Except for individuals facets of surface surface which determine flatness; there arent any requirements for appearance in the standard. Each one of these; mostly colour and colour alters must be agreed in between supplier and specifier or buyer. Colour is your very difficult property to standardise because that it adjustment with time both as part of the quarry and just after setting up and it may look very unusual in their give and on that roofing.

Colour properties of roof slate