Tile to metal roof conversion quotes: when two materials lists disagree, the contradiction becomes the customer's variation

Steve Parker
Jul 4
8 min read

Updated: Jul 9

The biggest quote in a batch we reviewed — a tile-to-steel roof conversion in the low-to-mid $30,000s including GST — contained two materials lists that contradicted each other. In a dispute, the ambiguity would have been read against the roofer. Catching it early cost nothing; losing that argument later meant several thousand dollars of unpriced scope.

By Steve Parker · Trueworks · NZ construction estimation · 7 min

What you'll learn in this case study

Why two conflicting materials lists in one quote hand the customer a claim to the fuller scope — at your price
How "edge protection" on a steep hip roof creates a safety, margin and dispute triple-risk in one line
The low-pitch, consent and over-spec checks a conversion quote must answer before it goes out

Quick answer: A tile-to-long-run-steel conversion on a large 1970s–80s house in a rural district north-west of Auckland — an inland site, the largest quote in the batch at low-to-mid $30,000s including GST. The quote carried a "quoted items" list of about 11 lines and a "specifications" list of about 6 that silently dropped purlins, ridge cap, chimney flashing, insulation and gutters. Under contra proferentem — ambiguity construed against the drafter — the customer can argue the fuller list is included in the price. We also flagged: only edge protection priced on a steep ~32° hip while the terms said site facilities are included unless stated otherwise; two low-pitch planes with the profile left "to be confirmed"; an insulation quantity roughly 5% of the ceiling area; several tonnes of tile coming off the structure with no mention of building consent; and marine-grade material on an inland site. All fixed before send.

The quote that came across the desk

A roofing business asked us to review a batch of outgoing quotes; this was the largest: a tile-to-long-run-steel conversion on a big 1970s–80s house in a rural district north-west of Auckland, priced in the low-to-mid $30,000s including GST. The site is inland — that matters later.

Conversions are the hardest residential roofing quotes to write: they change the structure's loading, the purlins, the flashings, often the gutters, and sometimes the consent status.

This quote had been assembled from a quoting system's line items plus a separate specifications section — and the two had drifted apart.

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What we found

Two materials lists that disagree. The "quoted items" list ran to about 11 lines: roofing, underlay, purlins, ridge, chimney flashing, ceiling insulation, gutters and more. The "specifications" list below it ran to about 6 — and had silently dropped the purlins, ridge cap, chimney flashing, insulation and gutters. Nothing in the document said which list governed. A customer would reasonably assume the fuller list is what the price buys; a roofer building from the shorter one treats five items as extras. That gap is a variation dispute pre-written into the quote — several thousand dollars of scope in play before the first tile comes off.

Scaffold priced as edge protection only. The roof is a hip at roughly 32° — steep enough that edge protection alone is a thin answer to the fall risk on a full strip-and-replace. And the attached terms said site facilities are included unless stated otherwise. The quote created a triple-risk: a safety exposure if the crew works a 32° hip off edge protection; a margin hole if full scaffold is needed and the terms gave it away; and a dispute if the roofer charges it later against terms that say it was included.

Profile "to be confirmed" on low-pitch planes. Two planes sat around 9–10° and about 14°. Corrugate is marginal at the bottom of that range under the metal roofing code of practice minimum-pitch tables. Leaving the profile "to be confirmed" treats a weathertightness decision like a colour choice. If corrugate goes on at the marginal pitch and the laps draw water, that is a leak the roofer owns.

Insulation quantity roughly 5% of the ceiling area. The quoted quantity covered about one-twentieth of the actual ceiling — either a token allowance not described as one, or a take-off error that survived into the price. The customer was being sold "ceiling insulation" the quantity could not deliver.

Several tonnes coming off the structure, and silence on consent. Converting concrete tile to long-run steel takes serious weight off the structure. Under Building Act Schedule 1, like-for-like replacement is treated differently from a change of roofing material, and whether this conversion needed consent was a question the quote neither asked nor answered. Silence does not make the question go away; it decides who is standing closest when it is finally asked.

Marine-grade material on an inland site. The quote called up marine-grade product for a rural inland property — over-spec: cost padded into the price with no durability benefit the site can use, and a less competitive quote for no reason.

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Contra proferentem: why the ambiguity belongs to the drafter

The principle that makes conflicting lists expensive is contra proferentem: ambiguity in a document is construed against the party who drafted it. The roofer wrote the quote, so the roofer wears the reading that favours the customer — here, "the 11-line list is what I bought."

In practice, the five dropped items are arguably inside the fixed price whether or not the roofer costed them — and on a conversion, purlins alone are a substantial line. The options after acceptance are all bad: absorb the cost, fight the customer over your own document, or attempt a variation for scope the quote appears to include — the weakest paper in construction.

The fix took an hour: one materials list, stated once and stated to define the included scope, with exclusions express. The profile was named per plane against the minimum-pitch tables; the scaffold line was repriced for the actual roof; the insulation quantity was corrected; the consent question went to the council before commitment; and the marine-grade spec was swapped for the zone-appropriate grade, which helped the price.

What it costs when it's caught late

| Stage caught | Cost range | Why | |---|---|---| | Before the quote goes out | The cost of a review, low hundreds | Reconciling two lists is edit work; nothing has been promised | | After acceptance | Low four figures | The customer holds a signed price arguably including five unpriced items | | Mid-job | Mid four figures | Full scaffold, purlins and gutters arrive at your cost while the job is open | | In a formal dispute | Five figures | Contra proferentem is argued against your own document, plus time, advice and relationship costs | | After a low-pitch leak | Five figures and a reputation | Remediation on a marginal-pitch profile choice you left "to be confirmed" |

Five checks before a conversion quote leaves the office

One materials list, stated once. If the quoting system produces both a line-item list and a specification, reconcile them word for word, state which defines the scope, and list exclusions expressly.
Price access for the roof you are actually stripping. A ~32° hip on a full conversion is scaffold territory, not edge-protection territory; and never let boilerplate terms give away site facilities your price has not included.
Fix the profile per plane against the minimum-pitch tables. Anything near the corrugate margin is a weathertightness decision — make it in the office, in writing, not on the truck.
Cross-check quantities against the building. Insulation at about 5% of the ceiling area is the kind of error a two-minute area check catches and a customer complaint does not forgive.
Answer the consent question before pricing. Tonnes of tile coming off a structure is a Schedule 1 conversation with the council — have it before the price is signed and record the answer in the quote.

FAQ — conversion quotes and conflicting documents

Q1: What is contra proferentem and why should a roofer care? It is the rule that ambiguity in a document is read against whoever drafted it. Your quote is your document: any contradiction inside it — two lists, a clause cutting across a line item — resolves in the customer's favour, converting sloppy drafting into unpriced scope.

Q2: Do I need a building consent to convert a tile roof to long-run steel? Sometimes. Building Act Schedule 1 exempts certain like-for-like roof replacement, but a change of roofing material — and tonnes of load coming off the structure — is territory where the council should be asked before you price. The answer varies; the mistake is not asking.

Q3: Is edge protection enough for a re-roof? On low-pitch, short-duration work it can be defensible; on a steep hip conversion it rarely is. The test is the actual fall risk of the actual work — and either way, the access method and its cost must appear in the quote, not hide inside a "facilities included" clause.

Q4: What is the minimum pitch for corrugate in NZ? The metal roofing code of practice minimum-pitch tables put corrugate at the low end of roughly the 8-degree region, rising with run length and exposure. Planes around 9–10° sit inside the marginal band where profile, lap treatment and run length must be checked — why "to be confirmed" is not an acceptable entry.

Q5: Is marine-grade material ever wrong? It is wrong when the site cannot use it. On an inland rural property, marine-grade product adds cost without service life the environment demands, and can price you out of the job. Spec to the exposure zone — no lower, no higher without a stated reason.

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About Trueworks

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

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