Roof areas
Main roof covering, underlay and many removal allowances follow actual roof surface area.
Roof measurement
How to measure a roof for an estimate
The first question is whether your measurement is plan-view or actual roof surface. Plan-view area needs a pitch conversion; an actual sloping measurement does not.
A useful roofing estimate also needs lineal quantities such as ridges, hips, valleys and barges because those components have different materials and labour from the main roof area.
- Area unit
- m²
- Flashings
- lineal metres
- Plan measurements
- apply pitch once
- Actual measurements
- no pitch conversion
Two measurement modes
Tell the estimator what you actually measured
Plan measurements are the roof as seen from directly above. They are convenient on architectural plans and satellite imagery but understate the sloping surface on pitched roofs.
Actual measurements already follow the roof surface/edge. Do not multiply them by a pitch factor again.
Simple plan-area conversion
- Plan area
- 180 m²
- Roof pitch
- 25°
- Pitch factor
- ≈ 1.103
- Approx. sloping surface
- ≈ 198.6 m²
This is the simple area conversion only. Eaves/overhangs and complex geometry must also be represented in the plan measurement or measured separately.
See the roof pitch guide for a full pitch-factor table.
What to measure
The component list that makes an estimate more useful
Ridges, hips, valleys & barges
These are measured as lineal lengths because flashing material and labour follow the edge/junction length.
Rainwater & custom items
Spouting is usually lineal; downpipes and some penetrations are counted; unusual items can be added separately.
Accuracy
When a plan measurement is enough — and when it is not
A plan can be excellent for preliminary quantities when it has a known scale and clear roof geometry. Satellite imagery can be useful for early planning but may contain perspective/imagery limitations. A site quote still needs actual conditions confirmed, especially access, penetrations, substrate condition and details hidden in the plan.
Safety: this guide is about measurement logic, not instructions to climb a roof. WorkSafe guidance applies to work at height; use plans, imagery and competent site measurement methods rather than putting yourself at risk to obtain a number.
Technical references
Why the detail still matters after the maths
Roof area is only one input. The New Zealand Building Code's external-moisture requirements and the selected roof manufacturer's details govern how claddings, flashings, junctions and penetrations need to perform.
- primaryBuilding Performance — E2 External Moisture — New Zealand Building Code external-moisture requirements · government
- primaryBuilding Performance — E2/AS1, 4th edition (2025) — Current acceptable-solution information for relevant buildings · government
- primaryWorkSafe — Working on roofs good practice guidelines — Fall prevention, roof access and edge-protection guidance · government
FAQ
Roof measurement questions
Is house floor area the same as roof area?
Usually not. Roof overhangs, plan geometry and pitch can all make the roof surface larger than the floor/footprint area.
Should I add pitch to ridge length?
A horizontal ridge length does not need the same pitch-area conversion as a roof plane. Hips and valleys can require geometric conversion when measured in plan view; the detailed RoofHub tool handles those measurement types separately.
Can I upload a plan and measure it digitally?
Yes. The detailed estimator includes a plan/image takeoff workflow where a known dimension can be used to calibrate measurements before tracing roof areas and components.