute coach screws for bolts without the engineer’s confirmation.
Concretor: Where post base brackets are cast into or bolted to a concrete slab, the concretor sets the anchor bolt or cast-in plate; the chippy then installs the post bracket and coach screws into the post. Sequence matters: confirm bracket bolt pattern matches the anchor layout before the pour.

Health and safety

Impact driver kickback: Coach screws are driven at high torque; unsupported timber or a binding screw can cause the driver to kick. Maintain firm grip and stable footing on scaffolding or decks under construction.
Swarf and metal fragments: Drilling pilot holes in HDG-coated members generates metal swarf; eye protection required.
Silica dust (masonry proximity): If drilling near masonry or concrete for bracket setting, P2 mask and wet drilling or on-tool extraction per WHS regulations.
Manual handling: Bulk boxes of M12 or M16 coach screws are heavy. Use mechanical assist for large deliveries to height.

Suppliers

ITW Proline (Zenith) (itwproline.com.au): Zenith coach screws in HDG, Tech-Shield (treated pine), zinc-plated, and 304/316 stainless; M6 to M12, lengths to 200 mm; meets AS/NZS 1393:1996 and property class 4.6.
Hobson Engineering (via AIMS Industrial, aimsindustrial.com.au): Metric hex coach screws in HDG, class 4.6, M8 to M20; compliant to AS/NZS 1393:1996.
Allfasteners Australia (allfasteners.com.au): HDG coach screws M8 to M20, lengths 30-200 mm; self-tapping gimlet point.
Scrooz Fasteners (scrooz.com.au): Galvanised coach screws with same-day dispatch; M6 to M16 range.
Bolt and Nut Australia (boltandnut.com.au): M6 to M16 HDG and stainless coach screws.

[Sponsor / preferred supplier slot. ACCC disclosure required.]

What can go wrong

Electroplated screw in treated timber: Corrosion begins within months in outdoor conditions; rust staining on the deck or pergola, then structural failure at the fixing as the shank deteriorates. Specify HDG or stainless at the purchase stage; you cannot retrofit coating once the screw is in.
Head shear in hardwood: Driving without pre-drilling, or using softwood pilot hole sizes in hardwood, overtorques the head before the thread reaches depth. The head shears off flush with the timber face and the stump is difficult to remove. Pre-drill to the correct diameter for the timber species.
Thread stripping in softwood: Pilot hole too large (or softwood too soft) means the thread does not grip. The screw spins without tightening. Fix: re-drill offset by 50 mm minimum and use a longer screw; do not try to re-use the stripped hole.
Missing washer: Coach screw head embeds directly into timber grain under load, compressing and crushing the wood fibre. The fixing appears tight but loses clamping force over time as the timber compresses. Washer is not optional on structural connections.
Wrong size for the application: M8 x 50 mm used where M12 x 100 mm is specified means inadequate thread engagement and shear capacity. Check the bracket manufacturer’s fixing schedule and any engineer’s spec before substituting a smaller or shorter screw.
Galvanic corrosion at bracket interfaces: HDG coach screw in a stainless bracket, or a bare steel screw in an HDG bracket. The more active metal corrodes at the contact point. See galvanic corrosion.

References

AS/NZS 1393:1996 (R2016), Coach screws - Metric series with ISO hexagon heads (Standards Australia Store) (verified 2026-05-10)
AS 4291.1:2015, Mechanical properties of fasteners made of carbon steel and alloy steel (Standards Australia Store) (verified 2026-05-10)
AS/NZS 1214:2016, Hot-dip galvanized coatings on threaded fasteners (Standards Australia Store) (verified 2026-05-10)
AIMS Industrial, Coach bolt and coach screw guide (aimsindustrial.com.au/blogs/product-guides/coach-bolt-guide) (verified 2026-05-10)
ITW Proline, Coach screws product data (itwproline.com.au/product/coach-screws/) (verified 2026-05-10)
Hobson Engineering / AIMS Industrial, Metric hex coach screw class 4.6 HDG (aimsindustrial.com.au) (verified 2026-05-10)