NZS 3404 hot-dip galv kettle length — the 11-13 m constraint nobody quotes

Steve Parker
May 30
7 min read

Updated: Jul 9

NZ hot-dip galv kettles run 11-13 m long. A 14 m PFC needs double-dipping under AS/NZS 4680 §4 — a 25-40% coating cost uplift the supplier's quote almost never names until the steel is fabricated.

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

NZS 3404 calls up AS/NZS 4680 for hot-dip galv durability. AS/NZS 4680 §4 allows double-dipping for members longer than the kettle, but the cost, the overlap appearance and the corrosion-performance compromise are rarely priced into the standard quote. The 11-13 m kettle constraint is one of NZ steel's largest hidden costs.

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

What you'll learn in this post

What NZS 3404 and AS/NZS 4680 actually say about hot-dip galv durability and the kettle constraint
The three failure modes from the 11-13 m kettle limit on NZ structural steel
A worked NZ example and the 5-item checklist before accepting a galvanising quote

Quick answer: NZS 3404 §5 references AS/NZS 4680 for hot-dip galvanizing of fabricated structural steelwork. Hot-dip galv kettle (bath) length in New Zealand is constrained by tank dimensions at the working galvanizers — typically 11 m to 13 m maximum. Members longer than the available kettle require double-dipping (dipping from one end then the other) under AS/NZS 4680 §4, which introduces an overlap band where coating thickness is uneven, corrosion performance is compromised, and the visual finish shifts. Most fabricators' quotes price single-dip galv at the standard $/m² rate; the double-dip uplift of 25-40%, the overlap-band touch-up and the longer cycle time arrive as a variation after the steel is cut to length. The 11-13 m kettle limit drives the variation.

NZS 3404:2009 is New Zealand's structural-steel design and construction standard. §5 covers durability, including hot-dip galvanizing. The cited standard for hot-dip galv is AS/NZS 4680, which specifies coating thickness, surface preparation and finish requirements for batched-dipped fabricated steel.

The standard is silent on kettle length — because the kettle constraint is a physical fact of New Zealand's galvanizing infrastructure, not a code rule. NZ's working hot-dip galv kettles run between 11 m and 13 m. A 14 m PFC, a 15 m UB, or a long-portal rafter cannot be dipped in a single immersion. The supplier quotes the galv at standard rate. The fabricator finds out at issue when the kettle availability check returns.

What NZS 3404 and AS/NZS 4680 actually say

NZS 3404 §5 names AS/NZS 4680 as the cited coating standard for hot-dip galv. AS/NZS 4680 specifies:

§3 coating thickness against the member's steel section thickness — typically 85 microns minimum local for thicker sections, 70 microns for thinner.
§4 application — including double-dipping, where a member too long for the kettle is dipped from one end, withdrawn, rotated end-to-end and re-dipped to coat the un-immersed portion.
§5 finish — including allowance for run lines, lumps and the overlap band where double-dipping produces a thicker / uneven coating zone.

AS/NZS 4680 §4.4 specifically addresses double-dipping. The clause permits the practice, but requires that the overlap region meet minimum coating thickness and that the coating remain continuous. In practice, the overlap region carries:

Coating thickness 1.5-2x the surrounding zone (excess zinc pickup at the second immersion line)
Run-line and drainage marks that are more pronounced
A visible aesthetic transition that is not always acceptable on architectural exposed steel

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Three failure modes from the 11-13 m kettle limit

1. Single-dip rate quoted on a member that requires double-dipping. A fabricator's quote for galvanising prices a single rate per square metre of surface area. A 14 m PFC quoted at the single-dip $/m² rate is priced 25-40% under the actual double-dip cost. Caught at fabricator-quote-stage, this is a single-line clarification. Caught at the galvanizer's intake desk, it's a variation on the steel order that ripples through the head builder.

2. Architectural exposed steel where the overlap band is not acceptable. The architectural specification might call up exposed structural steel — typically a long portal rafter or a long-span PFC in a feature opening — with a uniform hot-dip galv or duplex finish. The double-dip overlap band shows visibly. The architect can either accept the band, re-detail the member into two pieces with a site-welded splice (which then needs re-galv at the splice and creates a coating cold-joint), or upgrade to a fabricator-finish system with a different coating route entirely.

3. Programme impact on long-member galv cycle. A double-dip cycle takes ~50% longer than single-dip, and the kettle availability window for long-member double-dipping is constrained — most galvanizers schedule long members across specific shifts. A 14 m rafter that the programme assumed would galv in 5 working days typically takes 7-10 working days under double-dip. The programme impact under NZS 3910 §10.3 extension-of-time framework runs to the head builder and the QS.

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A worked NZ example: a 14 m portal rafter on a residential outbuilding

A residential outbuilding with a clear-span open front carries a 14 m × 310 UB structural rafter, hot-dip galvanized to AS/NZS 4680, architecturally exposed inside the structure.

Single-dip quote basis:

Member surface area: ~12.5 m²
Single-dip $/m² rate: $42/m²
Standard quote: ~$525

Actual double-dip cost under AS/NZS 4680 §4.4:

Surface area: 12.5 m² (unchanged)
Double-dip $/m² rate (NZ market): $58-62/m² (38-48% uplift)
Actual cost: ~$725-775
Overlap-band touch-up if required: $150-300
Programme uplift: 4-5 working days

| Cost item | Single-dip quote | Double-dip actual | Variation | |---|---|---|---| | Galv $/m² | $42 | $60 | +$18 | | Total galv on 12.5 m² | $525 | $750 | +$225 | | Touch-up overlap band | nil | $200 | +$200 | | Cycle time | 5 wd | 8 wd | +3 wd | | Total impact | — | — | +$425 cost + 3 wd |

On a residential outbuilding, the variation runs ~$425 plus a programme uplift. On a commercial project with a dozen members over 11 m, the same compounding logic runs $5-15k plus programme.

The 5-item checklist before accepting a steel galvanising quote

Member lengths over 11 m identified on the fabrication schedule — every PFC, UB, RHS or angle over 11 m needs the double-dip flag.
Single-dip vs double-dip rate confirmed per member — fabricator's quote should state which rate applies to each member.
AS/NZS 4680 §4.4 overlap-band acceptance confirmed — particularly for architectural exposed steel, the band should be reviewed against the architectural specification.
Programme allowance for double-dip cycle — typically +3-5 working days per long member, allocated under NZS 3910 §10 programme.
Coating system named (galv alone vs duplex) — for AS/NZS 2312.1 C3, C4 or C5 atmospheric zones, the system may be duplex with a paint top-coat that needs separate touch-up at the overlap band.

Each item caught at quote-stage is a single conversation with the fabricator. Each item missed is a variation through the steel order and a programme conversation with the head builder.

FAQ — NZS 3404 hot-dip galv kettle length

Q1: What's the typical NZ hot-dip galv kettle length? NZ's working hot-dip galv kettles typically run 11 m to 13 m. Specific facility dimensions vary; the longest commercial kettles in New Zealand sit around 13 m. Members longer than the available kettle require double-dipping under AS/NZS 4680 §4.4.

Q2: Does AS/NZS 4680 §4.4 permit double-dipping unconditionally? AS/NZS 4680 §4.4 permits double-dipping, subject to the overlap region meeting coating-thickness and continuity requirements. The overlap typically carries 1.5-2x the surrounding coating thickness, more pronounced run lines, and a visible aesthetic transition.

Q3: Can the architectural specification reject the double-dip overlap band? Yes — many architectural specifications for exposed structural steel call up a uniform finish, which the double-dip overlap band may not meet. The architect can specify a different coating route (typically a fabricator-applied paint system over a primed substrate) or re-detail the member to fit within the kettle length.

Q4: How does the kettle constraint interact with NZS 3404 §5 durability? NZS 3404 §5 sets the durability requirement; AS/NZS 4680 is the cited acceptable solution for hot-dip galv. The kettle constraint is a fabrication-route constraint, not a §5 durability constraint — the double-dipped member meets §5 if the coating-thickness and continuity rules under AS/NZS 4680 §3 are met.

Q5: What's the programme impact of a long-member galv cycle? A standard single-dip galv cycle runs 3-7 working days from receipt to despatch depending on the galvanizer's queue. A double-dip cycle typically runs 50% longer plus a kettle-availability wait — typically 7-12 working days. The programme allocation under NZS 3910 §10 should reflect the longer cycle.

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