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To me that sounds like it could be a good roofer who would rather use synthetic for its reliable uniformity than a poor grade-but equivalent price point - natural product. I've never seen a sample of synthetic but I had a contractor use spanish slate instead of welsh. It was darker and pretty uniform but worked fine. Any decent supplier will do samples so worth getting some and having a look before committing to anything.
Currently in discussion with a roofer and trying to decide between natural and synthetic slate. He says that there's not much difference in terms of fitting complexity but that the difference is "the cost to yourself as for natural slate the good ones that don’t twist are 3 times more expensive."
Does that ring any bells with anyone? I'd assumed that the difference in slates from different origins was in terms of durability and colour, not in the flatness of the slates as supplied.