Quick answer: A 10 m × 8 m footprint with a 6/12 pitch has a roof area of 80 × 1.118 = 89.44 m²; with 10% waste that is 98.39 m², or 10.59 roofing squares.
How it's calculated
Measure the footprint the roof covers, including the eaves overhang. A roof sloping at a pitch of r in 12 is longer than its plan by the factor √(1 + (r ÷ 12)²), so roof area = footprint × pitch factor. This holds for gable and simple hip roofs of one pitch.
Roofing material is sold by the square, which is 100 ft² (9.29 m²). Standard asphalt shingles come about three bundles to a square. For metal or fibre-cement sheets, divide the roof area by the cover area of one sheet (its effective width after overlap × length).
Most Gulf villas have flat concrete roofs, where the area is simply the footprint; pitched roofs are common on sheds, car parks and mountain areas such as Abha and Taif.
Worked examples
- 10 m × 8 m, pitch 6/12, 10% waste: 80 × 1.1180 × 1.10 = 98.39 m² = 10.59 squares
- Sheets with 1 m × 3 m cover: 98.39 ÷ 3 = 32.8 → 33 sheets
- 40 ft × 30 ft, pitch 4/12, no waste: 1,200 × 1.0541 = 1,265 ft² = 12.65 squares
Pitch, angle and factor
| Pitch | Angle | Area factor |
|---|---|---|
| 2/12 | 9.5° | 1.014 |
| 3/12 | 14° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 9/12 | 36.9° | 1.25 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45° | 1.414 |
Frequently asked questions
What is a roofing square?
100 square feet (9.29 m²) of roof surface. Roofers price and order materials by the square.
How do I find the pitch?
Measure how many inches the roof rises over 12 inches of horizontal distance, or read the angle from the drawings and use tan(angle) × 12.
How much waste should I allow?
About 10% for a simple gable roof and 15% for roofs with hips, valleys and dormers.
What is the pitch factor for common roofs?
1.054 for a 4-in-12 roof, 1.118 for 6 in 12, 1.202 for 8 in 12 and 1.414 for 12 in 12. Multiply the footprint by it and you have the roof area.
How many shingle bundles do I need?
Standard asphalt shingles run about three bundles to a square, so a 20-square roof takes around 60. Add some for waste, ridge caps and starter strips, and check the coverage printed on the bundle.
How much waterproofing does a flat roof need?
More than the floor area. The membrane turns up the parapet walls, usually at least 15 cm, and overlaps at every seam. Add the parapet length times the upturn height, plus the overlap the manufacturer asks for.