Coverage
Slates and plain tiles (double lap): gauge, number per m² and battens
- where
- batten gauge (mm)
- slate or tile length (mm)
- headlap (mm)
- slates or tiles per m²
- slate or tile width (mm)
- side joint gap (mm)
- battens per m² (m/m²)
Explain this equation
Gives the batten gauge and quantities for natural slates, fibre-cement slates and plain tiles, which are laid double lap. Use it to set out battens and order slates or tiles.
In double-lap work every point of the roof is covered by at least two thicknesses, and the headlap h is the amount the third course up overlaps the first. That is why the length less the headlap is halved to get the gauge g. Each slate covers its width plus any side gap j, so it occupies g × (w + j) square millimetres of roof. Battens are needed once per gauge, so 1000 / g metres per m².
Halving comes from the geometry of double lap: two gauges plus the headlap make up one slate length.
Sense check: a 500 × 250 mm slate with 100 mm headlap and 5 mm gaps gives a 200 mm gauge, about 19.6 slates and 5 m of batten per m².
Watch out: headlap depends on pitch and exposure. Take it from the BS 5534 tables or the manufacturer; too small a headlap lets wind-driven rain through.
For double-lap coverings (natural and fibre cement slates, plain tiles), every point of the roof is covered by at least two thicknesses, so the gauge is half of the length left after the headlap.
Enter your values to see the result.
Good to know
Read the headlap from the BS 5534 table (or the manufacturer’s) for the slate size, rafter pitch and driving-rain exposure. For example, a 500 × 250 mm slate at 30° needs 80 mm headlap below 56.5 l/m² per spell and 100 mm above it. Slates are laid with a small joint (about 5 mm) between them; plain tiles are butted, so enter 0. Concrete plain tiles: 267 mm long with 65 mm headlap gives just over 100 mm gauge; Marley gives 100 mm and 60 tiles/m².
Common mistake
Using single-lap thinking (gauge = length − lap). On a double-lap roof that doubles the gauge and the roof leaks.
Limits
Rectangular slates and plain tiles centre- or head-nailed, or hook-fixed. Random-width and diminishing-course slating is worked course by course. Does not choose the headlap: that comes from BS 5534 tables.
Sources
- Cupa Pizarras: Batten gauge for slates (gauge = (slate length − headlap) ÷ 2)
- Natural slate coverage tables (nominal sizes, 5 mm joint gap per BS 8000-6; at least 5% waste)
- Marley Rivendale fibre cement slate fitting instructions (gauge formula, 5 mm gap, 2 nails + 1 disc rivet per slate, coverage and weight table)
- Welsh Slate: UK annual driving rain index and headlap tables to BS 5534
- Marley concrete plain tile: technical data (267 × 168 mm, 65 mm headlap, 100 mm gauge, 60 tiles/m², 10 m battens/m², 73.8 kg/m²)
- BS 8000-6 Workmanship on construction sites. Slating and tiling of roofs and walls. Code of practice (BSI)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the roof covering manufacturer’s fixing specification, BS 5534, BS 8000-6, BS 6229, the Lead Sheet Training Academy guidance and Building Regulations. Wind uplift and structural design need a competent designer.