Page 8 – Shape, size and thickness
Standard specification and details
Shape, Sizes and Thickness
Natural Slates for roof coverings are rectangular with dressed edges; for vertical cladding they may be rectangular or square. Bull nose and other ornamental shapes can also be produced and are particularly effective for vertical cladding.
In British practice the length of the slate is typically twice the width, each of the most popular sizes having a traditional name. Thus Imperial slate sizes range from 12″ length by 6″ width (Small Doubles) and 14″ by 7″ (Narrow Ladies) up to 24″ by 12″ (Duchesses) and 24″ by 14″ (Princesses). Popular sizes are:
|Duchesses||24″ x 12″|
|Small Duchesses||22″ x 12″|
|Wide Countesses||20″ x 12″|
|Countesses||20″ x 10″|
|Wide Viscountesses||18″ x 10″|
|Viscountesses||18″ x 9″|
|Wide Ladies||16″ x 10″|
|Ladies||16″ x 8″|
Imported slates are commonly produced to metric sizes, a table showing the metric equivalents is available on Page 10. However, frequently metric sized slates are sold in rather smaller dimensions than these exact equivalents. Thus a 24″ by 12″ slate which would convert to 610mm by 305mm will often become 600mm by 300mm and a 20″ by 10″ slate would go from 510mm by 255mm to 500mm by 250mm. Be aware of this if you are trying to match slates with an existing roof. The vast majority of roofs in this country that are pre-1980 will be covered with Imperial sized slates.
Selection of the size of slate is important. Smaller slates give greater visual texture to a roof and are more readily available since only the very best pillars of slate can be cut to larger sizes. But they involve far greater labour in fixing and are not suitable with low pitches in exposed situations. In British best practice extra wide slates (usually referred to as doubles as they are twice the width of the standard slate) are required at verges, abutments, mitred hips and valleys. These doubles are laid in alternate courses to maintain the correct bond.
Variations in thickness are an inherent property of natural slate and a principal cause of texture and charm achieved in a surface expertly covered in natural slate. Depending on the quarry, slates are sorted at the works according to thickness within certain fairly wide tolerances. Thus “Bests” are on average 4mm thick but will typically vary between 3mm and 5mm. “Standards”, “Mediums” or “Strongs” average 5mm but vary between 3mm and 6mm. While “Heavies” are approximately 6mm and “Extra Heavies” approximately 8.5mm.
These thickness guides are based on British slate which is particularly strong and durable. Imported slate often does not have the same natural strength and therefore, under the current European Standards, has to be cut thicker in order to pass accreditation. Chinese slate in particular has to be cut very thick. More information on these standards is available on Page 7.