Bolts (M-series metric) for residential construction

TL;DR

M-series metric bolts are the standard structural fastener for Australian residential construction: M12 and M16 are the workhorse sizes for hold-downs, post bases, and structural steel-to-concrete connections. Property class 4.6 covers general-purpose applications; class 8.8 covers structural steel framing. Hot-dip galvanised (HDG) coating to AS/NZS 1214:2016 is the minimum for any bolt in an exposed or in-concrete location. The most common site failure is using a bare (uncoated) mild-steel bolt in a concrete slab where it will corrode and expand, spalling the concrete. Specify the coating, not just the size.

What it is

M-series bolts are ISO metric fasteners sized by nominal thread diameter in millimetres, from M5 up to M64. In residential construction, M8 through M20 cover nearly every application: from timber-to-timber connections through to cast-in anchor bolts in concrete footings.

Also known as: metric bolts, hex bolts, structural bolts, anchor bolts (when cast into concrete), hold-down bolts (tie-down application), threaded rod (when supplied as continuous rod rather than headed bolts).

Mechanical properties for general fasteners are governed by AS 4291.1:2015, Mechanical properties of fasteners made of carbon steel and alloy steel (verified 2026-05-08). High-strength structural bolt assemblies are governed by AS/NZS 1252.1:2016 (verified 2026-05-08).

Properties

Common M-series sizes in residential construction

Size	Nominal dia. (mm)	Common application
M8	8	Light timber connections, joist hangers, brackets
M10	10	Timber frame connections, batten fixings, hold-down light duty
M12	12	Hold-down bolts to slab, post bases, structural steel column base plates
M16	16	Heavy hold-down, post bases in cyclonic regions, portal frame base plates
M20	20	Engineer-specified heavy connections, mezzanines, industrial-residential hybrid

Thread form: ISO metric coarse pitch (the standard for construction). Fine pitch is for precision mechanical applications, not structural construction use.

Property classes and strengths (per AS 4291.1:2015)

The two-digit property class designation is a shorthand for strength. The first digit times 100 gives the minimum nominal tensile strength in MPa. The product of both digits times 10 gives the minimum yield strength (or proof stress) in MPa (verified 2026-05-08).

Property class	Min. tensile strength	Min. yield / proof stress	Typical use in residential construction
4.6	400 MPa	240 MPa	General-purpose: commercial hex bolts (AS 1111.1:2015), timber connectors, non-critical fixings
8.8	800 MPa	640 MPa	High-strength structural bolt assemblies (AS/NZS 1252.1:2016), steel portal frames, structural steel to steel connections
10.9	1000 MPa	900 MPa	Engineer-specified high-load connections; less common in standard residential work

Class 4.6 (M12, 4.6/S notation): The NCC 2022 ABCB Housing Provisions Part 6.3 specifies a minimum of M12 4.6/S bolts for structural steel connections in light residential steel framing (column-to-beam and base-plate-to-slab) (verified 2026-05-08, NCC 2022 HP Part 6.3).

Class 8.8: Required for AS/NZS 1252.1:2016 structural bolt assemblies. The “S” suffix (e.g. 8.8/S) denotes snug-tightened installation; “TF” denotes tension (fully tensioned) installation.

Grades and variants

By function

Variant	Description	Common sizes	Standard
Commercial hex bolt	Standard hexagon head, product grade C, property class 4.6	M8 to M24	AS 1111.1:2015
Structural bolt assembly	High-strength hex head with matching nut and washer, property class 8.8 or 10.9	M12 to M36	AS/NZS 1252.1:2016
Anchor bolt / hold-down bolt	Headed bolt or L-bolt cast into concrete; also post-installed mechanical/chemical anchors	M12, M16, M20	AS 5216:2021
Threaded rod (allthread)	Continuous rod cut to length, used with nuts each end; common in tie-down systems	M12, M16	AS 1111.1:2015 / AS 4291.1:2015
Coach bolt (carriage bolt)	Domed head with square shoulder under head to resist turning; timber-to-timber	M8, M10, M12	AS/NZS 1390:1997

By coating

Coating	Standard	When to use
Bare (uncoated)	None	Protected internal dry applications only. Do not use in concrete, outdoors, or in treated timber
Electroplated zinc (silver)	AS 1897	Lightly corrosive internal environments only; thin coating (~5 micron), not for external or in-ground
Hot-dip galvanised (HDG)	AS/NZS 1214:2016 (threaded fasteners M8-M64)	External, in concrete, in treated timber (H2-H3 zones), in contact with masonry
Stainless steel (316)	AS/NZS 1554.6	Coastal (within 1 km of surf coast), swimming pools, highly corrosive environments

Hot-dip galvanised coating per AS/NZS 1214:2016 is the minimum corrosion protection for any bolt cast into a concrete slab, embedded in treated timber, or installed outdoors (verified 2026-05-08, AS/NZS 1214:2016).

Where to use

Hold-down / tie-down systems: M12 or M16 HDG anchor bolts or threaded rod cast into the slab edge beam, connecting the wall bottom plate to the slab. Required by the NCC in cyclonic regions and engineer-specified in non-cyclonic wind classifications.
Post bases: M12 or M16 bolts through bolt-down post base connectors to footings (Simpson Strong-Tie ABU, Pryda, MiTek and similar). Keeps timber post end off concrete, reducing decay risk.
Structural steel base plates (NCC HP Part 6.3 DtS path): not less than 2 x M12 4.6/S post-installed mechanical anchors per column base plate to slab (verified 2026-05-08).
Timber frame connections: M10 or M12 bolts through LVL ridge beams, portal frames, verandah posts.
Masonry tie-down: M12 screw anchor (masonry tie-down pattern) cast through a coupler for threaded rod connection at top plate.

Where NOT to use

Do not use bare (uncoated) bolts in concrete. Carbon steel without a galvanised coating corrodes inside concrete, expanding and causing concrete spalling over time. Always specify HDG to AS/NZS 1214:2016 for any bolt cast in, or post-installed into, concrete.
Do not use property class 4.6 where 8.8 is specified. They look identical on-site. Check the head markings: 4.6 bolts are marked “4.6”, 8.8 bolts are marked “8.8”. Never substitute.
Do not use electroplated bolts in treated timber (H2 and above). The thin zinc coating is attacked by the copper-based preservatives (ACQ, CCA) used in treated pine. Use HDG or stainless.
Do not use bare threaded rod as a tie-down through a slab edge beam without HDG coating and an appropriate plate washer to distribute bearing load on the plate.
Do not use M8 or M10 for hold-down duty unless the engineer specifies it and the product has a valid compliance report (ETA or equivalent) under AS 5216:2021 for the required load.

Fixing and installation

Anchor bolts: cast-in vs post-installed

Two paths to embed a bolt in concrete:

Cast-in: bolt is positioned in the formwork before the pour and held with a template. Provides maximum holding capacity. Standard for slab edge hold-downs on new builds. Requires accurate surveying; difficult to rectify if the bolt is in the wrong position.

Post-installed: bolt is drilled and anchored into hardened concrete after the pour. Two types:

Mechanical (expansion or screw anchor): torque-set, immediate load. DynaBolt Plus, TruBolt (Ramset), and similar.
Chemical (adhesive): epoxy or vinylester resin set into a drilled hole. Higher capacity, used for engineer-specified connections (ChemSet, Hilti HIT-HY). Requires hole cleaning and cure time before loading.

Both types for safety-critical applications must comply with AS 5216:2021, Design of post-installed and cast-in fastenings in concrete (verified 2026-05-08). The engineer specifies the anchor type, size, embedment depth, and concrete strength.

Minimum bolt sizes (NCC 2022 HP Part 6.3 DtS path for light residential steel framing)

Connection	Minimum bolt	Notes
Universal beam to column (structural steel framing)	2 x M12 4.6/S	Per NCC 2022 HP Part 6.3.7 (verified 2026-05-08)
Column base plate to concrete slab	2 x M12 4.6/S post-installed mechanical anchors	Per NCC 2022 HP Part 6.3.7 (verified 2026-05-08)

For timber-framed construction, bolt sizes are specified in AS 1684.2 (non-cyclonic) and AS 1684.3 (cyclonic) fixing schedules, or in the structural engineer’s drawings. M12 is the most common hold-down diameter.

Installation requirements

Washer under nut and under bolt head: always use washers for structural connections. Oversized plate washers for timber connections distribute bearing load across the grain.
Thread engagement: minimum 1.5 times the bolt diameter for full thread engagement. A nut spun on three turns is not structural.
Tightening: structural bolt assemblies to AS/NZS 1252.1:2016 are snug-tightened (S) or fully tensioned (TF) per the engineer’s drawings. Commercial 4.6 bolts are typically snug-tight only.
Corrosion isolation: where a galvanised bolt bears on stainless steel fittings or aluminium flashings, isolate to prevent galvanic corrosion. See galvanic corrosion.

Tolerances and acceptance

Dimensional

Item	Tolerance	Source
Hold-down bolt position (cast-in)	Typically ±5 mm from drawing location	Engineer’s drawings / AS 1684 fixing schedules
Bolt protrusion above plate	Sufficient for full nut + washer engagement (min 1.5d thread)	Practical site rule
Bolt plumb in slab	Visual; not to have visible lean that affects nut engagement	Site inspection

Workmanship (HIA Guide pending)

Per current HIA Guide to Materials and Workmanship. Verified numerical value for bolt and anchor installation tolerances pending HIA member access. [HIA-067]

Visual acceptance

Galvanised coating intact on bolt shanks, head, and nut. No bare steel visible at cuts or thread damage.
Washer present under nut for all structural connections.
Nut fully tightened (no visible thread exposed above nut face for snug-tight installations).
No galvanic corrosion blistering between bolt and dissimilar metal connector.

Working with other trades

Concretor: cast-in anchor bolts must be set and templated before the pour. The concretor needs the bolt layout drawing before formwork is stripped. Once poured, position errors are expensive to fix (core drilling and chemical anchors).
Chippy / framing carpenter: installs the bottom plate over cast-in bolts, checks alignment, and installs oversized plate washers and nuts. Any bolt installed 5+ mm off position needs the engineer’s approval before the plate is slotted to fit.
Structural engineer: specifies anchor type, size, embedment, grade, and coating for all structural connections. The NCC DtS path (HP Part 6.3) gives minimum bolt sizes for standard light steel framing; anything heavier or atypical goes back to the engineer.
Steel fabricator: fabricates base plates with holes matching the anchor bolt layout. Verify the base plate drawing against the anchor bolt template before pouring the slab.

Health and safety

Manual handling: buckets of M16 x 600 mm anchor bolts are heavy. Use mechanical assist for bulk deliveries.
Protruding cast-in bolts: bolts standing proud from a freshly poured slab are a laceration and trip hazard. Protect with plastic caps (thread protectors) until the frame goes up.
Grinding and cutting: cutting threaded rod with an angle grinder throws sparks and metal fragments. Eye protection and gloves required. Thread the nut onto the rod before cutting, then remove it to clean up the cut end.
Chemical anchors: resin components (epoxy, vinylester) are skin and eye irritants. Follow the SDS; nitrile gloves and safety glasses for mixing and injection.

Suppliers

Ramset (ramset.com.au): DynaBolt Plus, TruBolt, ChemSet, and cast-in anchor products. Australian presence since 1952.
Allfasteners Australia (allfasteners.com.au): hold-down bolt assemblies, structural hex bolts, threaded rod, HDG range M12 to M42.
Bolt and Nut Australia (boltandnut.com.au): commercial and structural bolts, wedge anchors, M8 to M24 HDG range.
InfraBuild (infrabuild.com): threaded rod and structural fasteners in volume.
Simpson Strong-Tie (strongtie.com.au): MASA bottom plate anchors (alternative to M12/M16 cast-in for plates), structural connectors with matching bolt specifications.

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

What can go wrong

Uncoated bolt in concrete: bare steel corrodes inside the slab over years, the rust expansion cracks the concrete outward from the bolt. Not visible until the slab edge spalls. Always specify HDG to AS/NZS 1214:2016 for any in-slab bolt.
Wrong property class: 4.6 bolt used where 8.8 is specified. The bolt will pass a visual check but will fail at a lower load. Check head markings on delivery; 4.6 and 8.8 bolts are the same size and look similar in a bucket.
Short thread engagement: nut wound three turns onto a long bolt looks done; structural engagement requires 1.5 times bolt diameter. Common on long hold-down rods where the thread terminates below the top of the bottom plate.
Missing plate washer: a nut bearing directly on timber grain crushes and embeds into the wood under load. Plate washers distribute bearing load.
Cast-in bolt out of position: plate is slotted to fit, compromising the bearing area under the washer. Requires engineer’s sign-off and may need an additional post-installed anchor.
Galvanic corrosion at dissimilar metals: HDG bolt in a stainless fitting, or a mild-steel bolt in an aluminium threshold plate. See galvanic corrosion for the avoidance strategy.
Property class 4.6 threaded rod used as a tie-down: the yield strength (240 MPa) of class 4.6 rod is adequate for many residential hold-downs but the engineer’s load calculation governs. Do not substitute grades without re-checking the design.

References

AS 4291.1:2015, Mechanical properties of fasteners made of carbon steel and alloy steel (Standards Australia Store) (verified 2026-05-08)
AS/NZS 1252.1:2016, High-strength steel fastener assemblies for structural engineering (Standards Australia Store) (verified 2026-05-08)
AS 1111.1:2015, ISO metric hexagon commercial bolts (Standards Australia Store) (verified 2026-05-08)
AS/NZS 1214:2016, Hot-dip galvanized coatings on threaded fasteners (Standards Australia Store) (verified 2026-05-08)
AS 5216:2021, Design of post-installed and cast-in fastenings in concrete (AEFAC resources) (verified 2026-05-08)
NCC 2022 ABCB Housing Provisions Part 6.3, Structural steel members (ABCB) (verified 2026-05-08)
Galvanizing Association of Australia, Australian Standards for Bolting (gaa.com.au) (verified 2026-05-08)