Slate Roof Calculator — Slate Count, Nails & Battens

This depends on the slate size and headlap. For 600×300mm Duchess slates with 90mm headlap, you need approximately 13 slates per m² (Welsh Slate / BS 8000-6 coverage method). Smaller 400×200mm Lady slates at 75mm headlap need around 30 per m².

Headlap is the overlap between the tail of one slate and the head of the slate two courses below. Standard headlap is 75–90mm. Steeper roofs can use less headlap; shallower pitches need more.

BS 5534 requires a minimum of 2 nails per slate (single-nail fixing is not permitted). Use copper nails to BS 1202-2 or aluminium to BS 1202-3, with a head at least 10mm across. Copper is preferred because it does not corrode against natural slate. Nail length is typically 30–38mm depending on batten thickness.

Gauge = (Slate length − Headlap) ÷ 2. For 600mm Duchess slates with 90mm headlap: (600 − 90) ÷ 2 = 255mm gauge. Battens are then fixed at 255mm centres. Use our Felt and Batten Calculator to work out batten and membrane quantities. For standard tiles rather than slates, see our Roof Tile Calculator.

Allow 10% wastage for straightforward rectangular roofs. Increase to 15–20% for complex roofs with hips, valleys and dormers where more cutting is required.