Waiheke coastal residential case study — three steel coating misspecs caught at tender

Steve Parker
Jun 11
9 min read

Updated: Jul 9

A Waiheke coastal residential build had a structural steel quote with the right tonnage, the right fabricator, and three coating specifications that wouldn't survive C5 marine. AS/NZS 2312 and AS/NZS 4680 between them resolved all three at tender stage and saved $35-65k of re-coating exposure.

By Steve Parker · Trueworks · NZ construction estimation · 5 min

The steel was right. The tonnage was right. The fabricator was right. Three coating lines were quoted against C3 inland conditions on a site that's C5 marine. The fix was a tender-stage re-spec; the cost of not catching it was a coating warranty void within 5-8 years of practical completion.

By Steve Parker · Trueworks · NZ construction estimation · 5 min

What you'll learn in this case study

The three steel coating misspecs that appear on every coastal residential quote priced by mainland fabricators
The AS/NZS 2312 and AS/NZS 4680 clauses that govern the resolution
The dollar effect of catching the coating spec at tender versus at first inspection in year 5

Quick answer: A Waiheke coastal residential build had a structural steel quote where the column coating was named as "primer + 2-coat enamel" (a C3 inland spec), the beam coating as "hot-dip galv to AS/NZS 4680" without naming the coating thickness, and the cleat and fixings as "hot-dip galv to suit." All three are under-specified for C5 marine corrosivity. AS/NZS 2312 sets the corrosivity-zone-driven coating thickness; AS/NZS 4680 governs hot-dip galvanising minima. Catching all three at tender saved $35-65k of re-coating exposure at year 5-8.

The build

A two-storey contemporary residential on a Waiheke ridge site within 400 m of the high-water mark. Build value in the $2.5-3.5M range. Structural steel scope was a moderate portal-frame plus internal steel beams for an open-plan ground floor — roughly 7-10 tonnes total. The head contractor sent the steel fabricator's quote across at tender stage for review.

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What we found in the quote

The fabricator's quote was professional. The tonnage matched the engineer's framing plan. The connection details were called up against the engineer's standard schedule. The fabricator was an established Auckland-based steel shop with good warranty support. The pricing was within the band we'd expect for a 7-10 tonne portal-frame and beam package.

The coating specifications had three problems.

Problem 1: The column coating. The columns were called up as "primer + 2-coat enamel" — a standard inland Auckland spec for C3 corrosivity. AS/NZS 2312 sets the durability classes for paint systems against atmospheric exposure: a 2-coat enamel over standard primer is a C3-rated system with a typical durability of 5-10 years to first maintenance. On a Waiheke C5 site (within 1 km of the Hauraki Gulf, full salt-spray exposure) the same paint system has a durability of 2-5 years before substantial maintenance is required. The columns would need re-coating within the first 5 years of the build's life — and the columns are mostly interior or partially exposed under the roof, so the re-coating access would require scaffold and partial interior dust protection.

Problem 2: The beam coating. The beams were called up as "hot-dip galv to AS/NZS 4680." AS/NZS 4680 sets the hot-dip galvanising standard but allows for several coating thickness bands depending on steel article thickness and intended exposure. The default coating thickness for AS/NZS 4680 is 600 g/m² (about 85 µm). On a C3 site that's adequate. On a C5 marine site the appropriate thickness is at minimum 750 g/m² (about 105 µm) and ideally a duplex system (hot-dip galv plus a marine-grade paint over-coat). The quote didn't name a thickness, didn't reference C5, and didn't specify duplex.

Problem 3: The cleat and fixings. The connections were "hot-dip galv to suit." This is the catch-all spec that defaults to whatever the fabricator's standard is, which on most mainland shops is the same 600 g/m² minimum. On a C5 site, cleats and fixings are the parts most exposed to driven salt spray and are also the parts most prone to crevice corrosion at bolt threads and connection laps. The recommended spec is mechanical galv with a minimum 75 µm coating, or stainless 316 for the most exposed connections.

Together the three misspecs were sitting at $0 of variation on the quote and a $35-65k re-coating exposure at year 5-8.

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How the code resolves it

AS/NZS 2312 (Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings) is the controlling standard. The standard classifies external atmospheres into corrosivity categories C1 through C5 plus CX (industrial chemical), defines the expected first-maintenance interval for each coating system in each category, and provides selection tables for matching coating systems to required durability.

For Waiheke (C5 marine), the AS/NZS 2312 selection table for "high" durability (15-25 years to first maintenance) requires either:

A hot-dip galvanised coating to AS/NZS 4680 at minimum 750 g/m² plus a marine-grade two-pack epoxy paint system over-coat (duplex), or
A heavy two-pack epoxy paint system on prepared steel without galvanising (a "thick film" spec, typically 250-300 µm)

AS/NZS 4680 (Hot-dip galvanised (zinc) coatings on fabricated ferrous articles) governs the galvanising process itself. The standard sets minimum coating thickness as a function of steel article thickness (thicker steel articles get thicker galv coating naturally), with a default minimum around 600 g/m². For C5 service the standard allows the specifier to call up a higher class (typically 750 g/m² or higher), but the specification has to be explicit — the standard default isn't enough.

The reconciliation is straightforward at tender stage: the columns get re-spec'd to a duplex system (galv + 2-coat marine epoxy), the beams get re-spec'd to AS/NZS 4680 at 750 g/m² minimum with optional duplex over-coat for the most exposed locations, and the cleats get re-spec'd to mechanical galv at 75 µm minimum or 316 stainless for exposed connections.

What it would have cost if caught later

| Stage caught | Cost range | Why | |---|---|---| | At tender (paper review) | $1,500-3,000 (revised coating cost) | All three coating specs re-issued at tender; fabricator quotes the duplex system; price difference between C3 and C5 coating is typically 8-15% of fabricated steel cost | | At fabrication (post-award) | $4,000-9,000 | Coating spec changed mid-fabrication; possible stripping of inadequate coatings already applied; programme delay of 1-2 weeks while coating is re-done | | Pre-erection (post-fabrication) | $12,000-25,000 | Steel rejected and returned to fabricator for re-coating; full restart on coating; 3-6 weeks of programme delay; possible damages claim from erector for missed window | | Year 5-8 (under maintenance) | $35,000-65,000 | Substantial re-coating required; scaffold access on installed structure; interior dust protection; possible coating failure already visible at fasteners and laps; warranty disputes between fabricator and owner | | Year 10+ (under failure) | $80,000-150,000 | Substantial corrosion at exposed connections and possible structural section loss; engineering re-assessment required; possible partial replacement of corroded members |

The Waiheke build was 7-10 tonnes of steel. The cost premium for the correct C5 coating spec at fabrication was about $3-5k. The cost of getting it wrong and remediating at year 5-8 was 10-15 times that. Catching it at tender was the cheap option in every dimension.

The clarification we recommended

The clarification packet to the head contractor named all three coating misspecs, cited AS/NZS 2312 (corrosivity selection) and AS/NZS 4680 (galv thickness), and recommended a revised coating schedule:

Columns: duplex coating system — hot-dip galv to AS/NZS 4680 at 750 g/m² plus 2-coat marine-grade epoxy paint over-coat (total system thickness ~200 µm)
Beams: hot-dip galv to AS/NZS 4680 at 750 g/m² (single-system spec acceptable for protected beam locations; duplex over-coat for exposed beams under eaves)
Cleats and fixings: mechanical galv at 75 µm minimum, or 316 stainless for connections within 200 m of the high-water mark and for any exposed-to-weather fixings
Site touch-up: zinc-rich repair paint compatible with the duplex system, supplied by fabricator with installation instructions for the erector

The fabricator quoted the revised spec inside three working days. The price increment was about $3.5-4.5k on the steel package — well inside the contingency the head contractor had allocated. The engineer to the contract reviewed and approved the revised coating schedule.

What other Waiheke coastal residential builds should check

AS/NZS 2312 C5 corrosivity is the default assumption for any Waiheke site within 1 km of the Hauraki Gulf — the steel coating spec has to be priced against that, not against mainland C3
AS/NZS 4680 hot-dip galv has multiple coating thickness classes; "to AS/NZS 4680" without naming a class defaults to the lowest acceptable thickness, which is inadequate for C5
Cleats and fixings are the most exposure-critical connections on a coastal residential build — stainless 316 or mechanical galv at 75 µm minimum, not "to suit"
The cost premium for the correct C5 coating spec is typically 8-15% of fabricated steel cost at supply; the cost of remediation in years 5-10 is 10-15x that premium
The fabricator's coating warranty terms should be reviewed against the actual exposure — most fabricator warranties explicitly exclude failure where the coating was specified inadequately for the exposure

FAQ — structural steel coating on Waiheke coastal residential builds

Q1: How do I know if my Waiheke site is C5 under AS/NZS 2312? Most of Waiheke sits within 1 km of an unsheltered coast, putting it in C5 under the standard's distance-from-coast classification. The engineer's coating specification or a written corrosivity assessment against AS/NZS 2312 Annex E confirms the category for a specific site. Sites further inland on the island may be C4; few are below C4.

Q2: What's the price difference between a C3 paint spec and a C5 duplex coating spec? Typically 8-15% on the fabricated steel cost. On a 7-10 tonne residential package that's $2-5k of additional coating cost. The price isn't large; the issue is that it has to be specified, not assumed, because the standard fabricator default is the C3 spec.

Q3: Can a paint-only system (without hot-dip galv) work on a Waiheke C5 site? Yes — a heavy two-pack epoxy paint system at 250-300 µm total thickness is an alternative to a duplex galv system under AS/NZS 2312 for C5 service. The trade-off is that paint-only systems are more vulnerable to mechanical damage (chips, scratches) than galv, and the touch-up regime has to be more rigorous. Most coastal residential builds use the duplex system for that reason.

Q4: How long does correctly-spec'd C5 coating last before first maintenance? Under AS/NZS 2312 the "high" durability category targets 15-25 years to first maintenance. In practice, a well-applied duplex system on a Waiheke site typically delivers 18-25 years before significant coating maintenance is needed. A correctly-applied 750 g/m² galv alone (without duplex paint over-coat) typically delivers 12-18 years.

Q5: Who owns the warranty if the coating fails early? On a correctly-specified system, the coating warranty sits with the fabricator and/or the paint manufacturer. On a system that was specified inadequately for the exposure, both the fabricator and the paint manufacturer typically reject warranty claims and liability falls on the specifier (engineer or architect) or the head contractor under the design responsibility clause of the contract. Specifying correctly at tender keeps the warranty defensible.

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