Corrosion protection for steel fixings on coastal sites: choosing the coating

On a coastal build the fixings are the first part of the structure to rust and the last part anyone looks at. A nail, a screw, a bolt or a framing connector is a small amount of hardware in a job that runs to a great deal more, and it is the item most likely to be bought on price, substituted on the day or left unchecked once the cladding is up. When it fails, the damage is rarely confined to the fixing. Rust bleeds into the timber, the connection loosens, the cladding stains and the owner asks why nobody specified the right hardware for the place they chose to live.

This article is about choosing the coating and the material for steel fixings, brackets and framing connectors in a coastal environment: how to read the exposure, what the common coatings are for, and how to stop a cheaper item quietly taking the place of the one that was specified. It is not about galvanic corrosion, which happens when two different metals touch in the presence of moisture. That is a separate problem with its own rules, and it is covered in the article on galvanic corrosion between dissimilar metals. The question here is simpler: which coating suits which exposure, and how to prove it was used.

A coastal exposure is not a single condition. A fixing a few streets back from the beach and a fixing in the spray zone of breaking surf are both coastal, and they do not need the same protection. The job of the builder is to find out which one the fixing will meet, and to specify for that exposure rather than for the average day on the site.

Why the fixing fails before the frame does

Steel corrodes when it is wet and when salt is present, and the coastal atmosphere delivers salt to every exposed surface. The fixing is the part of the structure most exposed to that combination, because it is small, it sits in the most awkward corners and it is often hidden behind cladding that traps moisture against it. A bracket that is fine in a dry inland shed can be bleeding brown within a few seasons on a beachside deck. The loss of section is what matters structurally, and by the time it is visible the protection has usually been failing for some time.

There is also a timing problem. Fixings are bought early, stored on a site exposed to weather and installed late, sometimes after several weeks of salt air on the job. A coating that was adequate when it left the factory can be compromised before the frame goes up, particularly where the packaging has been opened and the items were left in a pile. Storage is part of the specification, not an afterthought.

Fixings set into concrete raise a related point. The steel in the fixing is only one part of the picture, and the cover over it and the quality of the concrete around it decide how much salt and moisture reach the metal. A bolt set shallow into a slab edge, or cast into concrete that was poorly compacted, will corrode from the inside out whatever coating it carries. The article on reinforcement cover and concrete durability covers that side of the problem, and the two should be read together wherever fixings are set into concrete at the edge of a coastal slab.

Reading the exposure, not the postcode

The exposure classification for a site is a matter for the designer or engineer, who should work it out from the site’s position, the distance from the breaking surf, the prevailing wind, the shelter provided by other buildings and the height of the element. A postcode is a poor guide, because two houses on the same street can sit in quite different conditions. The Australian Standards set out the exposure classes in words and in the relevant tables, and the current edition should be checked for the job rather than remembered from a previous one.

Where the exposure sits on the boundary between two classes, the conservative choice is usually the cheaper one to live with. Upgrading the coating on a few dozen connectors adds little to the cost of a frame, while replacing a corroded deck or a rotten bearing is a large and disruptive job. A designer may argue for the lower class to save money. The builder should ask for that argument to be made in writing, with the reasons, before the lower class is accepted.

The builder’s responsibility is to get that classification in writing before the hardware is ordered. If the drawings say only that the site is coastal, ask for the exposure to be stated in the terms the coating specification uses. A fixing specified for a sheltered inland position and then installed in the spray zone is a specification failure, even if the installer did exactly what the drawing said.

The coatings, and what each one is actually for

The common options fall into a small number of families. Plain zinc coating on mild steel is the baseline, and it is adequate only in the most sheltered positions. Hot-dip galvanising gives a much thicker coating and is the workhorse of coastal framing hardware in most positions. Stainless steel is the choice for the most severe exposures, but it is not a single product: the grade matters, and marine-grade stainless is the form that belongs in the spray zone. Polymer and powder coatings add a barrier over steel, which is useful only while the coating is intact.

The table below sets out where each option earns its place. It is a guide to the principle, not a substitute for the specification and the standard that applies to the job.

The useful discipline is to pick the coating for the worst exposure the item will meet, not for the most common one. A bracket that spends most of its life sheltered under a deck but is splashed by spray in a storm is a spray-zone bracket. A connector that is protected by the cladding for the first year and then exposed when the cladding is removed is an exposed connector from the day it is exposed. The specification should say which condition it was written for.

Timber, treatment and the metal that touches it

Coastal framing is often timber, and the preservative in the timber matters to the fixings. Some copper-based treatments corrode zinc coatings considerably faster than untreated timber would, so a galvanised bracket fixed into treated timber can fail earlier than the same bracket in untreated timber. The fixing has to be specified for the treatment, not just for the environment. Ask the timber supplier which treatment has been applied and check it against the fixing supplier’s guidance before ordering.

The timber is also a source of moisture. Fixings driven into timber that is wet at the time of fixing hold water against the metal, and the coating is then working in a damp crevice rather than in open air. Where timber is delivered wet or stored where it will take up water, the timing of the fixing should be part of the plan. A deck bracket fixed into a joist that is still drying out is a bracket that will be rusting from the inside.

Read the article on steel framing and cold-formed fixing details alongside this one. Light-gauge steel framing has its own connectors and fixings, and the coating decisions for those connectors follow the same exposure logic as the brackets on a timber deck.

Cut, drilled and damaged surfaces on site

Hot-dip galvanising protects the steel by sacrificing its own zinc, and that protection works well across the surface of the member. It works less well at the edges where a member has been cut, drilled, welded or scratched during installation. A galvanised member that has been drilled on site has an area of exposed steel, and that area needs the same kind of protection the original coating provided before it is covered by cladding or lining. The specification should name the touch-up material and the procedure, rather than leaving it to whoever happens to be on site with a tin of paint.

The same applies to any coated item that is damaged during fixing. A polymer-coated connector that has been driven with a hammer and scraped on a steel edge has a break in the barrier, and the break is where the rust will start. Set a rule that damaged coated items are replaced, not patched on the job, unless the coating manufacturer has specified a repair that is suitable for the exposure. Patching is a common shortcut and a common cause of later complaints.

Keeping the coatings apart on site

Mixed hardware is the quiet cause of many coastal failures. Plain mild steel screws, galvanised brackets and stainless fixings often look alike in a box, and a labourer working to a deadline will use what is nearest. The fix is physical as well as paperwork. Keep each coating in its own labelled container, do not open a box of the specified type until the item is needed, and return any offcuts of the wrong type to the store rather than leaving them in a tool bag. A stainless fixing in a galvanised bracket is a small mistake that becomes a corrosion problem wherever the two touch in moisture.

Check the coating on delivery as well. Packaging that shows a different coating from the order, a mix of finishes in a single box or a bracket with a visibly uneven zinc layer should go back to the supplier before any of it is used. The receiving check is cheap, and it is the only point at which a wrong item can be caught before it is inside the wall.

Bushfire and coastal requirements on the same site

Some coastal sites are also in bushfire-prone land, and the bushfire construction requirements can set their own expectations for the materials and fixings used in exposed elements. The NSW Rural Fire Service’s planning guidance explains how the bushfire framework applies to a development, and the designer should check both the bushfire and the coastal requirements before the fixings are chosen. The two sets of requirements usually point the same way, but where they do not, the stricter expectation should govern the specification.

Where the two overlap, the specification should say so in plain terms. A note that the fixings meet the coastal exposure and the bushfire construction rules, with the coating named, gives the installer and the certifier a clear reference. Without it, the installer will choose the item that looks right, and the certifier will have no way to check that the choice was correct.

A worked example: a deck bracket on the south coast

A builder on the New South Wales south coast was constructing a timber deck on a house a short walk from the surf. The brackets and the bolts had been specified as hot-dip galvanised, and the builder had ordered them with the framing. When a shortage delayed the brackets, the site supervisor used plain zinc-coated bolts from a local merchant to keep the deck on programme. The bolts looked the same and the drawing said only galvanised, so the substitution passed the site check.

Within a few seasons, rust stains were running down the posts below the bracket connections, and the timber around the bolt holes had begun to darken. The owner raised it at the handover inspection. The repair meant replacing the brackets and bolts, cutting out and replacing the affected timber, and making good the finish, which cost far more than the difference in hardware would have. The builder also had to explain to the owner why the specification had not been followed, and the answer was that nobody had checked the item against the drawing when it was installed.

The lesson the builder drew was procedural. The specification now names the coating and the exposure on the order, the supplier must confirm the coating in writing before dispatch, and a substitution is treated as a variation that needs the designer’s approval before it goes into the work. The cost of that process is small, and it removes the situation in which a shortage quietly becomes a defect.

What to keep on file for a coastal build

Keep the exposure classification, the coating specification, the supplier’s written confirmation of the coating delivered, and the record of any approved substitution. Keep the timber treatment details alongside them, and the touch-up procedure for cut and damaged galvanised surfaces. These records matter most years later, when a stain appears and the question is whether the builder specified and delivered what was required.

Coastal work is a good example of why the basics of specification and substitution control should be part of a builder’s professional development. The PointCPD guide to how NSW builder CPD works explains how that learning is structured for licensed builders. The coating decision is a small line on a drawing, but it governs whether a structure is still sound when the owner has forgotten who built it.

Questions builders ask

• Corrosion
• Durability
• Fixings

Published 23 October 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.