Roofing slate needs to be of an age, and to have been subject to a pressure, long-term load, and heat that allows it to be cleaved in flat plains free of faults. Roofing slate laid down at some time around 500+ million years ago is far superior to younger slates. Age ensures that roofing slate will be free of minerals that will degrade or oxidise when exposed to the air, water, sunlight or pollution.With younger slates there are two common problem minerals that may lead to short term issues, and definitely lead to a shortened roof life.

Thes are calcium carbonate (CaCO3) and soluble iron pyrites in the form of FeS2 (iron disulfide).Calcium carbonate in the stone will cause it to weather and change colour. Slate that does not weather and degrad in this fashion will last a lot longer than one that does. The presence of calcium carbonate in excess quantities is a common occurrence in slates from Spain, Italy, China, and some areas of the USA. Slates with iron disulfide present are to be avoided. The presence of „fools gold“ has two effects.

If the iron pyrites content is high, it will oxidise when exposed to rain and sun, and bleed rust over the roof as unsightly stains. If the granules of pyrites are of sufficient size, the end of the oxidation process will leave holes in the roof that will leak. The presence of short and mid-term oxidising minerals is a common occurence in slate from Spain and China. When the pyrites and calcium minerals are jointly present, the weathering of slates leads to a roof of varying brown and grey colours. It is not uncommon for geologically younger slates to have both of these minerals present.

Often specifiers rely solely on the British Standard in issuing specifications. This may cause unintended problems, particularly with slates from Spain, Itlay or China. The reason for this is that the British standards test was specifically designed for stone that rerely contain excess pyrites or calcium carbonate, and hence the presence of these is not tested for. The German, French and American standards address this as they have historically needed to deal with varying quality local slates, and the frequent presence in these slates of inappropriate minerals.

For the architect „The belt and braces“ approach of requiring compliance with the ASTM (American Society For Testing Material), as well as the BS (British Standard) goes towards ensuring that any roofing slate selected will provide the longest possible life, and maintain planned appearance troughout the life of the roof.