12 m × 8 m gable roof
Building: 12 m × 8 m
Overhang: 450 mm on every edge
Pitch: 20°
Projected roof area: 114.8 m²
Sloped roof area: 122.2 m²
With 10% waste: 134.4 m²
Roofing Calculator NZ: How Much Roofing Do I Need?
Calculate roof area from a building footprint and pitch, then estimate metal sheets, tiles or shingles, fixings, roof weight and an optional New Zealand cost. The calculator includes overhangs, waste, roof complexity, labour, removal, access costs and GST.
Suitable for homeowners, renovators, builders, roofers, property managers and quantity estimators preparing an initial roofing estimate.
Last reviewed by NZ Calculator: 5 July 2026 · Estimated completion time: under two minutes
- The roof-area mode converts the horizontal roof footprint into sloped area using the exact pitch factor.
- The materials mode estimates profiled metal sheets, roof tiles, shingles or an area-priced membrane system.
- Cost results use only the prices you enter. They are not live supplier prices or contractor quotations.
- Profile cover widths and minimum pitches vary. Use the selected product's current technical literature before ordering.
- This calculator does not design purlins, trusses, fixings, flashings, drainage, wind resistance or fall protection.
How to calculate roofing in New Zealand
Projected roof area including overhangs:(building length + end overhangs) × (building width + side overhangs)
Roof pitch multiplier:Pitch factor = 1 ÷ cos(roof pitch in degrees)
Actual sloped roof area:Projected roof area × pitch factor
Material allowance:Sloped roof area × [1 + waste % + complexity %]
For a simple rectangular roof where all planes have the same pitch, gable, hip and skillion styles use the same pitch-area relationship. Valleys, dormers, curved roofs, unequal pitches and irregular outlines need separate measurements or a professional take-off.
Your roof details
Roofing pitch calculator quick picks
The 8° label is a generic starting point for standard 16.5 mm corrugate. Other profiles can have different minimum pitches.
A planning allowance for hips, valleys, dormers and cut waste. It is not measured roof area.
These presets are examples from current manufacturer pages. Confirm the exact cover width, run length, lap requirements and minimum pitch for the product you are buying.
Use the laid coverage from the current product data sheet, including the required headlap and sidelap.
For estimating only. Actual fixing patterns depend on profile, purlin spacing, wind zone, spans and design.
Check the entries. Dimensions, area and material inputs must be valid positive numbers within the permitted ranges.
Your roofing estimate
Roof area estimateCalculating your roof estimate…
Comparison of projected, sloped and material-order areas.
Sloped roof area
0.0 m²
Order allowance
0.0 m²
Indicative entered-cost breakdown
How this Roofing Calculator NZ works
The roof area calculation starts with the horizontal projected area, including the overhang values you enter. It then applies the exact trigonometric pitch factor, 1 ÷ cos(pitch), to estimate the sloped surface area.
Roof style does not automatically change the total surface area of a simple rectangular roof when every plane has the same pitch. It does change the cutting pattern, flashing lengths, labour and waste. For that reason, hip, valley and complex roofs use a separate editable complexity allowance rather than an invented geometry multiplier.
The materials mode then converts the sloped area into an order allowance and estimates metal sheets, tiles, shingles or membrane area. Metal sheet counts use the effective cover width, not the total sheet width.
- Metal sheet estimate: order area ÷ (effective cover width × sheet length), rounded up.
- Tile or shingle estimate: order area × laid units per square metre, rounded up.
- Weight estimate: order area × the system weight entered from the product data sheet.
- Cost estimate: materials, labour, removal, access and other costs entered by the user, with GST handled according to the selected tax setting.
Roofing pitch calculator: pitch factors by roof angle
A roof pitch multiplier converts horizontal projected area into sloped surface area. The factor grows as the roof becomes steeper.
| Roof pitch | Pitch factor | Extra area above flat projection |
|---|---|---|
| 0° | 1.000 | 0.0% |
| 5° | 1.004 | 0.4% |
| 8° | 1.010 | 1.0% |
| 10° | 1.015 | 1.5% |
| 15° | 1.035 | 3.5% |
| 20° | 1.064 | 6.4% |
| 25° | 1.103 | 10.3% |
| 30° | 1.155 | 15.5% |
| 35° | 1.221 | 22.1% |
| 40° | 1.305 | 30.5% |
| 45° | 1.414 | 41.4% |
Roofing iron calculator NZ: profile cover widths and minimum pitches
In New Zealand, “roofing iron” commonly refers to profiled metal roofing. The calculator uses effective cover width, because the side lap means the full sheet width is not all exposed coverage. Minimum pitch also depends on the exact profile, rib height, roof run and manufacturer requirements.
| Example NZ profile | Effective cover width | Published minimum pitch | Use in calculator |
|---|---|---|---|
| Dimond Corrugate | 762 mm | 8° | Quick pick |
| Dimond Six Rib | 760 mm | 4° | Quick pick |
| Dimond Hi Five | 765 mm | 3° | Quick pick |
| Metalcraft T-Rib | 760 mm | 3° | Quick pick |
Important: these are product examples, not universal rules for all long-run or corrugated roofing. Check the current manufacturer manual and the New Zealand Metal Roof and Wall Cladding Code of Practice for the actual profile, run length, drainage and detailing requirements.
Sources: NZ Metal Roofing Manufacturers Code of Practice: roof pitch; Dimond Corrugate; Dimond Six Rib; Dimond Hi Five; Metalcraft T-Rib.
Roofing cost calculator NZ: materials, labour, removal and GST
There is no single reliable national re-roofing price per square metre. A roofing quote can change with the product, roof size, pitch, number of hips and valleys, access, scaffolding, coastal exposure, underlay, flashings, guttering, removal, disposal, timber repairs and whether the entered prices include GST.
The cost mode therefore does not insert unsupported market prices. Enter the actual material and contractor rates available for your project. The calculator can combine:
- sheet, tile, shingle or membrane material cost;
- labour per measured roof square metre;
- old roof removal and disposal per square metre;
- scaffolding or access as a fixed amount;
- underlay, flashings and other fixed costs; and
- GST-inclusive or GST-exclusive pricing.
Use the result as a scope and quote-comparison tool, not as a contractor quotation. A professional inspection may identify rotten framing, asbestos-containing materials, drainage problems or structural work that is not visible from dimensions alone.
Roofing calculator worked examples
Corrugate sheet estimate
Order area: 134.4 m²
Cover width: 762 mm
Sheet length: 6 m
Coverage per sheet: 4.572 m²
Equivalent sheet estimate: 30 sheets
Confirm cut lengths and layout with the rollformer.
Roofing weight estimate
Order area: 134.4 m²
Product system weight: entered from data sheet
Formula: 134.4 × kg/m²
If the system is 5 kg/m², the estimated material weight is 672 kg.
Can I measure my roof online or with Google Maps?
Satellite and aerial maps can help estimate the horizontal outline of a roof, but they usually do not measure the sloped surface. Trace the roof footprint, then apply the pitch factor. Verify the map scale, eaves, attached garages, porches and additions because imagery can be angled, outdated or partly obscured.
For a quote or material order, use building plans, on-site laser measurements, drone photogrammetry or a professional roof take-off. WorkSafe recommends considering remote inspection and measurement methods before accessing a roof because falls are the main serious hazard associated with roof work.
NZ roofing consent, licensed work and safety
Comparable repair or replacement work can be exempt from building consent in some circumstances, but all building work must still comply with the Building Code. Consent may be required when a roof has failed prematurely, the replacement is not comparable, structural work is involved or specified systems are affected. Check the current Building Performance guidance and ask the local council when the exemption is uncertain.
Roofing that affects a home's structure or weathertightness may be restricted building work and can require an appropriately licensed building practitioner to carry out or supervise it. Roof access and installation also require proper controls for falls and other hazards.
Official guidance: Building Performance: repair, maintenance and replacement; Licensed Building Practitioners: when an LBP is needed; WorkSafe: working safely on roofs.
How to use the Roofing Calculator NZ
- Measure the building footprint. Enter the building length and width in metres.
- Add the roof overhangs. Enter the overhang at each end and each side so the projected area includes the eaves.
- Enter the roof pitch. Use degrees from the plans, a pitch gauge or a verified measurement. The calculator applies the exact pitch factor.
- Add waste and complexity allowances. Keep material waste separate from the allowance for hips, valleys, dormers and irregular cuts.
- Calculate materials and cost. Open Materials & Cost, select the roofing system, enter current product coverage and optional project prices, then review the quantity, weight and cost estimate.
Common roofing estimating mistakes
- Using floor area without adding eaves and other roof overhangs.
- Using a rough pitch band instead of the exact trigonometric pitch factor.
- Assuming a hip roof automatically has more measured area than a gable roof with the same projected outline and pitch.
- Using full sheet width instead of the effective cover width after side laps.
- Counting sheets by area without checking actual cut lengths, joins and end laps.
- Using a generic minimum pitch instead of the selected profile's current technical requirements.
- Forgetting valleys, dormers, penetrations, apron flashings, ridge and hip caps, underlay, gutters and disposal.
- Applying one fixing rate to every wind zone, profile, span and purlin layout.
- Relying on an online cost estimate without inspecting access, framing and the old roof.
Roofing Calculator NZ Frequently Asked Questions
Calculate the projected roof area including overhangs, multiply it by 1 divided by the cosine of the roof pitch, then add separate waste and complexity allowances. Convert the resulting order area into sheets, tiles or shingles using the selected product's laid coverage.
For a simple roof with one uniform pitch, sloped roof area equals horizontal projected roof area multiplied by 1 ÷ cos(pitch). Add the roof overhangs to the projected dimensions before applying the pitch factor.
A roofing calculator converts roof dimensions and pitch into an estimated surface area, then uses product coverage, waste, weight and prices to prepare an initial material or cost estimate. It does not replace a measured take-off, technical design or contractor quote.
Measure the horizontal roof footprint, including eaves, and multiply it by the pitch factor. For example, a 20° roof uses a factor of about 1.064, so a projected area of 100 m² has an estimated sloped area of about 106.4 m².
Divide the material order area by the effective cover width multiplied by the sheet length, then round up. This is an equivalent-sheet estimate. The actual count can change with roof geometry, cut lengths, hips, valleys, end laps and reusable offcuts.
The generic NZ Metal Roofing Manufacturers table gives 8° for 16.5 mm corrugate, while deeper corrugate profiles can have lower published minimums. Dimond Corrugate and Metalcraft Corrugate publish 8° for their standard products. Always use the requirements for the exact profile and roof run.
You can use satellite imagery to estimate the horizontal roof outline, but it normally does not include slope. Apply the pitch factor and verify overhangs, scale and hidden roof sections before relying on the result.
They can provide a useful early budget when the dimensions and rates are accurate, but they cannot inspect access, framing, moisture damage, asbestos, drainage, flashings or local labour requirements. Use the result to compare quotations rather than as a fixed price.
There is no universal New Zealand price. The total depends on measured area, material, roof complexity, access, scaffold, old-roof removal, disposal, underlay, flashings, gutters, timber repairs, location and GST. Enter project-specific rates in the cost mode and obtain itemised contractor quotes.
Comparable repair or replacement in the same position can be exempt in some circumstances, but consent may be required for premature failure, non-comparable replacement, structural changes or work affecting specified systems. All work must comply with the Building Code, so check with the council when uncertain.
There is no single replacement interval. Service life depends on the material, coating, exposure zone, roof design, installation, maintenance and corrosion or damage. Arrange an inspection when there are leaks, widespread corrosion, loose units, failing flashings or uncertainty about the roof condition.
Yes. It is free to use and does not require an account. The result is an initial planning estimate rather than a professional roof measurement, structural design or contractor quotation.
This calculator is a high-level planning tool. Actual roof dimensions, profile coverage, minimum pitch, wind design, fixing patterns, flashing requirements, drainage, material quantities, costs, consent status and safety controls can differ. Confirm the result with current product technical literature, the project designer, the local council and an appropriately qualified roofing professional. Do not access a roof without suitable training, equipment and fall controls.