Head Laps

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

The table below shows Minimum Recommended Head Laps.

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

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.