Mt Eden villa renovation case study — the cut-in roof flashing junction that nearly killed the timeline
Updated: Jul 9
A Mt Eden villa renovation roofing quote read clean on the inclusions list — until the cut-in flashing junction between the new addition and the original 1920s roof was tested against MRM CoP §8 and E2/AS1 §9.1.6. Catching it at tender saved 3-6 weeks of programme and $20-35k of variation cost.
By Steve Parker · Trueworks · NZ construction estimation · 5 min
The headline quote priced the roof. It didn't price the junction. The junction was where the project was going to lose 3-6 weeks of programme — and the variation was already drafted in the original architect's notes if anyone had read them against the code.
By Steve Parker · Trueworks · NZ construction estimation · 5 min
What you'll learn in this case study
The cut-in flashing scope ambiguity that surfaces at tender on every villa addition
The MRM CoP §8 and E2/AS1 §9.1.6 clauses that resolve it
The dollar and programme effect of catching it on paper versus catching it after the new roof is laid
Quick answer: A Mt Eden villa renovation roofing quote priced the new addition roof clean but left the cut-in junction between the new roof and the existing 1920s corrugated roof undefined. MRM CoP §8 (flashings) and E2/AS1 §9.1.6 govern how that junction is detailed and warranted; without an explicit scope line, both the existing-roof roofer and the new-roof roofer disown it. Catching the ambiguity at tender stage cost a one-page clarification. Catching it at install would have cost $20-35k of variation and 3-6 weeks of programme delay.
The build
A two-storey rear addition to a Mt Eden character villa, build value in the $1.2-1.8M range. Original roof was 1920s corrugated profile in serviceable condition; the addition was a trapezoidal long-run metal sheet in a different profile and colour. The architect's drawings showed the new roof footprint and the existing roof footprint side by side, with a single line where they met and the note: "new roof to flash into existing — installer to detail." Tender was at the price-comparison stage when the head contractor sent the roofing quote across for review.
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What we found in the quote
The roofing quote was tidy. The headline rate per square metre was within the expected band, the underlay was named, the fastener class was right for the inland Auckland C3 exposure, the warranty term was stated. The quote priced the new addition roof in full.
The quote did not price the cut-in flashing.
That sounds like a small thing. It isn't. The cut-in flashing on a villa-to-addition roof junction is the single most failure-prone detail on the entire roofing scope. It carries every drop of water that lands on the new roof and meets the existing roof. The detail has to: (a) terminate the new long-run sheet under a flashing that's mechanically fixed and sealed to the existing corrugated profile; (b) accommodate two different sheet profiles with different rib heights and pitches; (c) maintain the manufacturer's warranty on the new sheet at the point where the new sheet is cut on site; and (d) handle thermal movement between two roof structures that may not have been designed to move together.
The quote treated the junction as if it would be detailed by "the installer." It wasn't priced. The existing-roof roofer (a different subbie engaged for any flashing touch-ups on the original roof) wasn't quoted on the junction either. The architect's note pushed it to "the installer" without naming which installer.
This is the variation pattern: when two trades each assume the other owns a detail, the detail belongs to the first one on site holding the bag. On a roof junction that bag is the head contractor's, and the cost lands as a variation against the build budget.
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How the code resolves it
MRM CoP §8 (flashings) sets the design and material standards for cut-in flashings on metal roofs. The clause requires: minimum upstand of 150 mm onto the existing roof surface, minimum 50 mm horizontal cover onto the new sheet, mechanical fixing at 300 mm centres maximum, sealant compatibility between flashing and both substrate materials, and explicit treatment of the cut edge of the new sheet (factory-coated edge preferred; cut edges to be touched up with manufacturer-approved coating to maintain warranty).
E2/AS1 §9.1.6 governs roof-to-roof junctions in renovations — specifically the scenario where a new roof meets an existing roof on a different pitch or profile. The clause requires that the detail be designed and documented before construction, that the responsibility for the design sit with a named party (architect, engineer, or manufacturer's technical service), and that the as-built junction be inspected and signed off.
Together these clauses make the cut-in flashing a designed and named scope item, not an "installer's discretion" item. A roofing quote that doesn't price it has either left the scope incomplete or assumed someone else will. Both are tender-stage problems if caught; both are variations if not.
What it would have cost if caught later
| Stage caught | Cost range | Why | |---|---|---| | At tender (paper review) | $0-500 (clarification fee) | One clarification clause added to the quote; flashing design referenced to manufacturer technical service; cut-in scope assigned and priced | | At pre-start (post-award) | $3,000-6,000 | Variation order to add cut-in flashing to scope, manufacturer technical sign-off requested, programme adjusted | | Mid-install (after new roof laid) | $20,000-35,000 | Roofer on site without flashing materials; specialist flashing materials freight-expedited; potential lift-and-relay of edge sheets to achieve compliant upstand; 3-6 weeks of programme delay; weather exposure on partially complete roof; potential consequential damage to ceiling linings below | | Post-completion (warranty year 1-3) | $40,000-80,000 | Leak presents at junction; insurance claim disputed because junction wasn't designed; remediation requires partial roof replacement at both sides of junction |
The headline quote was around $35-45k for the new addition roof. The variation cost at mid-install would have been 50-80% of the original roof scope. Caught at tender, it cost a clarification email.
The clarification we recommended
The clarification packet to the head contractor named the cut-in flashing junction as an unpriced scope item, cited MRM CoP §8 and E2/AS1 §9.1.6, and recommended one of three resolution paths: (1) instruct the new-roof roofer to price and detail the junction with manufacturer technical sign-off and a back-to-back warranty on the cut-in; (2) instruct a specialist flashing subcontractor to price and own the junction with both adjacent roofs in their scope; or (3) instruct the architect to fully detail the junction on a revised drawing and have the new-roof roofer price against the revised drawing.
Path (2) is generally the cleanest on a villa renovation because the specialist owns both sides of the junction and the warranty for the cut-in itself. Path (1) is workable if the new-roof roofer has experience with villa cut-ins. Path (3) is the most expensive in design time and the most likely to still leave gaps if the architect isn't fully across the flashing manufacturer's technical requirements.
The head contractor went with path (2). The variation never happened. The roof was watertight at first wet weather and has remained so.
What other Mt Eden villa renovation builds should check
Every villa-to-addition roof junction is a designed scope item under MRM CoP §8 and E2/AS1 §9.1.6 — not an installer's-discretion item
The roofing quote must explicitly price the cut-in flashing with a named responsibility (which subbie owns it) and a referenced manufacturer detail
If two roofing subbies are involved (one on existing, one on new), the junction belongs to one of them by named scope line — never to "both" or "the installer"
The cut edge of the new long-run sheet at the junction must be treated with manufacturer-approved coating per MRM CoP §8 to maintain the sheet warranty
The junction detail should be designed and sealed before the new roof framing closes in — retrofitting a compliant junction after the new roof is laid is the most expensive way to do it
FAQ — roofing on Mt Eden villa renovations
Q1: Why is the cut-in flashing junction such a high-risk detail on Mt Eden villas specifically? Because the original roofs are 1920s-40s corrugated profiles with non-standard rib geometry, the new long-run trapezoidal sheets sit at a different rib pitch and height, and the junction has to bridge that geometry plus typically 80-100 years of differential settlement in the existing roof structure. The flashing has to be designed for those specific conditions, not pulled off a generic detail.
Q2: Can the new-roof roofer detail the cut-in themselves, or does it need engineer sign-off? A new-roof roofer with manufacturer technical service support can detail and warrant the junction. Engineer sign-off is required only if the junction also involves structural connection between the two roofs (rare in residential additions). The MRM CoP §8 and E2/AS1 §9.1.6 framework lets a manufacturer-approved detail stand without independent engineer review.
Q3: What happens to the manufacturer's warranty on the new roof sheet at the cut-in? The factory coating on a long-run sheet is warranted at full term. A cut edge made on site is not warranted unless treated with the manufacturer's approved touch-up coating per MRM CoP §8. Without the touch-up, the warranty at the cut edge typically reduces to 5-10 years from 25-30 years.
Q4: How long does a properly detailed cut-in flashing add to the roofing programme on a Mt Eden villa renovation? Typically 1-3 days of additional roofer time at the junction stage, plus 5-10 working days of lead time on the specialist flashing if it isn't a standard manufacturer profile. The cost of those days is small compared to the variation cost of getting the detail wrong.
Q5: What's the warranty term we should be asking for on the cut-in flashing itself? The flashing material warranty (typically 25-30 years on the metal) is one number; the installation warranty (typically 5-10 years on workmanship) is the other. Both should be named in the quote, with the responsibility for any defect during the workmanship period assigned to a named subbie.
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