How to Calculate Roofing Iron Quantities for a New Roof

Calculating roofing iron quantities accurately before you buy can save you hundreds of dollars and avoid frustrating delays mid-project. The process involves measuring your roof's actual surface area, applying the correct pitch factor, and adding allowances for overlaps and waste. Here's how to do it properly for a typical New Zealand home.

Understanding Roof Pitch First

Before you measure anything, you need to know your roof's pitch (slope), because a steeper roof has more actual surface area than its footprint suggests. Pitch is usually expressed as a ratio — for example, 25° or as a rise-over-run (e.g., 1:3).

The pitch factor converts your horizontal (plan) area into the actual sloped surface area. Common pitch factors are:

If you're unsure of your pitch, a simple way to measure it is with a spirit level and tape measure: hold the level horizontally against the rafter, measure 300 mm along the level, then measure the vertical drop to the rafter below. Divide the drop by 300 to get the tangent, then use a calculator to find the angle.

Note: MBIE Building Performance sets minimum pitch requirements under the NZ Building Code. For most profiled metal roofing (corrugate or tray), the minimum is typically 3° to 5° depending on the profile — always check the manufacturer's installation guide.

Step 1 — Measure the Roof Footprint

From the ground or from your building plans, measure the plan area (the horizontal footprint) of each roof plane separately. For a simple gable roof:

• Measure the length of the building (including any eave overhang)
• Measure the width from eave to eave (including overhangs)
• Multiply length × width to get the plan area of both slopes combined

For hipped roofs, L-shapes, or complex roofs, break the roof into rectangles and triangles, calculate each section separately, then add them together.

Tip: Always work from your building consent plans if you have them — they'll show accurate dimensions and roof geometry.

Step 2 — Apply the Pitch Factor

Multiply your total plan area by the pitch factor for your roof:

Sloped surface area = Plan area × Pitch factor

For example: A 10 m × 12 m house (120 m² footprint) with a 25° pitch: 120 m² × 1.103 = 132.4 m² sloped area

Step 3 — Account for End Laps and Side Laps

Roofing iron sheets overlap each other, which means you need more material than the bare surface area suggests.

• End laps (where sheets overlap end-to-end going up the slope): typically 150–200 mm per lap, depending on pitch and manufacturer requirements. Steeper pitches can use a smaller lap; shallower pitches need a larger lap to prevent water ingress.
• Side laps (where sheets overlap side by side): most profiled iron overlaps by one corrugation or one rib, which is built into the sheet's effective cover width.

Calculating Sheet Lengths

Most roofing iron in NZ is available in custom cut-to-length sheets (up to around 12 m from most suppliers), so for simple roofs you can often order full-length sheets that run from ridge to eave with no end laps at all — this is the preferred approach as it eliminates a potential leak point.

If your rafter length (ridge to eave, measured along the slope) is within one sheet length, order sheets at rafter length + eave overhang + any ridge cap allowance.

Calculating Number of Sheets

1. Measure the eave length (the width of each roof plane along the gutter line)
2. Divide by the effective cover width of your chosen profile (not the sheet width — check the manufacturer's spec sheet, as effective cover is typically 760 mm to 900 mm for common NZ profiles)
3. Round up to the nearest whole sheet
4. Multiply by the number of roof planes

Example: Eave length 12 m, effective cover 762 mm (0.762 m): 12 ÷ 0.762 = 15.75 → round up to 16 sheets per slope

Step 4 — Add a Waste Factor

Always add a waste allowance on top of your calculated quantity:

• Simple gable roof: 5–8%
• Hipped or complex roof with lots of cutting: 10–15%

This covers off-cuts, any damaged sheets, and minor measurement errors.

Step 5 — Don't Forget Accessories

Roofing iron is only part of what you need. Also calculate quantities for:

• Ridge capping — measure total ridge length and divide by the effective length of each cap piece
• Barge flashings — measure all barge (verge) edges
• Valley iron — measure all valley lengths and add 10% for laps
• Fixings — typically one screw per corrugation per purlin; your supplier can help calculate this
• Butyl tape or foam closures — for ridge and barge sealing

Check BRANZ guidance and your chosen product's installation manual for fixing patterns and flashing requirements under NZ conditions.

Choosing Your Roofing Iron

Common NZ options include corrugated iron (5-rib or 6-rib), trapezoidal tray profiles, and standing seam systems. James Hardie and various steel manufacturers supply roofing through merchants like Mitre 10, Bunnings, and PlaceMakers. Always confirm the effective cover width and minimum pitch for the specific profile you choose.

When to Call a Professional

For complex roofs — hips, valleys, dormers, skylights, or multi-level structures — the calculations above can get complicated quickly, and errors are costly. A licensed quantity surveyor can produce an accurate material schedule and help you avoid over- or under-ordering.

Roofing installation itself is not restricted work under the NZ Building Code for owner-builders in most cases, but a building consent may be required for a full re-roof or new roof — always check with your local council first. Licensed Building Practitioners (LBPs) must carry out or supervise certain restricted building work, so confirm what applies to your project.

If your project involves any roof penetrations for plumbing vents or gas flues, those connections must be completed by a licensed tradesperson registered with the Plumbers, Gasfitters and Drainlayers Board.

You can find qualified quantity surveyors, roofers, and other tradespeople in your area at tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance — NZ Building Code
• BRANZ — Roofing and weathertightness guidance
• Licensed Building Practitioners — LBP register
• PlaceMakers — Roofing products and advice
• Bunnings — Roofing iron and accessories
• Standards NZ — NZS 3604 and related standards