How the roof tile calculator works
The calculator first works out the true sloping area of the roof. For a pitched roof that's the plan area divided by the cosine of the pitch, so steeper roofs have more area than their footprint suggests. It then multiplies by the number of tiles per m² for your chosen tile and batten gauge.
Typical coverage by tile type
| Tile / slate | Approx. per m² | Typical gauge | Typical min. pitch |
|---|---|---|---|
| Plain tile 265 × 165mm | 60 | 100mm | 35° |
| Large interlocking concrete | 9.5–10.5 | ~345mm | 15–17.5° |
| Clay pantile | 15–17 | ~265mm | ~30° |
| Natural slate 500 × 250mm | ~18.5 | ~212mm | 22.5° |
| Natural slate 600 × 300mm | ~12.5 | ~262mm | 20° |
Gauges and coverage vary between manufacturers and with pitch and exposure. Use these to budget, then confirm with the tile maker's fixing specification (in the UK, BS 5534 sets the rules for fixing tiles and slates).
What else a re-roof needs
- Battens: treated, graded tile battens (typically 25 × 50mm for 600mm rafter centres), sold in 4.8m lengths.
- Underlay: breathable membrane laid under the battens, lapped at least 150mm.
- Ridge and hip tiles: about 2.2 per metre for 450mm ridge tiles, either bedded or on a dry-fix system.
- Fixings: tile nails and clips to BS 5534. Most modern roofs need every tile mechanically fixed.
- Extras: eaves vents, verge trims or dry verge, valley troughs and lead flashings.
Working at height is dangerous. Use proper scaffolding or roof access equipment, and check whether your project needs building regulations approval (replacing more than 25% of a roof normally does).
Worked example: an 8m × 6m house with a 35° gable roof
With 300mm overhangs, each rafter runs (3m + 0.3m) ÷ cos 35° = 4.03m, so the two slopes cover 2 × 4.03 × 8 = 64.5m², a third more than the 48m² footprint.
- In plain tiles (about 60 per m²) with 10% for cuts, that's 4,255 tiles, about 709m of battens (148 lengths of 4.8m), 2 rolls of membrane and 19 ridge tiles.
- In large interlocking concrete tiles (about 10 per m²), it's only 710 tiles and around 43 batten lengths. That's why interlocking tiles are so much quicker to lay.
Switch the roof shape to "Hipped" to see how hips change the area and add hip tiles. For an awkward roof, measure each slope from eaves to ridge and use "Measured slope".
Changing the roof covering
Different coverings weigh very different amounts. Swapping lightweight slates for heavy concrete tiles, or the other way round, can affect the roof structure. Your roofer or building control may need structural calculations. Re-roofing usually also means new battens and membrane, and is the ideal time to improve loft ventilation and insulation. Use the loft insulation calculator for that, or the loft conversion cost calculator if you're thinking bigger.
Measuring the pitch
You can often find the pitch on the original drawings or the tile maker's spec. Otherwise, measure it in the loft with a digital angle finder on a rafter, or estimate it from the ground with a phone inclinometer app held level with the slope. Many UK houses have pitches between 30° and 45°. The unit converter turns degrees into percent and rise per metre if you need them.
Common mistakes
- Using the footprint as the roof area. At 35° the roof is about 22% bigger than its plan, and at 45° about 41% bigger.
- Too low a pitch for the tile. Every tile has a minimum pitch. Below it, wind-driven rain gets under the laps. The calculator warns you.
- Too few spares. Hips, valleys and dormers mean lots of cut tiles. Allow 10–15%, and keep a few spares for later repairs, as tile ranges get discontinued.
To finish the eaves, the guttering calculator covers gutters and downpipes. See all roofing calculators.