Manukau 12-dwelling group home case study — the H1.2 vs H3.2 purlin gap caught on dwelling 1

Steve Parker
May 30
8 min read

Updated: Jul 9

A Manukau 12-dwelling group home had the carpentry scope and the roofing scope quoted separately. Both spec'd H1.2 in some places and H3.2 in others. The crossover — purlins under the metal roof sheet — must be H1.2 only per MRM CoP §4.5. Caught on dwelling 1 it saved $80-120k of warranty exposure across the remaining 11.

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

One trade scope spec'd H3.2. Another spec'd H1.2. Read separately, both looked code-compliant. Read together against MRM CoP §4.5, the overlap voided the metal-roof warranty on every dwelling.

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

What you'll learn in this case study

The H1.2 vs H3.2 purlin treatment trap that only appears when two trade scopes are read against each other
The MRM CoP §4.5 and NZS 3640 clauses that resolve it
The dollar effect of catching it on dwelling 1 versus catching it after dwelling 12 is roofed

Quick answer: A Manukau 12-dwelling group home had the carpentry framing scope quoted with H3.2 noted on "all structural framing in moisture-exposed locations" and the roofing scope quoted with "H1.2 purlins" as a standalone line. The two scopes overlapped on the purlins immediately under the metal roof sheet. MRM CoP §4.5 requires H1.2 only at that location; H3.2 timber reacts with the steel sheet underside and voids the manufacturer's 30-year warranty. Catching the gap on dwelling 1 saved the warranty across all 12 dwellings — an exposure of roughly $80-120k in replacement-cost terms.

The build

A 12-dwelling group home development in Manukau, build value in the $5-7M range. Single-storey dwellings, identical floor plan repeated 12 times, trapezoidal metal roof sheet on every roof, light timber framing throughout. The head contractor sent both the carpentry framing quote and the roofing quote across for review at tender stage as part of a standard quote-check engagement.

Roofing quote or roof spec on your desk right now? Get it checked in writing before you sign or submit — first check free. Send us your stamped drawings + the supplier's quote. We'll return a code-cited Quote-Check packet within 24 hours. Free for first-time customers. NDA available, NZ-hosted processing. → Get the free check at trueworks.co.nz/contact — or email hello@trueworks.co.nz

💡 Caught something similar on your job?

What we found in the quote

The carpentry framing scope was internally consistent. The framing schedule named timber treatment per location: H1.2 to all internal framing, H3.2 to "external wall framing and any structural framing exposed to weather or moisture during construction," H3.2 to deck framing, H5 to in-ground posts. The specifier had taken a conservative reading of NZS 3640 and pushed up-treatment wherever there was a defensible case for moisture exposure.

The roofing scope was also internally consistent. The roofing quote named the sheet substrate, the underlay, the fastener class, and a single timber line: "purlins H1.2 to MRM CoP §4.5." The roofer was reading the metal roof manufacturer's Code of Practice and quoting against the treatment level the manufacturer requires under the sheet.

Read separately, both scopes were code-compliant.

Read together, they conflicted. The purlins are structural framing in a moisture-exposed location (per the carpentry spec interpretation) and are the timber immediately under the metal roof sheet (per the MRM CoP). The carpentry scope said H3.2; the roofing scope said H1.2. On site, whichever trade installed the purlins first would install them to its own scope. If the carpenter went first — which on a 12-dwelling repeat build is normal — the purlins would go in as H3.2.

H3.2 under a metal roof sheet is not just a warranty issue. The copper-chrome-arsenate (CCA) treatment chemistry in H3.2 timber reacts with the zinc-aluminium coating on the steel sheet underside. Over 5-10 years the reaction degrades the coating from the underside, leading to early sheet failure that presents externally as rust streaks at fastener heads and along sheet laps. The metal roof manufacturer's 30-year warranty explicitly excludes failure caused by contact with incorrectly-treated timber. The standard manufacturer position is: H3.2 in contact with the sheet underside voids the sheet warranty across the affected area.

On a 12-dwelling repeat build, "the affected area" is every dwelling. If the gap had gone uncaught and the framing had been built out across all 12 dwellings, the warranty exposure would have been the full roof replacement cost on every dwelling — typically $80-120k of warranty exposure across the development.

The Trueworks Per-Project pack (full estimation, risk register, contingencies table and pre-trade-start sheets across all major trades) is quoted per build. A Trueworks quote check starts at NZ$179 per quote — and your first one is free. Builders typically save NZ$10,000–20,000 of their own time per job. Get your first quote check →

📋 Want this kind of risk review on every trade?

How the code resolves it

MRM CoP §4.5 is the operative clause. The Code of Practice for metal roof and wall cladding (issued by the Metal Roofing Manufacturers Association) requires that timber in contact with the underside of metal roof sheets be H1.2 treated boron, LOSP, or untreated kiln-dried — and explicitly excludes H3 and above treatments due to chemistry incompatibility. The clause is a manufacturer's warranty condition, not a Building Code requirement per se, but it's referenced in most CodeMark certifications for metal roof systems and is therefore the controlling standard for warranty purposes.

NZS 3640 (the timber treatment standard) defines the H-class hazard levels (H1.2 through H6) and the chemistry and depth-of-penetration requirements for each. Under NZS 3640, H1.2 is appropriate for protected internal applications and is the standard treatment for roof framing where the framing is protected from direct wetting. H3.2 is appropriate for exposed external timber subject to wetting but not in-ground.

The reconciliation is that purlins, when read under NZS 3640 alone, can be argued either way — they're external in elevation but protected from wetting under the roof sheet. Under MRM CoP §4.5 the argument resolves to H1.2 only. The MRM CoP wins because it's the warranty-controlling document for the metal sheet, and the warranty is the asset being protected.

What it would have cost if caught later

| Stage caught | Cost range | Why | |---|---|---| | At tender (paper review) | $0-500 (clarification fee) | Purlin treatment line corrected in carpentry scope to H1.2; both trades aligned on a single specification; one note added to the framing schedule | | At dwelling 1 framing | $1,500-3,500 | Carpenter swaps purlin stock to H1.2 before installation; small premium on faster delivery of H1.2 stock if H3.2 was already on site; 0.5-1 day of programme noise | | At dwelling 1 roof (post-purlin install) | $8,000-15,000 | Strip and replace purlins on dwelling 1 before sheet install; carpenter back on site; framing inspection re-issued; 1-2 weeks of programme delay | | Across dwellings 1-12 (gap uncaught until close-out) | $80,000-150,000 | Full warranty exposure on every dwelling; manufacturer technical review on every roof; remediation involves stripping the metal sheet on every dwelling to replace purlins or accepting a voided warranty; potential litigation between owner, head contractor, and either the carpenter or the roofer depending on contract terms |

The cost asymmetry is the most extreme we see in repeat-build work. On dwelling 1 the fix is a 0.5-day swap of timber. Uncaught across 12 dwellings the fix is a development-scale remediation. The discipline is to catch it on paper before dwelling 1 framing starts.

The clarification we recommended

The clarification packet to the head contractor named the H1.2 vs H3.2 conflict, cited MRM CoP §4.5 and NZS 3640, and recommended a single amendment: the carpentry framing schedule purlin line to be amended from "H3.2 external structural framing exposed to weather" to "H1.2 purlins per MRM CoP §4.5 (overriding general external framing treatment specification)." The amendment was a single annotation on the framing schedule that put the controlling clause explicitly on the drawing.

The head contractor circulated the amendment to the carpenter and the roofer before dwelling 1 framing started. The carpenter swapped the purlin stock for the development from H3.2 to H1.2. The cost difference at supply was minor (H1.2 is cheaper per linear metre than H3.2). The roofer's warranty position on every dwelling was preserved.

What other Manukau group home developments should check

On any repeat-build development, the carpentry and roofing scopes must be read against each other before dwelling 1 framing starts — internal consistency within each scope is necessary but not sufficient
MRM CoP §4.5 overrides general external-framing treatment specs at the purlin location — the framing schedule must call out H1.2 at the purlins specifically
The cost of getting the purlin treatment wrong scales linearly across dwellings in a repeat build, so the catch must happen before dwelling 1 framing, not after
Any timber in contact with the underside of metal roof sheets (purlins, top plates if exposed under unlined eaves, fascia where it sits behind the sheet edge) is subject to MRM CoP §4.5 — not just the purlins
The carpenter's scope should explicitly note "MRM CoP §4.5 controls at all roof-sheet contact locations" so the trade hierarchy is clear on site

FAQ — carpentry and roofing crossover on Manukau group home builds

Q1: Why does H3.2 react with metal roof sheets when H1.2 doesn't? H3.2 typically uses copper-chrome-arsenate (CCA) or copper-azole treatments, which contain copper compounds that react electrochemically with the zinc-aluminium coating on the steel sheet. H1.2 uses boron-based or LOSP treatments without the reactive copper, so there's no galvanic reaction at the contact surface.

Q2: Is H1.2 actually compliant with NZS 3640 for purlin applications? Yes — NZS 3640 categorises purlins as "protected from wetting" because they sit under the roof sheet and don't see direct rain. H1.2 is the correct hazard level for that exposure. The confusion arises when the framing specifier reads "external" geometrically rather than functionally.

Q3: Can the roofer install metal sheet over H3.2 purlins if a separating membrane is used? Some manufacturers permit a separating membrane (typically a polyethylene barrier) between H3.2 timber and the sheet underside, but the warranty terms become more restrictive and the membrane has to be specified by the manufacturer's technical service. On a 12-dwelling build, swapping to H1.2 is cheaper and cleaner than membrane-and-warranty-negotiation on every dwelling.

Q4: What's the cost difference per linear metre between H1.2 and H3.2 purlins? H1.2 is typically 5-15% cheaper per linear metre at supply because the treatment chemistry is less expensive. On a 12-dwelling development the supply-cost difference is small relative to the warranty exposure, but it's worth noting that getting it right is also the cheaper option at supply.

Q5: Who's liable if the gap goes uncaught and the warranty is voided? Liability typically sits with whichever party's scope contradicted the MRM CoP — usually the carpenter if H3.2 was installed under the sheet, or the roofer if they accepted incorrectly-treated framing without raising it. On a contract with NZS 3910 administration the engineer to the contract may also have exposure if the conflict was visible on the drawings and not flagged.

Get a free Quote-Check on your next job

Drawings + one supplier quote = a code-cited risk packet within 24 hours, ready for your pre-start meeting.

Free for first-time customers. No commitment. NDA available. Files NZ-hosted, deleted after 30 days unless you ask us to retain them.

→ Get the free check at trueworks.co.nz/contact — or email hello@trueworks.co.nz

About Trueworks

Trueworks is built by Steve Parker — 20 years on the analytical side of NZ construction. Variation reviews, contract advisory, programme review, and AI-augmented document workflows. Trueworks is the productisation of that practice for builders: same defensible analysis, at a price and pace a NZ builder can actually use.

Every report is checked and signed off by me personally before it goes out. If you've got a quote you want a second opinion on, the easiest way to find out if Trueworks is useful is to send it.

📧 hello@trueworks.co.nz · 🌐 trueworks.co.nz