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Choosing fasteners and screws: the right fixing for each part of a building

17 October 2026

A building is held together by thousands of fixings that cost a few cents each. Most are never seen again after the trade packs up. When one is wrong, it does not fail on day one. It rusts behind the cladding, pops through the ceiling, backs out of the deck or lets a roof sheet lift in a storm, usually a few years later and usually after the warranty argument has become harder.

The frustrating part is that the right fixing is rarely more expensive. It is a matter of asking four questions before anyone opens the box. This article walks through those questions, explains how to read what is printed on a fastener, and then goes application by application: timber framing, metal roofing, treated timber and decks, plasterboard, wet areas and anchors into concrete and masonry. We covered what happens when different metals meet in materials compatibility: order, direction and junctions. This article is about picking the fastener itself.

Four questions before you choose

  1. What is it holding, and what happens if it lets go? A fastener that carries structural load (a tie-down strap, a roof sheet, a balustrade post, a bottom plate anchor) is specified by a standard or an engineer and cannot be swapped. A fastener holding a skirting board can.
  2. What is it going into? Timber, light-gauge steel, structural steel, concrete, brick, hollow block and plasterboard each want a different point, thread and length. A screw designed for one usually performs poorly in another.
  3. Where is it, and what will it be exposed to? A dry internal wall, a sheltered roof space, an exposed roof and a site near the coast are four different environments. Coating class follows exposure, and so does design life.
  4. What is it touching? Copper-treated timber, wet cement, dissimilar metals and some rubber washers all shorten a fastener's life. The fastener and the material it sits in should have a similar life expectancy.

If a fastener satisfies all four, it is probably right. If someone on site proposes a substitute, run the same four questions again.

Reading the fastener

Fastener boxes carry a lot of information in a short code, and most of it matters.

  • Size. Australian screws are still mostly described by gauge and threads per inch, for example 12-14 x 50 means a 12 gauge screw with 14 threads per inch and 50 mm overall length. The industry is converting to metric: #12 becomes roughly M5.5, so you may see both systems on the same shelf.
  • Point. A Type 17 point has a cut-away tip that clears chips and is made for timber. A self-drilling (drill point) screw has a small drill bit formed into the tip and is made for steel, with longer drill points for thicker steel. A needle point is for thin sheet and light-gauge steel. Using a timber screw to drill steel, or a drill-point screw in timber, gives a poor hold either way.
  • Thread. Coarse thread bites into timber; fine thread suits thin steel. The exception is plasterboard, where testing has shown both coarse and fine thread work in timber, as discussed below.
  • Head. Hex heads take a socket and sit on a washer (roofing, framing). Bugle heads sink slightly below a surface without tearing it (plasterboard). Countersunk heads sit flush in timber. Wafer and button heads sit flat on sheet. Trim heads are small and meant to be hidden (decking, cladding).
  • Coating class. This is the one most often missed. Self-drilling screws for building are covered by AS 3566, and Part 2 (reissued in 2025) sets four corrosion-resistance classes. Broadly, Class 1 and 2 are for dry, protected or internal use, Class 3 is the usual minimum for anything exposed to the weather, and Class 4 is for severe marine exposure. Beyond that, the choice is stainless steel, usually grade 304 inland or grade 316 near the coast, pool chemicals or heavy industry. Hot-dip galvanising (AS/NZS 4680) is a separate route to the same goal and is common for nails, bolts and connectors.

A useful way to think of coating class is as a clock. A thin zinc plating runs out in months outdoors. A Class 3 or 4 coating, or hot-dip galvanising, buys decades. Stainless does not rely on a coating at all, which is why it is specified where nothing else lasts.

Structural timber framing

Framing fixings are not a judgement call. For houses, NCC 2022 Volume Two clause H1D6 accepts AS 1684.2 or AS 1684.4 for timber framing in non-cyclonic areas and AS 1684.3 for cyclonic areas, with AS 1720.1 as the engineered alternative.

AS 1684 separates two kinds of fixing:

  • Nominal fixings (clause 9.5) are the standard nailing that holds members in position: not less than two nails at each joint unless the standard says otherwise, with sizes set out in the tables.
  • Specific tie-down (clause 9.6) is the extra connection needed to resist wind uplift and racking: straps, framing anchors, bolts, cyclone rods and nail plates, chosen from the tables for the site's wind classification. These are in addition to nominal nailing, and the standard requires a continuous load path from the roof sheeting down to the footings.

Three practical points follow. First, gun nails are fine if they are the specified diameter and length, galvanised where the standard or the exposure requires it, and not blown through the timber. Second, proprietary connectors (nail plates, joist hangers, framing anchors) carry a published capacity that only holds if the specified nails are used in all the holes; a hanger fixed with half its nails has far less than half its capacity. Third, a bottom plate fixed to a slab with proprietary masonry anchors relies on the anchor manufacturer's published uplift capacity, which must be matched to the AS 1684 table for the wind class, so the brand and model matter.

If a carpenter proposes a different connector or a shorter fixing, it is a design change and needs the engineer or the standard, not a tape measure on the day. See carpentry standards and structural integrity for how this fits into the frame inspection.

Metal roofing and wall cladding

Roof screws are the clearest example of coating class following exposure. BlueScope's Lysaght product advisory bulletin on fasteners (PAB04, January 2024) sets the following for its roofing and walling products:

  • AS 3566 Class 3 (or better) for most external applications not closer than 400 m from the ocean or severe marine influence, on COLORBOND Metallic and ZINCALUME steel;
  • AS 3566 Class 4 for severe exposure: COLORBOND steel 200 m to 400 m from a marine environment, and COLORBOND Ultra steel 100 m to 200 m from a marine environment;
  • stainless steel fasteners only with SUPERDURA stainless steel cladding, not with COLORBOND or ZINCALUME; and
  • painted or powder-coated fasteners where colour match is an overriding consideration, provided the coating system still meets Class 3 or 4.

The same bulletin covers the details that leak when they are ignored:

  • Washers. Sealing washers must be EPDM, not neoprene, and must not contain significant conductive carbon black, particularly in marine environments. A conductive washer sets up a cell between the screw and the sheet.
  • Length. The screw must have at least one full thread showing past the support; three visible threads confirm the penetration is met. With insulation blankets or boards under the cladding, screws get longer, not shorter, so the same embedment is reached.
  • Setting. Tighten only until the washer is gripped firmly enough to seal. Over-tightening splits the washer or dishes the sheet, and either lets water in. Drive the screw perpendicular to the sheet.
  • Rib fixing. Where screws pass through the rib, a top-thread section is recommended to pull the sheet up to the washer, and the shank protection must not reach the support.
  • Rivets. Aluminium blind rivets for COLORBOND, ZINCALUME and GALVABOND steels; stainless rivets with stainless mandrels only for stainless cladding.
  • Nails are limited to fixing the clips of some concealed-fixed profiles to timber, and should be galvanised.

The installation standard behind all of this is AS 1562.1, called up by NCC Volume Two clause H1D7. In cyclonic regions the clause goes further: the roof assembly, its connections and supports must stay in position through the Low-High-Low fatigue test in Table H1D7, which means the fastener type, spacing and pattern tested with that profile is the only one that complies. In every region, the fastener count and spacing per support come from the cladding manufacturer's installation manual for the wind class, not from habit.

Two more things we insist on. Swarf (the metal filings from drilling) is swept off the roof at the end of each day, because filings rust into the coating and look like the sheet is failing. And the metals in the fastener, flashing, sheet and gutter are checked against NCC Housing Provisions clause 7.2.2 and its tables, as described in our materials compatibility article. Our roof plumbing article covers the gutter and downpipe side.

Treated timber, decks and anything outside

Most outdoor timber in NSW is treated against decay and termites, and the common modern treatments (ACQ, copper azole and others) are copper-based. Copper is more aggressive to zinc than the older CCA treatment. New Zealand's building research body BRANZ has measured mild steel corroding several times faster in ACQ and copper azole treated pine than in CCA, with the effect increasing in wetter timber. Hot-dip galvanised or stainless fasteners are the usual recommendation with copper-based treatments, and plain zinc-plated screws are a poor choice outdoors.

Our working rules:

  • Hot-dip galvanised or stainless steel for anything into treated timber outdoors. Where a coated screw is proposed instead, get the maker's written confirmation that it suits the treated timber and the exposure, because not all coatings are tested for it.
  • Grade 316 stainless near the coast, around pools, and in tannin-rich hardwoods, where lesser fasteners can leave black streaks. Suppliers report that grade 304 can develop tea staining in salty air.
  • Keep the metals consistent. Stainless screws in a galvanised joist hanger, or galvanised nails in a stainless strap, create a corrosion cell at the connection. The fastener should match the connector, and both should match the exposure.
  • Pre-drill hardwood, always at board ends and usually everywhere, to prevent splitting and to avoid snapping stainless screws, which have lower shear strength than hardened steel. Two fixings per board at each joist is the usual pattern, set in from the board edge; follow the decking supplier's guide for the species.
  • Keep coated steel away from copper-treated timber. Roofing manufacturers advise against contact, as noted in our materials compatibility article.

The same logic applies to fence brackets, pergola connectors, balustrade posts and cladding battens. If it is outside, it is hot-dip galvanised, Class 3 or better, or stainless, and the choice is written down before the order goes in.

Plasterboard and internal linings

Internal fixings seem low risk until a ceiling sags or a line of screw heads appears through the paint. Gypsum linings are installed to AS/NZS 2589, and manufacturers such as Siniat and Gyprock publish installation guides that apply it.

  • Screw type. Plasterboard screws have a bugle head and are driven to just below the paper surface without breaking it. They must comply with AS 3566, and a Class 1 finish is acceptable in dry internal areas. For external walls and humid rooms, manufacturers specify hot-dip galvanised or Class 3 fasteners.
  • Thread. Fine thread was traditionally used for steel studs and coarse for timber. Siniat's testing shows both 6g and 8g coarse and fine thread screws perform in timber, and AS 3566 does not restrict the thread form, so either is acceptable in timber provided the length is right. Manufacturers have traditionally recommended fine thread for steel studs, with a drill point for heavier steel.
  • Length. The number most often missed: 10 mm plasterboard ceilings on timber frames need 30 mm screws. Walls and other thicknesses have their own minimums in the standard and the manufacturer's guide.
  • Spacing. Framing supporting the lining is at no more than 600 mm centres, and 450 mm for 10 mm ceilings. Screw centres, edge distances and the pattern at butt joints come from the manufacturer's system sheet, which is also where fire-rated and acoustic systems specify screw type and spacing as part of the tested system. Changing the fixing changes the rating.
  • Overdriving. An overdriven screw that has broken the paper has no hold and will pop. The fix is a second screw about 20 mm away, not more torque. Keep screws at least 200 mm from adhesive daubs.
  • Treated and green timber are corrosive to plain steel fasteners, especially with moisture, so linings on treated framing in humid rooms need the higher-class screw.

For fire-rated walls, shaft walls and ceilings in apartment buildings, the fastener is part of the tested system. Our apartment floor acoustics article explains why "tested as a system" matters, and the same applies to fire.

Wet areas

In bathrooms, laundries and balconies the simplest rule about fixings is: do not put one through the waterproofing. A screw through a membrane is a hole in a membrane, whether it holds a shower screen, a grab rail, a towel rail or a floor waste grate.

In practice that means wall and floor penetrations are planned before the membrane goes down. Shower screen channels are fixed above the waterline or sealed by the method the membrane manufacturer specifies. Fixings into the wall behind tiles are set out so they land in framing and above the membrane turn-up where possible. Where a fixing has to pass through, the membrane maker's detail for sealing it is followed, using products tested with that membrane, as AS 3740 requires. Linings in wet areas are fixed with screws, not staples, and the screws are corrosion-rated for a humid room. See wet area waterproofing for the full picture.

Anchors into concrete and masonry

Fixings into concrete and masonry range from a plastic plug holding a towel rail to a chemical anchor holding a balustrade post or a bottom plate. The NCC treats the serious end as structural. Clause 2.2.4 of the ABCB Housing Provisions, Part 2.2 lists AS 5216 as the Deemed-to-Satisfy route for post-installed and cast-in fastenings in concrete. AS 5216 covers the design of mechanical and chemical anchors in safety-critical applications, where failure could cause injury or significant damage, and relies on anchors assessed under a recognised test regime (a European Technical Assessment under the relevant assessment document, or equivalent).

What this means on site:

  • Safety-critical anchors are specified, not chosen from the shelf. Balustrades, awnings, handrails on stairs, heavy wall-mounted plant and anything overhead fall into this category. The anchor type, brand, diameter, embedment, edge distance and spacing come from the engineer or the manufacturer's design tables, and are not interchangeable between brands.
  • Installation is part of the design. Hole diameter, drilling method, hole cleaning (brushed and blown, for chemical anchors), cure time and installation torque are all conditions of the published capacity. A chemical anchor set in a dusty hole is a different, weaker anchor.
  • Masonry is not concrete. Clay brick, hollow block, aerated concrete and old lime mortar each need anchors tested for that substrate. An anchor rated in solid concrete can have a fraction of that capacity in a hollow block.
  • Plastic plugs and light-duty screws are for light, non-structural items only, and even then into the brick, not the mortar joint.
  • Bottom plates to slabs follow the AS 1684 tie-down tables, using the anchor manufacturer's published limit-state uplift capacity, as noted above.

When we fix anything to a strata building's facade or slab edge, the anchor specification is recorded, and for safety-critical items a proof test or an engineer's sign-off goes into the file. See strata facade and balcony waterproofing for why penetrations through a facade also need a waterproofing detail.

Red flags on site

Any of the following is worth a conversation before the next layer goes on:

  1. One box of screws being used for everything from framing to roofing.
  2. Bright zinc-plated screws in decking, pergolas, fence brackets or any outdoor treated timber.
  3. Stainless screws into COLORBOND or ZINCALUME, or galvanised fixings into stainless cladding.
  4. Roof screws dished into the sheet, washers split or squeezed out, or screws driven on an angle.
  5. Metal filings left on a new roof.
  6. Joist hangers, nail plates or straps with empty nail holes.
  7. A connector or anchor "equivalent" to the one on the drawing, without the engineer's agreement.
  8. Screw heads through plasterboard paper, or 25 mm screws in a 10 mm ceiling on timber.
  9. Fixings through a wet-area membrane that nobody has a sealing detail for.
  10. Fasteners with no coating class, standard or maker on the box.

Questions worth asking

  1. Which standard or system sheet specifies the fasteners for this element, and does the order match it?
  2. What coating class, or stainless grade, is specified for each outdoor fixing, and what is our distance from the coast?
  3. Are the fasteners rated by the maker for contact with the treated timber, cladding or substrate they are going into?
  4. For roofing, which washer material is being used, and what is the fastener count per sheet per support for our wind class?
  5. For anchors in concrete or masonry, which product, embedment and installation method, and who checks it?
  6. Who inspects the fixings before they are covered, and where is that recorded?

How we approach it

Fasteners are listed with the materials they go into, by element, before ordering, and a substitute is treated as a design change. Framing tie-down, roof fixings, deck fixings and anchors are checked at the hold points before the next trade covers them, and the product data for coating class and anchor capacity goes into the handover file. If you are planning a build, a renovation or a repair and want the fixings checked rather than assumed, get in touch.

Sources

This article is general information, not technical or legal advice for your project. Fastener selection depends on the loads, the substrate, the exposure and the manufacturer's current guidance, so confirm it with the product data sheets, your engineer, your certifier and the relevant standards. Full website disclaimer

© 2026 Precision Projects Group Pty Ltd. All rights reserved. This article may not be reproduced, republished or adapted without written permission.

Codes and standards change. This article reflects the code editions and standards cited at its publication date. Check the current edition and your state's variations before relying on it. See ourfull disclaimer.

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