Roof Area Calculator
True sloped roof area, rafter length and ridge rise from pitch.
Quick answer
With the default inputs: Plan length 12 · Plan width 9 · Roof pitch 30° etc.True roof area 150.11 m²
Edit the inputs below to recalculate.Inputs
Result
True roof area
150.11m²
| Projected area with eaves | 130.00m² |
| Slope factor | 1.1547 |
| Slope percent | 57.74% |
| Horizontal run per slope | 5.00m |
| Rafter length | 5.77m |
| Ridge rise | 2.89m |
| Ridge length | 13.00m |
| Eaves length | 46.00m |
| Membrane at 1.15 laps | 172.63m² |
Slope factor is 1 ÷ cos(pitch) — 1.1547 at 30° and 1.4142 at 45°. A 2% flat-roof fall gives 1.0002 and can be ignored, but beyond 15° the factor is essential or you under-order tiles by more than 15%. Eaves are part of the roof area and a significant share of the quantity.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Projected area | A = (L + 2e) × (W + 2e) |
| Slope factor | k = 1 ÷ cos(θ) |
| True roof area | A = projected × k |
| Half span | b = width ÷ 2 for a gable |
| Rafter length | L = b ÷ cos(θ) |
| Ridge rise | h = b × tan(θ) |
A 12 × 9 m plan at 30° with 0.5 m overhangs: 13 × 10 = 130.00 m² projected, slope factor 1.1547, true area 150.11 m², rafters 5.77 m and a 2.89 m ridge rise.
How to work it out
- Compute the horizontal projection including eaves: (length + 2 × eave) × (width + 2 × eave). Eaves are normally 0.4-0.6 m, and longer overhangs need a structural check.
- Slope factor k = 1 ÷ cos θ. At 30° it is 1.155; at 45° it is 1.414. Note that a slope angle is not a percentage — a 20% fall is about 11.3°.
- Actual roof area = projection × k. This is the area that governs tiles, membrane and insulation, not the plan area.
- For a single pitch on a duo-pitch roof, the horizontal width is half the width including eaves, and the rafter length is that divided by cos θ. This dimension sets purlin and batten lengths.
- Ridge height difference = horizontal width × tan θ, which sets gable height, ridge level and gable area.
- Tile quantity = actual area × (1 + waste), with 5-10% waste — the upper end for hips, valleys and verge cuttings — then converted to piece count per square metre.
Where it goes wrong
- Using plan area as actual area. A 30° pitch is 15.5% larger, so 100 m² of plan under-orders 15 m² of tile and membrane and inevitably runs short.
- Confusing slope percentage with degrees. A 20% fall is arctan(0.2) = 11.3°, not 20°; k is 1.02 against 1.064, a 4% difference in tile.
- Forgetting the eaves. With 0.5 m all round, a 12×9 m plan goes from 108 m² to 130 m² — 20% more. Omitted eaves are the leading cause of tile shortages.
- Costing one pitch only. A duo-pitch roof has two pitches; a hipped roof has four plus two gables. Counting one halves the order.
Material and cost
Clay tile 40-90 CNY/m², concrete tile 25-45, asphalt shingle 35-70, metal roofing 80-200. SBS membrane 30-50 CNY/m², XPS insulation at 50 mm 25-40. Tiling labour 30-50 CNY/m², with pitched work costing over 50% more than flat.
On site
- Pitches below about 10° are unsuitable for tiles and will leak; use membrane or metal roofing instead.
- Waste depends heavily on shape: 5% for a simple rectangle, up to 12% for L-shaped, multi-pitch roofs with valleys. Price by shape, not by average.
- Pitched roofs generate significant horizontal thrust, which the trusses or gables must resist — a common structural weakness in self-built housing.
- Valleys, hips and eaves need additional material measured by linear metre, and these are routinely left outside a per-square-metre rate.
Basis
- GB 50345-2012 Technical code for roof engineering, roof design
- GB 50207-2012 Code for acceptance of construction quality of roof works
- GB 50693-2011 Technical code for pitched roof engineering
FAQ
Why is roof area larger than floor area?
A pitched roof is a slope, so the same plan projection develops into a larger surface — by exactly 1/cos(pitch). Eaves, parapet inner faces, gutters and upstands add more. At 30° with 0.5 m overhangs, roof area typically runs 30-40% above floor area, so estimating tiles from floor area loses money.
What pitch keeps a roof watertight?
It depends on the covering: plain tiles 30° minimum, shingles 15°, profiled metal 5°, membrane flat roofs 2% with gutters at 1%. Too shallow cannot be fixed by adding layers — water sits under the tiles and rots the battens first. Above 45° add mechanical fixings or the tiles slide.
Is the rafter length the same as the sloped length?
Close but not equal. Rafters also need the eaves overhang, the ridge lap and cutting allowance, so cut lengths run 300-500 mm longer. Timber also depends on purlin positions; for steel, divide the sloped length by the purlin spacing and round before working back, otherwise the last sheet needs cutting on site.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Roof Area Calculator. 2026-10-25. https://jianzhujisuan.com/en/roof-area
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.