Step 1: slope length from the pitch
Drawings give the plan width; tiles go on the slope. Slope length = plan distance from eaves to ridge ÷ cos(pitch). A 4.5 m plan run at 30° is 4.5 ÷ 0.866 = 5.20 m of rafter.
Step 2: tiles per m² and gauge
| Tile | Tiles per m² | Typical gauge |
|---|---|---|
| Concrete interlocking | about 10 | about 345 mm |
| Plain clay tile | about 60 | 100 mm |
| Natural slate 500 × 250 | about 20 | about 200 mm |
The gauge is the batten spacing, set by the tile length and the headlap the manufacturer requires for your pitch and exposure. Always check the manufacturer's figures; low pitches need more headlap, which means more tiles.
Worked example: a gable roof
Two roof slopes, each 8 m along the eaves, 4.5 m plan run at 30°, concrete interlocking tiles at 10 per m² and 345 mm gauge.
| Step | Working | Result |
|---|---|---|
| Area per slope | 8 × 5.20 | 41.57 m² |
| Tiles per slope, +5% | 41.57 × 10 × 1.05 | 437 |
| Courses per slope | 5196 ÷ 345 = 15.1, round up to 16, plus 1 | 17 |
| Batten per slope | 17 × 8 m | 136 m |
| Battens to order (3.6 m) | 272 m ÷ 3.6 | 76 lengths |
| Underlay, +10% laps | 83.1 m² × 1.1 ÷ 50 m² rolls | 2 rolls |
Total: 874 tiles, 76 battens and 2 rolls of underlay, plus ridge tiles (one per tile length along the ridge), eaves and verge details, and fixings.
UK rules worth knowing
- BS 5534 sets batten sizes: 25 × 50 mm is typical for rafters at up to 600 mm centres for most tiles, larger for heavier tiles or wider spacing.
- BS 5534 also requires tiles to be mechanically fixed (nailed or clipped) to resist wind uplift, and ridge, hip and verge tiles to be mechanically fixed too: bedding mortar on its own is no longer enough.
- Breathable underlay must be laid with the laps the manufacturer gives, and the roof void ventilated as Approved Document C requires.