Roof tiles are counted per square metre of roof slope. The trap is measuring the plan, the area you see from above, which is always smaller than the slope itself.
Tiles per square metre
The count depends on the tile and the headlap, the overlap up the slope. Marley publishes its Modern interlocking tile at 9.9 per m² with a 75 mm headlap and 10.7 with a 100 mm headlap. Baglist starts at 10.
Plain tiles are much smaller and lap twice. Marley lists concrete plain tiles at 60 per m².
Plan area to slope area
If you only have the plan, the slope area is the plan area divided by the cosine of the pitch. That gives a slope factor to multiply by.
A 28 m² plan at 22.5° is 28 × 1.082 = 30.31 m² of slope. At 10 tiles a square metre with 10% waste, that is 333.4 tiles, so 334.
| Pitch | Slope factor | Plan 28 m² becomes |
|---|---|---|
| 22.5° | 1.082 | 30.31 m² |
| 30° | 1.155 | 32.33 m² |
| 35° | 1.221 | 34.18 m² |
| 40° | 1.305 | 36.55 m² |
| 45° | 1.414 | 39.60 m² |
What the count includes
It is a count of main tiles only. Ridge, hip, verge and valley pieces are bought separately by the metre or by the piece, and the manufacturer's guide gives the gauge and lap for your pitch.
This is a shopping estimate, not a roof design. The minimum pitch, the batten gauge and the fixings come from the tile maker's literature.
Figures are estimates for a shopping list. Check the pack or datasheet before you buy.