Hong Life CalculatorsUnderstanding the metal roof cost calculator — concept and principle
This calculator estimates the roof area of a gable or shed roof from the house size, overhang and pitch, the number and length of metal panels, and the total cost of panels, trim and labour. Homeowners use it to sense-check quotes before re-roofing.
How it is calculated
- Level run per slope: gable = width ÷ 2 + overhang; shed = width + 2 × overhang.
- Slope (panel) length = run ÷ cos(pitch).
- Eave length = house length + 2 × overhang.
- Roof area = slopes × eave length × slope length; squares = m² ÷ 9.290304.
- Panels per slope = ⌈eave length ÷ panel cover width⌉.
- Cost = area × (1 + waste) × panel price + trim/fasteners/underlay (% of panels) + labour × area.
The principle
A pitched roof surface is larger than the floor plan it covers. Each slope is a tilted rectangle, and its true length along the slope is the level run divided by the cosine of the pitch, from right-triangle trigonometry. Steeper roofs therefore have more area. Metal panels run from eave to ridge, so their length is the slope length and their number depends on the eave length and each panel's cover width (the width left after side laps). Trim, fasteners and underlay scale roughly with the panel cost, and labour with the area.
Worked example
Default inputs: house 12 × 8 m, overhang 0.4 m, pitch 25°, gable, panel cover 900 mm, panels $22/m², trim 25%, labour $20/m², waste 7%.
- Run = 4 + 0.4 = 4.4 m; slope length = 4.4 ÷ cos 25° = 4.85 m; eave length = 12.8 m.
- Area = 2 × 12.8 × 4.855 = 124.3 m² = 13.4 squares.
- Panels = 2 × ⌈12.8 ÷ 0.9⌉ = 2 × 15 = 30.
- Panels = 124.29 × 1.07 × 22 = $2,925.66; trim etc. = 25% = $731.41; labour = 124.29 × 20 = $2,485.69.
- Total = $6,142.76, or $49.43 per m².
| House length along the eaves (m) | Estimated total cost |
|---|---|
| 3 | $1,823.63 |
| 6 | $3,263.34 |
| 9 | $4,703.05 |
| 12 | $6,142.76 |
| 15 | $7,582.47 |
| 18 | $9,022.18 |
| 24 | $11,901.60 |
How to read the result
Compare the area and cost per m² with your quotes. Hips, valleys, dormers, chimneys, skylights and tear-off of old roofing add cost and are not included. Higher waste allowances are sensible for complex roofs or short panel runs.
Common mistakes
- Using the floor area instead of the sloped area.
- Forgetting overhangs at the eaves and gables.
- Using a panel's full width instead of its cover width.
- Comparing quotes that include different trims or underlays.
Frequently asked questions
How much roof area does a 12 × 8 m house have?
About 124 m² for a 25° gable roof with 0.4 m overhangs.
How do I calculate roof area from pitch?
Divide the plan area of each slope by the cosine of the pitch.
What is a roofing square?
100 ft², or about 9.29 m².
Sources: Right-triangle trigonometry (slope length = run ÷ cos pitch), any geometry textbook · NIST Special Publication 811 (2008), Appendix B (1 ft² = 0.09290304 m²)