Roofing isn't a typical DIY project — but understanding how estimates work puts you in a far better position when contractors arrive with quotes. It helps you verify whether numbers are reasonable, understand what drives material costs, and ask the right questions.
The Key Challenge: Pitch Inflates Area
A roof's surface area is always larger than the floor area directly below it — because the roof slopes. The steeper the pitch, the larger the difference. This is the most common source of confusion in roofing estimates. Our roofing calculator applies pitch factors automatically. Use our area calculator for the floor footprint first.
Pitch Factors
- 15° pitch: × 1.04 | 20°: × 1.06 | 25°: × 1.10
- 30° pitch: × 1.15 | 35°: × 1.22 | 40°: × 1.31 | 45°: × 1.41
Example: 30° pitch over a 10m × 8m footprint (80 m²): 80 × 1.15 = 92 m² per slope. Symmetrical gable roof: × 2 = 184 m² total.
Tile Quantities
Coverage depends on tile type and batten gauge. Manufacturer data sheets specify tiles per m² (typically 9-16 tiles/m² for standard interlocking tiles; up to 60+ for small plain clay tiles). Always use the manufacturer's figure. Add 10% waste for standard gable roofs; 15% for hip and valley roofs where additional cutting is needed.
Underlay
Breather membrane is sold in rolls (typically 1m wide). Calculate total roof area plus 15% for overlaps. Courses typically overlap by 150mm horizontally.
Ridge and Hip Tiles
Ridge tiles run along the apex. Measure total ridge length and divide by individual tile length (300-450mm) for quantity. Hip tiles are measured along each hip rafter length.
What Drives Cost
- Tile type: concrete tiles cheapest; handmade clay most expensive
- Roof complexity: hips, valleys, dormers increase labour and waste significantly
- Scaffolding: required for all roof work, adds £800-£2,000 to project cost
- Labour: typically 50-70% of total project cost
Further reading: The National Federation of Roofing Contractors helps find qualified, registered roofers. Find a registered roofer via the NFRC.