MRM CoP §4.5 + NZS 3640 — Why H3.2 Purlins Void the Metal Roof Warranty on NZ Residential Builds

sp8002
3 hours ago
6 min read

Purlins under NZ metal roof sheet must be H1.2 only — MRM CoP §4.5. A carpenter substituting H3.2 voids the 30-year manufacturer warranty on the entire roof. On a typical residential roof that's $30-50k of avoidable exposure.

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

What you'll learn in this post

Why H1.2 only, not H3.2
The treatment-class reference table
Where the substitution gets missed on a typical NZ residential build

Quick answer: Purlins directly under NZ long-run metal roof sheet must be H1.2 treatment only under MRM Code of Practice §4.5 — the standard treatment is set by NZS 3640. Substituting H3.2 (which is the correct treatment for exposed deck joists right next door) voids the manufacturer's 30-year warranty on the entire roof. The copper in H3.2 reacts with the back of the sheet at the contact line and accelerates corrosion. On a typical NZ residential metal roof that's $30-50k of avoidable warranty exposure hidden in a single timber-treatment line.

Purlins directly under a long-run metal roof sheet must be H1.2 treatment only per the NZ Metal Roof & Wall Cladding Code of Practice §4.5, with treatment classes defined under NZS 3640. Substituting H3.2 — which is the standard for exposed deck joists and external timber framing right next door — voids the manufacturer's 30-year warranty on the entire roof.

On a typical NZ residential metal roof, that's roughly $30-50k of avoidable warranty exposure hidden in a single treatment-class line on the timber order.

This is the kind of find that only shows up when the carpentry scope and the roofing scope are read side-by-side against the code. Reading either trade alone, the issue doesn't surface.

Why H1.2 only, not H3.2

The treatment classes are different chemistries for different exposure scenarios, under NZS 3640:

H1.2 — light fungicide treatment for interior framing protected from weather. Standard for wall studs, ceiling joists, and purlins under a continuous roof envelope.
H3.2 — heavier copper-based treatment for timber exposed to weather. Standard for deck joists, exposed timber posts, exterior fascia.

The metal roof CoP requires H1.2 specifically because the copper-based components in H3.2 react chemically with the back of the steel sheet, accelerating corrosion at the contact line. This is well-documented in MRM CoP §4.5 and is the reason manufacturers exclude H3.2 from their warranty coverage on all major NZ metal roof brands.

A purlin doesn't know whether it's "under a roof" or "exposed timber" — the spec lives in the drawings and the carpenter's scope. If the carpenter is also doing the deck joists in H3.2, it's natural to consolidate the timber order to one treatment class. That's the trap.

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The treatment-class reference table

| Treatment | NZS 3640 use case | Suitable under metal roof? | Failure mode if mis-used | |---|---|---|---| | H1.2 | Interior framing, purlins under sealed roof envelope | Yes (required) | None — code-compliant | | H3.2 | Exposed deck joists, fascia, external timber | No | Copper reaction with steel sheet → accelerated corrosion | | H4 | Posts in ground contact | No | Not applicable to roof framing | | H5 | Heavy ground-contact / critical structural | No | Not applicable to roof framing |

The "round-up to H3.2" instinct on a single yard order is the single biggest cause of warranty-void purlin installs we see on NZ residential.

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Where the substitution gets missed on a typical NZ residential build

Most NZ residential carpentry is quoted as a single line-item — "wall framing + floor joists + roof framing + deck framing — all timber, fixings, treatment as drawn." The carpenter buys the timber from a single yard order. If the order line for purlins doesn't explicitly call H1.2, and the bulk of the deck order is H3.2, the yard will commonly fulfil purlins in H3.2 because it's "stronger / more durable."

Three failure modes:

Carpenter doesn't know the warranty rule. Most NZ residential carpenters work from the architect's specification, the engineer's specification, and the timber merchant's product listing — not from the MRM CoP.
Architect or engineer doesn't call it out explicitly. The detail typically reads "70 × 45 purlins" without specifying treatment, on the assumption the contractor knows H1.2 is the only acceptable treatment under metal roofing.
Roofer doesn't inspect the substrate. By the time the roofer arrives, the purlins are already up and the treatment stamp is hidden against the framing.

The defect only surfaces if a leak develops at year 8-12 and the manufacturer's warranty team inspects the substrate. By then it's the head contractor's problem and the original carpenter is long gone.

Three checks before the carpentry order is placed

Each one takes under five minutes:

Read the carpenter's quote line by line for treatment class. If purlins are listed in the same line as deck joists, ask explicitly: "Are these purlins H1.2?"
Cross-reference against MRM CoP §4.5 and the metal roof sheet manufacturer's published warranty document. Both will name H1.2 as the only acceptable purlin treatment under their sheet.
Add a written clarification to the carpentry scope. Single line: "All purlins under metal roofing to be H1.2 treatment per MRM CoP §4.5 and NZS 3640. H3.2 substitution voids the roof sheet warranty and will be rejected at inspection."

What it costs to fix at each stage

Caught at carpentry tender stage: $0. Clarification added to scope. Treatment class is no harder to source.
Caught after framing is up but before the roof is on: ~$2k. Re-frame the purlin layer. Programme cost: 1-2 days.
Caught after the roof is on: $40k+. Strip the roof, replace purlins, re-sheet, re-flash. Programme cost: 2-3 weeks plus engineering and consent re-issue.
Caught at year 8-12 by the manufacturer's warranty team: the warranty is void. The head contractor wears the full re-roof cost — typically $30-50k on a moderate residential — plus any consequential damage from the leak.

FAQ — H1.2 vs H3.2 purlins on NZ metal roofs

Q1: Is H1.2 cheaper than H3.2? Typically marginally cheaper per linear metre at the yard, but the price gap is small enough that "round up to H3.2" is the default for carpenters who don't track the warranty rule. The cost differential isn't the trap — the workflow differential is.

Q2: Does the H1.2-only rule apply to every NZ metal roof brand? Yes — every major NZ metal roof manufacturer's warranty references MRM CoP §4.5 directly or names H1.2 as the only acceptable purlin treatment. Substituting H3.2 voids the warranty on all dominant NZ profiles.

Q3: What if the purlins are sarked / building paper installed below them? Sarking and building paper sit above the purlins (between purlins and roof sheet) and provide a moisture barrier — but they do not isolate the purlin from the sheet sufficiently to prevent the H3.2 copper reaction at the fixing contact points. The MRM CoP §4.5 rule applies regardless of sarking.

Q4: Can I get a written waiver from the manufacturer if H3.2 has been installed? On all dominant NZ metal roof brands, no — the warranty exclusion for H3.2 is non-negotiable. Some manufacturers will issue a one-off limited-warranty product on case-by-case basis but the standard 30-year cover is void.

Q5: How do I prove the purlins are H1.2 after install? Treatment stamps are visible on every length of treated framing — visible end-face stamping with the treatment class. Best practice: photograph the stamps before sheet install and file in the project documentation. Some head contractors require the carpenter to supply the treatment producer statement from the timber merchant as part of the producer-statement chain.

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