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 | - | - | 95 | 85 | 80 | 70 | 60 | 55 | - |
| 500 x 300 | 115 | 105 | 95 | 85 | 80 | 70 | 60 | 55 | - |
| 500 x 250 | - | - | 95 | 85 | 80 | 70 | 60 | 55 | 50 |
| 460 x 225 | - | - | - | - | 80 | 70 | 60 | 55 | 50 |
| 400 x 250 | - | - | - | - | 80 | 70 | 60 | 55 | 50 |
| 400 x 200 | - | - | - | - | 80 | 70 | 60 | 55 | 50 |
| 350 x 250 | - | - | - | - | 80 | 70 | 60 | 55 | 50 |
| 350 x 200 | - | - | - | - | 80 | 70 | 60 | 55 | 50 |
| 300 x 200 | - | - | - | - | 80 | 70 | 60 | 55 | 50 |
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 | - | - | - | - | 100 | 90 | 80 | 70 | - |
| 500 x 300 | - | 130 | 120 | 110 | 100 | 90 | 80 | 70 | - |
| 500 x 250 | - | - | - | 110 | 100 | 90 | 80 | 70 | 65 |
| 460 x 225 | - | - | - | - | 100 | 90 | 80 | 70 | 65 |
| 400 x 250 | - | - | - | - | 100 | 90 | 80 | 70 | 65 |
| 400 x 200 | - | - | - | - | 100 | 90 | 80 | 70 | 65 |
| 350 x 250 | - | - | - | - | 100 | 90 | 80 | 70 | 65 |
| 350 x 200 | - | - | - | - | 100 | 90 | 80 | 70 | 65 |
| 300 x 200 | - | - | - | - | 100 | 90 | 80 | 70 | 65 |
(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.