Famitchel

Slate is a fine grained metamorphic rock that can be split into layers. The mineralogical composition of slate includes chlorite, quartz, mica, feldspar, and deceasing quantities of calcite, pyrites and carbonates.

Click to see Del Carmen Slate photos

Although a rock may be classified as slate there is no guarantee that it will be of suitable quality for roofing applications unless thorough testing has been applied. Visible defects that can make slate unsuitable are non-flatness, irregular thickness, cracks, transverse grain, non-squareness, flaking or delamination. Other defects hat are only apparent through testing are water absorption, non resistance to acid attack, inclusions such as pyrites that cause rust and possible failure, and too high a calcium carbonate content.

Thus it is not possible to tell the quality of a natural roofing slate by its appearance alone. What may look like a perfect slate tile at the time of laying it on the roof may eventually rust, delaminate, crack, or at worst break and fall off the roof within a few years or even months. In order to avoid the problem of specifying defective slates many countries have established their own quality standards which require that the slate comply with certain minimum criteria before it can be allowed on the roof. The UK, France, Germany, Belgium and the US all have their own standards but the stringency of these standards varies from country to country. In the UK the standard for natural roofing slate is the BS 680 part 2 which tests slates to three basic criteria: water absorption, acid resistance and wetting and drying cycles. Some of the inadequacies of the BS Standard can be highlighted when comparing it with the French P32/302 Norm which is widely recognized as being the most stringent of all international standards.

In addition to the three tests performed under B 680, the French standard also requires tests analysing both the quality of the rock and the performance of the slate itself. A vital aspect of the French norm is that samples are selected at source by testing authorities and a classification system has been developed which classifies the whole quarry as A, B, C etc depending on the consistency of the slate in terms of colour, absence of inclusions, appearance etc. Classification is obligatory for any quarry exporting to the French market and the classification is reassessed annually. The American ASTM test is also a good guide to the life expectancy in excess of 75 years. A quarry with a French Class A and American grade S1 classification will ensure a good quality roof.

Requirements of French Class A

• Increase in thickness: No nodules or thickness increase more than half the thickness of the slate
• Fines: Sectioning, flaking – not permitted
• Steps: Not permitted
• Colour: Slates shall be of the same colour
• White deposits: Not permitted
• Pyrites: Non oxidising – not permitted if passing through the slate. Oxidising – not permitted.
• Flatness: Bow shall not exceed a value equal to the thickness of the slate. Concavity shall not be permitted on the undersurface of the slate.

Advantages of a Good Quality Slate

• Life expectancy of at least 75 years
• Ease of fixing
• Less wastage
• Few or nil harmful inclusions such as pyrites making the slate less susceptible to environmental pollution
• No maintenance
• No discolouration
• No erosion or delamination
• More beautiful roof thus adding value to property

Natural slate is the perfect roofing material being environmentally friendly, low maintenance, pollution resistant, durable, flexible in design and long life.