Structural screws for timber construction: Australia

TL;DR

Structural screws are high-carbon steel, fully or partially threaded fasteners designed for load-bearing timber connections. No pilot hole required in most applications: self-tapping or self-drilling points cut their own path and pull the joint tight in one pass. The result is faster installation and comparable or better withdrawal and shear capacity than an equivalently sized coach screw, without the risk of splitting from an under-sized pilot hole. Design to AS 1720.1-2010 (Amdt 3:2015): characteristic withdrawal capacity comes from Table 4.13, lateral (shear) capacity from Tables 4.9 and 4.10. Engineers specify these by joint group and screw diameter, not by brand. The main corrosion trap is using a zinc-plated screw in ACQ or CCA treated pine: copper-based preservatives attack zinc coatings; Class 4 galvanised or stainless 316 is the minimum for H3 and above.

What it is

A structural screw is a purpose-engineered timber fastener with hardened steel construction, aggressive thread geometry, and a self-tapping or self-drilling tip designed to achieve rated load capacities without pre-drilling. The category covers products marketed under names such as ASSY VG (Wurth), SPAX Timber Construction, Simpson Strong-Tie SDWS, HDC-P (Allfasteners), and similar engineered screw ranges.

What it is not: a standard wood screw (no rated capacity), a Type 17 framing screw (light-gauge only), or a Tek screw (metal-to-metal or LGS applications). Structural screws are a distinct product family with third-party load data and defined design procedures.

Key reference: AS 1720.1-2010 (Amdt 3:2015), the Australian design standard for structural timber connections including coach screws, bolts, nails, and structural screws (verified 2026-05-10, Standards Australia).

Properties

Available diameters and lengths

Brand / Range	Diameters (mm)	Lengths (mm)	Drive
Wurth ASSY VG (countersunk, cylinder, reverse head)	5, 6, 7, 8, 10	80 to 800	RW (Wurth proprietary)
SPAX Timber Construction (washer head, cylinder head, CSK)	6, 8, 10	80 to 600	T-Star Plus (Torx T40 / T50)
Simpson Strong-Tie SDWS22	5.6	76 to 254	T-40 6-Lobe
Allfasteners HDC-P	M6, M8, M10	50 to 340	Torx T30 / T40

All ranges are fully threaded or partially threaded variants. Fully threaded screws maximise withdrawal capacity; partially threaded screws clamp the outer member against the main member for tight bearing joints.

Material and coating

Coating / Grade	Corrosion class equivalent	Suitable for
Zinc-plated yellow	Class 1 to 2	Internal dry only. Not for treated timber.
Mechanically galvanised or WIROX (Spax proprietary)	Class 3 to 4	External non-coastal; treated pine H2 to H3 in non-marine zones
Hot-dip galvanised (HDG)	Class 3 to 4	External; treated pine H3. Preferred over mechanical galv for exposed applications.
Stainless 304	Class 4+	Coastal (non-surf). H3/H4 treated timber.
Stainless 316	Class 5 equivalent	Marine, surf-coast, pool surrounds, H4 and above treated timber

Treated timber: ACQ and CCA copper-based preservatives attack zinc coatings, with corrosion rates one and a half to two times higher than non-treated exposure (verified 2026-05-10, BRANZ SR153, BRANZ). Stainless 316 is the safest option for H3 and above. See galvanic corrosion and H3 treated timber.

Approvals

The primary approval pathway for structural screws in Australian use is the European Technical Assessment (ETA) issued by EOTA (European Organisation for Technical Assessment). ETAs are recognised internationally and provide third-party load data used for engineering design.

Product	Primary approval
Wurth ASSY (all variants)	ETA-11/0190 (current edition 2026, Wurth NZ/AU)
Allfasteners HDC-P	ETA-18-0817
SPAX Timber Construction	ETA-12/0114 (Scribd copy), design guide to AS 1720.1 available from SPAX Pacific
Simpson Strong-Tie SDWS22	ESR-3046 (ICC-ES), ER-192 (IAPMO), AU technical data sheet available (strongtie.com.au)

ETA is a European approval. In Australian residential construction these products are applied via the NCC performance pathway using manufacturer technical data and engineering design to AS 1720.1. Confirm with the supplier whether a CodeMark certification exists before specifying on a council-approved project (verified 2026-05-10, NCC 2022 performance pathway).

Grades and variants

Full thread vs partial thread

Type	Characteristic	Best use
Fully threaded (FT)	Thread runs entire shank length. No smooth shank. Maximum withdrawal capacity. Screws pull the members together by thread engagement only.	Timber reinforcement, cross-grain withdrawal loads, thin members where shank bearing is not desired.
Partially threaded (PT)	Unthreaded shank below head. Thread only in main member. Shank clamps the outer member tightly against the main member.	Timber-to-timber clamping joints (deck ledger, bearers to stumps, beam-to-beam), where tight bearing is needed and withdrawal is the secondary load.

Head types

Head	Application
Countersunk (CSK)	Flush finish in visible timber. Joinery.
Cylinder/Pan head	General timber-to-timber. Broad bearing.
Washer head	Outdoor structures, treated timber; larger bearing area reduces pull-through. Preferred for deck framing and landscaping.
Reverse head (RH)	Pull-the-joint loading where the screw is in tension pulling two members together; the reverse thread resists backing out.

Where to use

Timber-to-timber structural connections: bearer to stump, ridge to wall plate, ledger to wall frame. Replace coach screws where single-sided access or speed is critical.
LVL and glulam beams: beam-to-structure, multi-ply LVL lamination. Confirm compatibility with the EWP manufacturer’s documentation.
Deck framing: ledger to wall blocking; angled installation (45 degrees) maximises axial capacity per AS 1720.1.
Pergola and outdoor structures: post-to-beam, beam-to-ledger. Class 4 galvanised or stainless 316 depending on exposure zone.
Timber reinforcement (perpendicular-to-grain): ASSY VG screws are rated for cross-grain reinforcement per ETA-11/0190, resisting splitting loads in notched members. Standard screws are not rated for this.
CLT and mass timber: fully threaded screws at 45 degrees are a common CLT-to-CLT shear connection. Specify per manufacturer engineering data.

Where NOT to use

Do not substitute for through-bolts without engineering sign-off. Bolted connections in AS 1720.1 (Tables 4.5 to 4.8) have different design procedures. Structural screws are not a 1:1 bolt replacement.
Do not use zinc-plated (Class 1/2) screws in treated timber (H2 and above). ACQ and CCA copper preservatives corrode zinc rapidly. Class 4 or stainless 316 is the minimum.
Do not drive into end grain as the primary load path. End-grain withdrawal is capped at 60% of side-grain capacity per AS 1720.1 Clause 4.5.2.2.
Do not use standard Type 17 framing screws in place of structural screws. Type 17 screws have no rated structural capacity.
Do not mix stainless screws with unprotected carbon steel angles or plates. Bimetallic galvanic corrosion attacks the carbon steel. See galvanic corrosion.

Fixing and installation

Pre-drilling

Most structural screws self-tap in softwood and engineered timber (MGP10, MGP12, LVL, glulam) without a pilot hole. Pre-drilling is recommended:

In dense hardwoods (JD1 to JD2 joint group species) to prevent head shearing.
Near board ends (within 3D to 5D of the end) in any species to prevent splitting.
In dry, fully seasoned hardwood where splitting risk is elevated.

Manufacturer installation guides specify when pre-drilling is mandatory for each range. Check the product technical data sheet before installing in hardwood.

Spacing and edge distances (AS 1720.1-2010)

For coach screw and screw-type fasteners in timber (verified 2026-05-10, calcs.com AS 1720.1 guide):

Parameter	Minimum distance
End distance (tension joint)	7D (D = screw diameter)
End distance (compression/bending joint)	5D
Spacing perpendicular to load (a), b/D = 2	2.5D
Spacing perpendicular to load (a), b/D >= 6	5D
Minimum penetration into main member (JD4, e.g. MGP12 pine)	8D
Minimum penetration into main member (JD1/JD2, e.g. hardwood)	7D
Minimum outer member thickness	3D

D = nominal screw diameter in mm. These minimum distances are for the design capacity tables in AS 1720.1 to be valid. Reducing distances reduces capacity; do not mix and match without recalculating.

Drive tools and installation angle

18 V impact driver (minimum); underpowered driving stalls mid-shank.
Use the specified drive bit: RW (ASSY), T-Star Plus Torx T40/T50 (SPAX), T-40 6-Lobe (Simpson SDWS). Wrong bit = cam-out and a stripped head.
Set torque limit to stop the head flush; over-driven structural screws in hardwood can snap the head.
Install perpendicular to grain for shear connections. At 45 degrees to grain, axial load capacity increases significantly (inclined screw utilises tensile strength); this angle is specified by engineers for deck-to-ledger and beam-to-plate connections. Use a jig: free-hand angling produces variable embedment.

Tolerances and acceptance

Dimensional

Item	Requirement
Screw head proud of surface	Not acceptable. Must sit flush (countersunk) or bearing flat (washer head).
Screw head over-driven	Excessive countersink reduces bearing area. Check against manufacturer limit.
Edge/end distance	Per AS 1720.1 minimums above.
Penetration depth	Must meet minimum per joint group (see table above). Under-length is a defect.
Screw spacing	Per AS 1720.1 minimums. Reduced spacing reduces capacity.

Workmanship

Per current HIA Guide to Materials and Workmanship and the relevant state Guides to Standards and Tolerances. Verified numerical workmanship tolerances for structural screw connections (pull-through, head bearing, edge distance deviation) pending HIA member access. [HIA-153]

Visual acceptance

No screw heads proud of surface on finish-face timber. On structural members, proud heads are a design concern only if they conflict with a bearing plate or hardware item.
No rust staining from screw heads within the defects liability period (DLP). Rust staining indicates wrong corrosion class for the exposure.
No cracks radiating from screw entry point. Splitting indicates wrong entry point (too close to edge/end) or wrong tip type for the substrate density.
No spinning (screw not biting): indicates stripped hole or wrong thread for substrate. Fix: relocate 50 mm minimum from failed location.

Working with other trades

Chippy (framing): confirm screw range, drive bit, and installation angle before starting. Wrong product = no engineering basis for the connection.
Engineer (structural): signs off on anything beyond the manufacturer’s prescriptive tables. Nominates diameter, count, spacing, and angle per AS 1720.1 and ETA load data.
Concretor: structural screws are timber-to-timber only. Concrete anchorage needs masonry anchors or epoxy rod.
Roofer: rafter-to-wall-plate ties may specify structural screws. Confirm the specified product; no on-site substitution.

Health and safety

Eye protection: swarf from impact-driven self-tapping screws. Safety glasses mandatory.
Hand injuries: cam-out on the wrong bit slips into the hand. Use the correct bit and keep centred pressure.
Silica (engineered timber): LVL and some plywood products generate fine dust. P2 mask where ventilation is poor.
Electrical services: locate services before driving. Structural screws at 80 to 300 mm will penetrate further than standard framing screws.

Suppliers

Wurth Australia (wurth.com.au): ASSY VG, ASSY 4, ASSY Kombi (ETA-11/0190). Trade account and eshop.
SPAX Pacific (spaxpacific.com): Timber Construction in WIROX and stainless. AS 1720.1 design guide downloadable.
Simpson Strong-Tie (strongtie.com.au): SDWS22 and SDWS27. AU technical data sheet available.
Allfasteners (allfasteners.com.au): HDC-P Class 4 (ETA-18-0817), M6 to M10.
Bunnings (trade desk): Simpson SDWS in 50-pack and 250-pack.

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

What can go wrong

Wrong corrosion class in treated timber: ACQ pine with a zinc-plated screw corrodes within 2 to 3 years; connection fails invisibly. Use stainless 316 or Class 4 rated for ACQ.
Under-length screw: penetration less than the joint-group minimum; design capacity table does not apply. Common when a builder substitutes an in-stock shorter screw. Check penetration against the connection design.
Too close to end or edge: splitting on first load, especially in LVL and glulam. Pre-drill and observe AS 1720.1 minimum distances.
Wrong drive bit: RW bit in a T40 socket strips the head immediately. Match bit to product, label the boxes.
No engineering basis: screws selected from a brochure for a connection that needs engineering calculation (large deck ledger, cyclone zone). Prescriptive tables have limits; outside them, an engineer must confirm.
Galvanic corrosion at dissimilar metals: stainless screw into a galvanised bracket corrodes the bracket. See galvanic corrosion.
Bolt substitution without redesign: a structural screw in shear carries less load per fastener than a through-bolt of equivalent diameter. Not a 1:1 substitution.

References

AS 1720.1-2010 (Amdt 3:2015), Timber structures, Part 1: Design methods (Standards Australia) (verified 2026-05-10)
AS 3566.2-2002, Self-drilling screws for the building and construction industries, Part 2: Corrosion resistance requirements (Standards Australia Store) (verified 2026-05-10)
AS 4312:2019, Atmospheric corrosivity zones in Australia (referenced via Buildex corrosion class guide) (verified 2026-05-10)
BRANZ SR153, The corrosion of metallic fastener materials in untreated, CCA-, CuAz- and ACQ-treated timber (BRANZ) (verified 2026-05-10)
Wurth ASSY VG, ETA-11/0190 (2026 edition), European Technical Assessment (Wurth NZ) (verified 2026-05-10)
Allfasteners, HDC-P Class 4 Landscape Screws (allfasteners.com.au) (verified 2026-05-10)
Simpson Strong-Tie Australia, SDWS22 Timber Screw (strongtie.com.au) (verified 2026-05-10)
Calcs.com, AS 1720.1 Timber Screw Connections: Design Guide for Australian Engineers (calcs.com) (verified 2026-05-10)
AIMS Industrial, Coach Bolt and Coach Screw Guide (aimsindustrial.com.au) (verified 2026-05-10)