Hansa Systems has recently been offering slate roofs for sale in addition to their tile roofs. Which set me to wonder-wouldn't slate roofs be quite expensive, if durable? I thought one might still suit a public building or expensive home.

Expensive for sure, and durable. I roof part time with a friend and when we re-roof a slate one, we save the good removed slate ( with no scale, flake) and resell it at $3 to $4 a piece per slate shingle. It is quit a job to roof with it and most are going with demensionall shingles now adays. If you see a house getting slate shingles, you know there is money there.

During a trip to Wales I was visiting a former slate mine near Mt Snowdon that showed a film called „they stole a mountain" and in the old days when labour was cheap slate roofs where more common. And it was cheap well into the 20th century. And in Europe the alternates are burned clay and straw. Clay was local until the 19th century due to the transport efforts (fuel,clay etc)

Yes, that's the point. For exemple slate roofs are very common in Liguria and Piedmont (northern Italy) because is quite easy finding and extracting this material in that regions, so this constructive solution was a lot cheaper than transporting clay tiles from other zones. Furthermore you can see slate roofs in some Italian mountain regions. In this case trasporting clay tiles wasn't just too expensive...Considering the buildings position it was almost impossible...

Slate roofs are also very common in the Ardennes and Luxembourg.
Until today all the roofs are covered with slate roofs. Now they are imported from somewhere else, but there have been extracted in the region before.

In Martelange (on the B and L border) they started a museum not so long ago

some pictures here, the direct link to the museum is not worth it as nothing is online

http://www.industrie.lu/

type "ardoise " in the search field and choose the first or second result for a lot of pictures.

Hint for everyone

Simply surf a while through this site. It is an extraordinary collection of old industries in Luxembourg. Even without understanding to much, you will find a lot of interesting pics.
If someone needs translation, please ask.

Here is a picture taken by me recently during a walk in the northern Lux region (the very south, my region, is dominated by iron ore)

Notice the slate in the ground!
Here is a partial pic of the church of Clervaux


Cheers
Claude

Like all building materials, slate is most common where it's found. It's actually quite cheap to make as a roofing material. So long as you have the right type of stone, you can quarry it readily and it can be split and trimmed by machine. Large areas of Wales, Cornwall, Cumbria and Leicestershire have slate roofs on all sorts of buildings, including cow sheds. It's hardly reserved for the top end.

I don't know about the rest of Europe, but in the UK slate roofs (and brick walls) became much more common all arond the country when the railway network was developed. That made transportation much cheaper - and smoother than the roads, to minimise breakages. It's telling that slate spread much more than clay tiles did: proof that slate was the cheaper option once you could transport it easily.

The reason prices are now higher than they ought to be is mainly a shortage of supply. There's only so much good slate and a great deal of it is already on roofs, leaving less than there used to be for new buildings. This is why reclaimed slates find a market, the same as reclaimed bricks (that, and people think they look more "authentic").