So good that it has been transported across the world and has been the roofing material of preference on some Chinese pagodas.
If fixed down using wooden pegs, a Welsh slate roof can have a useful lifespan of around 400 years. Iron nails are not good as the deterioration in the nails over time also affects the slate. There is no roof more perfect in appearance than a Welsh slate roof, and if you ever go to North Wales, you will know exactly what I mean. 
Not just roofing, slate floor tiles have a quality all of their own.
Slate was mainly quarried, but in the case of the slate of Blaenau Ffestiniog, it was mined.
Was it easy work? About as easy as any quarry or mine can be. Open cast on the sides of the Dinorwig escarpments in the middle of winter or any time really would have been difficult on poor quality food, and many fell to their deaths because they no longer had the strength to hold on.
In the Llechwedd deep mines, explosions as the slate was blasted always presented obvious dangers, and like coal miners, the dust was unforgiving.
Quarrymen and miners were only paid for good workable slate blocks. There was always a considerable amount of waste that came away with the blocks, and it all had to be cleared manually and for no wages.
Weather in North Wales is changeable to say the least, in no small part due to its proximity with the Irish Sea, a particularly rough part of the North Atlantic Ocean. Blaenau Ffestiniog has been recorded as having more rain then anywhere else in England and Wales, and there are days in the area when it is impossible to see where the clouds stop and the mountains of waste begin.
If ever you find yourself in this part of the world, I strongly suggest that you do the Llechwedd tour, and while in Dinorwig a visit to the Miners Hospital. The records there will give you a very clear idea of just how tough it was. In the records, you will see the word ‘Marusmus’ in many of the entries. See here for details of the condition..
http://en.wikipedia.org/wiki/Marasmus
Don’t worry. It is not contagious and your visit will be a safe one. For more on Welsh slate, see here.. http://en.wikipedia.org/wiki/Slate_industry_in_Wales
.. and what you can still buy..http://www.welshslate.com/
Other mining in Wales..
There is a coal mine in Blaenavon, South Wales which has public tours. Here too is a real eye opener into the life of a miner.. see here..
http://en.wikipedia.org/wiki/Big_Pit
As nasty a job as mining or quarrying may seem to you, the miners and quarrymen of North and South Wales were all proud of what they did, and all will tell you that they would do it over again.
It would be good to end on a high note, but there is a scar on the face of coal mining in South Wales. The name of the village was Aberfan..
