Roof Tiles

NATURAL ROOFING SLATE DESIGN AND FIXING GUIDE

Effective design of a slate roof must take into account a number of inter-related factors including site exposure, the pitch of the roof, the type of slate selected and the slate lap.

Reference should be made to AS4597:1999, Roof slates installation. Some of the more important considerations are given below. In all applications good workmanship and adherence to appropriate building codes should be undertaken.

Environmental conditions

Rain exposure
The degree of exposure of a building to driving rain determines the minimum lap which should be specified. The anticipated degree of exposure can be found in AS. Localised factors such as high buildings, buildings on slopes or tops of hills and coastal sites, can increase the exposure grading which should be applied in a specific project

Wind uplift
Adequate resistance to wind load and wind lift can be provided by following the fixing details.

Design calculations for wind load and wind uplift are given in AS1170.2.

Minimum recommended laps in mm for different exposures and roof pitches

Moderate exposure index between 3m/s and 7m/s
Slate size (mm)	Roof pitch
Length x width	85	45	40	35	30	25	22	20
500 x 250	50	65	65	75	75	90	105	120
400 x 250	50	65	65	75	75

Severe exposure index greater than 7m/s
Slate size (mm)	Roof pitch
Length x width	85	45	40	35	30	25	22	20
500 x 250	65	65	75	95	100	115
400 x 250	65	65	75	90	95

Pitch of roof
In general the lower the roof pitch the greater the lap. This longer lap will help to resist both capillary action and wind uplift.

Lap
The lap is calculated by taking into account wind uplift, exposure to driving rain and the roof pitch.

Battens
Recommended timber battens for natural slate roofs are 50x25mm

Battens should be set out horizontally across the roof at a gauge calculated from the formula:

Gauge = (length of slate - lap)
        -----------------------
        2

Battens should be supported on a minimum of three rafters with each end fully supported.

Underlay
A good quality sarking meeting the requirements of AS……. The sarking should be installed to AS.

Fixing method
Natural slates should be fixed using the traditional holing and nailing system.

Nails
Nails should be copper or stainless steel. They should be at least 20mm longer than two thicknesses of slate.

Coverage of slates

Slate size (mm)	Lap (mm)
Length x width	50	65	75	80	90	100	110	115
Number of slates/m
500 x 250	17.8	18.4	18.8	19	19.5	20	20.5	20.8
400 x 250	22.9	23.9	24.6	25	25.8	26.7	27.6	28.1

Weight of slate roof

Thickness	Size	Weight/1000 (kg)
5 - 6mm	500 x 250	1675
	400 x 250	1445

The weight of a slate roof can be determined from the above tables taking into consideration slate size, roof pitch and exposure.