What is the minimum headlap for fixing slates?

Minimum headlap for fixing slates with nails or hooks

The installation of a slate roof involves a number of aspects that must be considered in the design process. Most significant is the environment, with the two main factors being exposure to wind and rain. The primary aspects to be considered are:

Site Exposure

Buildings positioned on slopes, hills or coastal areas as well as tall buildings will be graded with higher exposure.

Type of slate

Small slates are more suitable for steep roofs, while the more exposed the site and on lower roof pitches, wider slates should be used and the lap increased. (See table)

Slate lap

This is established by measuring the roof pitch and considering the rain and wind exposure. Where the thickness of natural slates reduces their pitch by 3O or more, the next lower rafter pitch should be selected to determine the recommended head-lap.

Geographic categories of driven rain exposure around the uk

Sheltered: Less than 33

Moderate: 33 less than 56.5

Severe: 56.5 to less than 100

Very severe: 100 or more

In general any area within 30 miles of a west facing coastline is considered to be one of severe exposure.

*Please note that hook fixings should not be used on any pitch below 25º.*
SHELTERED/MODERATE EXPOSURE (LESS THAN 56.5L/M²)
SIZE (mm)	RAFTER PITCH
	20º	22.5º	25º	27.5º	30º	35º	40º	45º to 75º	85º
600x300	-	-	95	85	80	70	60	55	50
500x300	115	105	95	85	80	70	60	55	50
500x250	-	-	95	85	80	70	60	55	50
450x250	-	-	-	-	80	70	60	55	50
400x250	-	-	-	-	80	70	60	55	50
400x200	-	-	-	-	80	70	60	55	50
350x250	-	-	-	-	80	70	60	55	50
350x200	-	-	-	-	80	70	60	55	50
300x200	-	-	-	-	80	70	60	55	50

*Please note that hook fixings should not be used on any pitch below 25º.*
SEVERE/VERY SEVERE (56.5L/M² OR GREATER SPELL)
SIZE (mm)	RAFTER PITCH
	20º	22.5º	25º	27.5º	30º	35º	40º	45º to 75º	85º
600x300	-	-	120	110	100	85	80	70	65
500x300	-	130	120	110	100	85	80	70	65
500x250	-	-	-	110	100	85	80	70	65
450x250	-	-	-	-	100	85	80	70	65
400x250	-	-	-	-	100	85	80	70	65
400x200	-	-	-	-	100	85	80	70	65
350x250	-	-	-	-	100	85	80	70	65
350x200	-	-	-	-	100	85	80	70	65
300x200	-	-	-	-	100	85	80	70	65

Minimum pitch for slate roof

When considering the minimum pitch for a slated roof it is necessary to establish the exposure of the site, the length of rafter and the height of the building.

The recommendations shown above are based on a maximum height to eave of 12m and a maximum rafter length of 9m in moderate exposure and 6m in severe exposure.