About Slate: The Roofing Material

For decades slate has been considered the best roofing material in the world. Real slate can last for over a century without maintenance. Slate is so durable in many instances it has outlasted the structure of the building it was on, then subsequently has been recycled for a new building or restoration work. Slate is a 500 million year old stone which is largely unaffected by severe climatic conditions. Slate is a hard, dense stone with rugged structural integrity. The intrinsic value of slate as a supreme roofing material can be attributed to the following qualities:

• Lasts a lifetime • Low maintenance • Fire proof
• Non staining • Impermeable to moisture
• Non-combustible • Resistance to acid
• Environmentally friendly
• It's a very popular choice for many architects

Coloured slate can be classified as one of three types….Fading, Unfading and Weathering. The weathering of slate is induced by the oxidation of minerals as they are exposed to the elements. The weathering process slowly changes the colour of the individual slate. Weathering slates will often exhibit changes that result in the colour changes. Slates rated non-weathering would exhibit the least amount of colour change. Semi-weathering slates are where some of these slates will exhibit colour change while others remain their original shade. The percentage of semi-weathering slates that will experience colour change is variable depending upon the location in the quarry from which the slate is extracted. The term weathering refers to slates that will exhibit the largest number of individual pieces that will transform from the original colour to an earth tone. Slates are classified as fading or unfading solely according to colour permanence. Fading is characterized by a chalk-ashen residue which will appear over time. While coloured slates do not fade, some will experience some colour change. Unfading refers to slate that will experience the least amount of colour change within its lifetime.

The most renowned slate comes from Wales however there are many other quarries around the world now producing good quality slate including Glendyne and Trinity in Canada, Vermont in America, Del Carmen from Spain and some others in Spain, Argentina, Brazil, China and Vietnam.

The testing method most used for slate is the British Standard 680 and French Standard P32-301 and  ASTM. This measures degrees of water absorption, its resistance to acids and wetting/drying cycles to check for signs of flaking, softening and delamination that may occur under stress.