The single biggest error in roof estimating is using the plan area. A roof is sloped, so its true surface area is always bigger than its footprint, and the steeper the pitch, the bigger the difference.
The pitch factor
Pitch factor = √(1 + (rise ÷ run)²). Multiply the plan area (including overhangs) by this factor to get the true roof area.
| Pitch | Degrees | Factor |
|---|---|---|
| 4/12 | 18.4° | 1.054 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45° | 1.414 |
For hip roofs where every slope has the same pitch, the same factor applies. A hip roof over a given footprint has the same area as a gable. The edges are what differ: hips instead of verges.
Tiles, slates and gauge
Tiles per m² depend on the gauge (the exposed length of each course) and the tile's cover width. Large interlocking concrete tiles run about 10 per m², plain tiles about 60 per m², and 500 × 250 slates about 20 per m². Battens are needed at every course: batten run = roof area ÷ gauge.
The edges
Measure eaves, verges, ridge, hips and valleys separately. They drive:
- Eaves course, under-cloak and verge system
- Eaves trays, fascia vents and drip
- Ridge and hip tiles, plus dry-fix or mortar
- Valley troughs or lead
- Ventilation at eaves and ridge
Underlayment
Breathable membrane comes in 1 m × 50 m rolls. Allow about 15% for laps. HR felts need more ventilation than LR membranes.
Check the minimum pitch
Every tile and slate has a minimum pitch that depends on exposure zone and headlap. Plain tiles typically need 35°, large interlocking tiles much less.
The roofing calculator works all of this out for gable, hip and mono-pitch roofs.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the Building Regulations and Building Control for your project.