Moderate Exposure (less than 56.6 l/m2 per spell)

The table below shows Minimum Recommended Head Laps
    Minimum Rafter Pitch   
Size -
Nominal (mm)
20˚22.5˚25˚27.5˚30˚35˚40˚45˚ to 75˚85˚
600 x 300--958580706055-
500 x 300115105958580706055-
500 x 250--95858070605550
460 x 225----8070605550
400 x 250----8070605550
400 x 200----8070605550
350 x 250----8070605550
350 x 200----8070605550
300 x 200----8070605550

Values calculated using nominal sizes and incorporating a 5mm joint gap as per BS8000: part 6: 1990. We recommended the addition of at least 5% wastage allowance.

Severe Exposure (56.6 l/m2 or greater per spell)

The table below shows Minimum Recommended Head Laps
    Minimum Rafter Pitch   
Size - Nominal (mm)20˚22.5˚25˚27.5˚30˚35˚40˚45˚ to 75˚85˚
600 x 300----100908070-
500 x 300-130120110100908070-
500 x 250---11010090807065
460 x 225----10090807065
400 x 250----10090807065
400 x 200----10090807065
350 x 250----10090807065
350 x 200----10090807065
300 x 200----10090807065

(Based on BS5534: 2003 – All headlaps have been rounded up to the nearest 5mm)

Wind Loads & Weather Resistance

Slates correctly fixed will have adequate resistance to wind loads, wind uplift and rain penetration under most conditions. The tables above give minimum recommended laps according to exposure, roof pitch and slate size.

Detailed guidance on wind load calculations is given in BS5534: 2003 and BS6399, Part 2: 1997 and Part 3: 1998.