Roof slate calculator: headlap, gauge and slates
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Slate length | 500 mm |
| Slate width | 250 mm |
| Headlap | 75 mm |
| Side gap between slates | 5 mm |
| The area is | Measured on the slope |
| Roof area | Slope: rectangle 8 m × 5 m |
| Nails per slate | 2 nails |
| Verges | 2 |
Order: 739 slates at a 212.5 mm gauge
| Result | Value |
|---|---|
| Slates | 739 |
| Batten gauge | 213 mm |
| Slates per m² | 18.45 slates/m² |
| Batten per m² | 4.71 m/m² |
| Battens | 188.24 m |
| Roof slope area | 40 m² |
| Slate nails | 1,478 |
Show the working
- Gauge: (500 − 75) ÷ 2 = 212.5 mm
- Slates per m²: 1 ÷ (0.212 m × (0.25 + 0.005) m) = 18.45
- Batten per m²: 1 ÷ 0.212 m = 4.71 m
- Slates: 40 m² × 18.45 × 1 (0% waste) = 738.2, rounded up = 739
- Battens: 40 m² × 4.71 m/m² = 188.2 m
- Nails: 739 × 2 = 1478
How it works
- Gauge = (slate length − headlap) ÷ 2
- Slates per m² = 1 ÷ (gauge × (slate width + side gap)), all in metres (10⁶ ÷ (gauge × (width + gap)) in mm)
- Batten per m² = 1 ÷ gauge (1000 ÷ gauge in mm)
- Slope area = plan area ÷ cos(pitch) when measured on plan; slates = slope area × slates per m² × (1 + waste), rounded up
- Nails = slates × nails per slate; slate-and-a-half = verges × courses, where courses = rafter length ÷ gauge, rounded up
Headlap depends on the slate, the pitch and your exposure: take it from the slate maker or BS 5534, never a rule of thumb.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Side gap between slates | 5 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 R2 (test RS1 uses a 5 mm side gap). Researcher to confirm against BS 5534 and slate makers. |
| Waste allowance | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 R2 gives no waste figure. Researcher to set from slate makers' guidance; blank adds none. |
| Nails per slate | 2 nails | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 R2: 'nails (2 per slate)'. Researcher to confirm against the maker's fixing specification. |
Regulations and standards
- BS 5534 (slating and tiling): headlap by pitch and exposure, from the slate maker
Explained: BS 5534 (slating and tiling).
BS 5534 is used across the UK; headlaps and minimum pitches come from the slate maker for your exposure.