Mt Eden re-roof case study — the H3.2 purlin pattern that survived a manufacturer warranty audit

Steve Parker
Jun 24
7 min read

Updated: Jul 9

A Mt Eden villa re-roof retained existing H3.2-treated purlins under a new metal roof — the wrong treatment class for direct contact with the sheet. Caught at quote review, the substrate correction protected the manufacturer's 30-year warranty before sheets went down.

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

The metal roof manufacturer's warranty has one specific timber-treatment requirement at the substrate. Get it wrong on a re-roof with retained purlins and the warranty audit at year 8 finds it. The pattern is simple, the consequence isn't.

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

What you'll learn in this case study

The H1.2 vs H3.2 purlin ambiguity on retained existing framing
The MRM CoP §4.5 and NZS 3640 resolution
The cost-of-catching-it-later table from quote review to a manufacturer warranty audit

Quick answer: On a Mt Eden 1920s villa re-roof, the existing 75 x 50 purlin pattern was retained under a new long-run trapezoidal metal roof. The retained purlins were H3.2-treated (correct for the original tiled-roof era but wrong for direct contact with a modern metal sheet). MRM CoP §4.5 requires H1.2 only for timber in direct contact with the sheet — H3.2's copper content reacts with the steel back. Caught at quote review the resolution cost $2,500-4,000 in a separator-strip retrofit. Caught at a year-8 manufacturer warranty audit it would have run $35-55k in early re-roof costs.

The build

A Mt Eden character-overlay 1920s villa re-roof: 145 m² of replacement metal roof over the existing rafter-and-purlin frame, retaining the existing 75 x 50 H3.2-treated purlin pattern at 900 mm centres. Standard residential head-contract, total re-roof in the $42-58k envelope including a small dormer addition. The villa carries the character-overlay protections of the Auckland Unitary Plan, which constrained the visible profile to a low-pitch trapezoidal sheet that read close to the original.

The carpenter and the roofer were separate trades. The carpenter scope included only minor purlin replacements at decay locations identified during the strip. The roofer scope included the sheet, the underlay, the flashings, and the fasteners.

The roofer's quote crossed our desk before the strip-and-re-sheet sequence started.

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What we found in the quote

The roofer's quote was clean on the substrate spec. Standard sheet (correctly named for Auckland inland C3), Class 4 fasteners (correct for C3), galvanised flashings (correct for C3), self-supporting underlay over the rafters per the manufacturer's specification.

The quote also included a single line we wanted to look at more carefully: "Existing purlins retained except where replacement required at strip stage. New purlins H1.2 treated. Underlay direct to purlins per manufacturer's specification."

The villa's original purlins were H3.2-treated. The 1920s era used a copper-based timber preservative, and subsequent maintenance and partial replacement work through the 1970s and 1990s carried through the H3.2 treatment class. The retained purlins under the new sheet would be H3.2.

The MRM Code of Practice for metal roof and wall cladding §4.5 is explicit: purlins, battens, or any timber framing in direct contact with the metal sheet must be H1.2-treated only. The copper compounds in H3.2 (CCA, ACQ, or copper azole) react with the steel back of the sheet — the technical mechanism is a galvanic reaction at the contact interface, exacerbated by any moisture migration. The result is accelerated corrosion of the sheet's back coating around fastener penetrations and along purlin lines.

The manufacturer's 30-year warranty data sheet carries the same H1.2-only requirement explicitly. Any departure voids the warranty.

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How the code resolves it

NZS 3640 Chemical preservation of round and sawn timber sets the hazard-class treatment system. H1.2 is the treatment class for interior, dry, no-direct-weather-exposure timber framing — limited to specific preservative formulations (typically permethrin-based) that do not contain copper. H3.2 is the higher hazard class for exterior, exposed-to-weather timber typically using copper-based preservatives, suited to outdoor framing but not to direct steel contact.

The MRM CoP §4.5 cross-references NZS 3640 and is explicit: "Timber in direct contact with the sheet shall be H1.2-treated only. H3.2 or higher hazard class is not permitted in direct contact with the sheet substrate." The clause carries the technical reasoning: copper-based preservatives accelerate the corrosion of the sheet's back coating, voiding the manufacturer's warranty.

On a re-roof with retained framing, the practical fix is one of three: (a) replace the H3.2 purlins with new H1.2 purlins, (b) install a separator strip (DPC, bitumen tape, or a manufacturer-approved isolating membrane) between the H3.2 purlin and the sheet, or (c) install a battens-over-purlins system with H1.2 battens between the existing H3.2 purlin and the sheet.

Option (b) is the typical retrofit solution. A 50 mm wide self-adhesive bitumen separator tape on the top edge of each retained purlin isolates the H3.2 timber from the sheet contact. The manufacturer accepts the retrofit as equivalent to H1.2-only contact.

What it would have cost if caught later

| Stage caught | Cost range (ex GST) | Why | |---|---|---| | Quote review (where we caught it) | $2.5-4k | Separator strip retrofit across 145 m² of purlin runs, half a day's extra carpenter labour, materials | | Pre-sheet (strip exposed, no sheet down yet) | $4-6k | Same retrofit but with a return visit by the carpenter and small standdown of the roofer | | Sheet down, before flashings | $9-14k | Partial strip-back of sheet to access purlin tops, retrofit, re-fix sheet, paint touch-up at fastener heads | | Practical completion to year 5 | $15-25k | No visible failure but the warranty exposure sits in the file; remediation deferred but the manufacturer's audit risk is live | | Year 8 manufacturer warranty audit | $35-55k | Audit finds H3.2 contact via fastener-penetration photographs and a moisture-meter test; warranty position contested; early re-roof negotiated or owner-funded |

The year-8 row is the realistic worst case. Most warranty audits happen on a claim — a localised corrosion failure at a fastener around year 7-10 — and the audit response back-tests the entire substrate.

The clarification we recommended

One scope edit to the roofer's quote and one to the carpenter's scope. The roofer's quote amended to include "self-adhesive bitumen separator tape (manufacturer-approved compatibility) to all retained existing purlins prior to underlay laydown, per MRM CoP §4.5 and the sheet manufacturer's data sheet for retrofit substrate compliance." The carpenter's scope amended to flag any further purlin replacement during the strip as H1.2 only.

Cost: $2,500-4,000 added to the roofer's quote. Programme: nil — the tape laydown happens between strip and underlay, which is already on the critical path with slack. Warranty: written manufacturer endorsement confirming the retrofit meets the warranty conditions.

What other Mt Eden re-roof builds should check

The treatment class of the retained existing purlins — H1.2 only is the rule for direct sheet contact
The separator-strip retrofit option where retained H3.2 is unavoidable, per MRM CoP §4.5
The new purlins (where replacement is needed) named as H1.2 only on the carpenter's quote
The manufacturer's written warranty endorsement confirming the substrate retrofit meets the warranty conditions
The fastener pattern aligned to the retained purlin spacing — re-sheet over old purlins often needs the new sheet's fastener pattern reconciled to the existing centres

FAQ — H1.2 vs H3.2 purlin substrate on Mt Eden re-roofs

Q1: Why is H3.2 incompatible with metal roof sheet when it's the standard treatment for exterior timber elsewhere? H3.2 contains copper-based preservatives (CCA, ACQ, or copper azole). Copper compounds in contact with the steel back of a metal sheet create a galvanic cell. Moisture migration through the timber accelerates the reaction. The sheet's back coating corrodes preferentially around the contact zone, voiding the manufacturer warranty.

Q2: Does the same H1.2-only rule apply to battens under tile or shingle roofs? No. The H1.2-only rule is specific to timber in direct contact with metal sheet. Tile-roof battens, shingle battens, and any timber under non-metal roof systems can use H3.2 per the relevant manufacturer's installation guide.

Q3: What does the manufacturer warranty audit actually test? A typical audit on a claim involves a moisture-meter reading at the failure zone, a photographic record of the fastener-penetration and purlin-contact detail, a treatment-class confirmation (sometimes by lab analysis on a sample), and a review of the installation documentation. H3.2 in direct contact will be flagged.

Q4: Is the separator-strip retrofit accepted by all metal sheet manufacturers? Most accept it where the product is named in their compatibility schedule. A written confirmation from the manufacturer for the specific build is the safer path — generic acceptance is not the same as a project-specific endorsement.

Q5: Why is this pattern common on Mt Eden villa re-roofs specifically? The 1920s villa stock in Mt Eden was originally tiled, with H3.2 purlins as the era-standard treatment. Conversion to metal long-run in the 1980s-2000s often retained the existing purlin frame to save cost. The pattern of H3.2 retained purlins under metal sheets is widespread across the Mt Eden character overlay — and the warranty exposure is widespread with it.

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