How to quote a re-roof

1. Measure the roof — at the pitch, not on the flat

Everything starts with the plan shape: the outline of the building, and the internal lines that make it a roof — ridges, hips, valleys, barges, aprons. You can take that off an aerial photo, off the plans, or off a tape on site.

Then correct for pitch. A roof drawn in plan is flat, and the real roof is longer wherever it slopes. The multiplier is 1 / cos(pitch):

It applies to anything that slopes: sheet runs, rafter lengths, hip and valley lengths, barge lengths, and the roof area. It does not apply to the ridge or the gutter line, which are level. Applying it to everything is as wrong as applying it to nothing.

Hips and valleys need care: they run diagonally, so their true length is longer again than a simple rafter at the same pitch. If you are doing it by hand, measure them in plan and apply the pitch factor for the diagonal, not the rafter.

2. Count the sheets by run length

Long-run steel is cut to length, so what you are ordering is a set of lengths and quantities, not an area. For each roof face:

1. Find the run length — ridge to gutter, at the pitch, plus the overhang into the gutter
2. Divide the face width by the profile's cover width and round up
3. Repeat per face, then group identical lengths together for the order

Cover width is the number that matters, not sheet width. Corrugate covers less than it measures because of the side lap — a nominal 860 mm sheet covers about 762 mm — and a trapezoidal 5-rib covers differently again. Using sheet width instead of cover width leaves you short on every roof: on corrugate you order 88.6 per cent of what you need — short by about one sheet in every nine.

One trap worth naming: a sheet run must be measured to the gutter that actually catches it. On a roof with a wing or a step-down, the nearest gutter in a straight line is often not the one the water reaches, and a run measured to the wrong one comes out metres long. It is a mistake that survives right through to the delivery.

3. Count the flashings — as pieces, not metres

This is where re-roof quotes go wrong, and it goes wrong in the direction that costs you.

Every roof line is a flashing: ridge along the top, hip down each external corner, valley up each internal one, barge along each gable edge, apron where the roof meets a wall, and change-of-pitch wherever the slope breaks.

Write them down as pieces at lengths — 1 @ 6.8 m, 1 @ 3.7 m — not as a total. Forty-two metres of barge could be six pieces or fifteen, and the merchant folds and cuts to length. A total is not an order.

Then the ones that get forgotten:

• Back-trays. Every pipe penetration needs a dektite and a back-tray behind it. The tray is the item that gets left off the docket and found on the roof.
• Boxed penetrations. A boxed chimney is five pieces, not one: a back flashing, two sides, a front apron, and a saddle running from the ridge down to the top of the box.
• Shared edges. Where two roofs meet, that edge is one flashing. Counting it on both roofs is the standard way a take-off comes out over.

4. Consumables follow from the counts

Once the sheets and flashings are counted, everything else is arithmetic off those two numbers:

• Underlay — by roof area, with the lap allowed
• Screws — by sheet metres and purlin spacing; count the ridge and eave rows separately, they are denser
• Rivets — by flashing metres
• Sealant — by the number of laps and penetrations
• Gutter and downpipes — gutter by the gutter line, downpipes by catchment; a downpipe serves a limited length of gutter and the count is a site decision

5. Price labour, access and disposal

Material is the easy half. Labour on a re-roof is driven by:

• Area and pitch. Above about 35° the work slows down noticeably and the access requirements change.
• Cut-up-ness. Hips, valleys and penetrations cost hours that area alone does not predict. A 200 m² hip-and-valley roof with four penetrations is a different job from a 200 m² straight gable.
• Storeys and access. Whether the truck can get to the eave.
• The strip. Removing the old roof, and carting it. Tile-to-steel means over-purlining as well.

Scaffolding or edge protection is its own line, and worth showing separately — it is a real cost the homeowner did not expect, and burying it in the rate makes the whole quote look expensive rather than the access looking expensive.

On disposal: the scrap value of the old steel roughly offsets the labour and time to strip and cart it. Whether the customer keeps it or you take it away tends to be a wash, so it is a genuine choice to offer rather than a charge to argue about.

6. Write the scope, and write the exclusions

A quote that lists a price and a product invites every argument later. Say what you are doing:

• Strip and dispose of the existing roof
• New underlay
• New long-run steel — by profile, grade, gauge and colour
• All new flashings, listed by type
• Guttering and downpipes, or explicitly not
• All penetrations reflashed
• Scaffolding or edge protection
• Site clean and make-good

Then what you are not doing, because this is the half that protects you: rotten purlins or framing found once the roof is off, asbestos or lead discovered in the strip, anything needing a building consent, painting, and internal damage from pre-existing leaks. Price the variation honestly when it appears; exclude it clearly before it does.

Choosing the material

Two decisions carry most of the cost difference, and both depend on where the house is.

Grade is about corrosion. Distance to breaking surf sets the zone, and the zone sets the warranty: mild is roughly 5 km or more inland, moderate 500 m to 1 km, severe 100 to 500 m, and very severe 25 to 100 m. Colorsteel MAXAM holds up best close to the sea; ColorCote is comparable; ColorZen is a cheaper pre-painted option that gives away corrosion protection in coastal zones; plain Zincalume is unpainted and self-weathering, the cheapest and the longest-lived where appearance does not matter.

Gauge is 0.40 or 0.55. On a normal residential roof 0.40 is standard. 0.55 buys stiffness underfoot, better performance across wider purlin spacings, and it is worth having in a severe coastal zone or on a roof people will walk on regularly.