Welsh slate makes the perfect roofing material. It cleaves readily into thin sections, absorbs hardly any water and reacts with practically nothing. The same properties also make it ideal for any number of construction uses.

The slate from Penrhyn and Cwt-y-Bugail quarries in north Wales makes such good roofing tiles that Welsh//Slate, the company that makes the tiles from the slate, is prepared to guarantee them for 100 years.

There is no doubt it will keep the rain out for that long because slate taken from the quarries in Victorian times continues to do so all over the world, including in Wales itself, which can be pretty damp and gets its fair share of snow and ice in Snowdonia, where the slate comes from.

The same properties that make it so resilient a roof covering also make it ideal as a building stone and it is also sold as walling, paving, worktops and flooring. Welsh slate is even used as table settings. It makes fine plates and bowls – and they are even dishwasher safe.

But undoubtedly when most people think of slate they think of roofing. It has even become the generic term for roofing tiles, even when they are made of other materials.

And if you want Welsh roofing slate, the chances are you will buy it from Welsh//Slate because it produces about three times more roofing from its two quarries than the other remaining Welsh slate producers put together.

Most of the slate Welsh//Slate extracts comes from Penrhyn, which is where the company has its headquarters. The slate there is from the Cambrian, laid down about 500million years ago, whereas the slate from Cwt-y-Bugail is a little bit younger. It is from the Ordovician, which is the geological age that followed the Cambrian.

The older slate has a greater history of use in service and therefore provenance, however, the slate from Cwt-y-Bugail does split into thinner sections for roofing and is available in larger format for architectural purposes. Nevertheless, Welsh//Slate produces 25-30,000 Cwt-y-Bugail roofing slates a week and three-to-four times that number at Penrhyn.

Slate is a metamorphic rock. It started as a sediment washed into the sea where it eventually became a mudstone. Over the eons the mudstone was transformed by heat and pressure within the earth into slate. It is the transformation that gives it its cleaving planes, which bear no relationship to the orientation of the bedding planes formed when the sediments were deposited.

It is a dense, siliceous stone. It has the same issues of RCS dust as sandstone, granite or engineered quartz, which Welsh//Slate addresses through dust suppression and PPE. It also conducts regular health checks on all its employees.

Roofing slate has traditionally been cleaved by hand. And still is – there are 30 or so people splitting slate at Penrhyn and Cwt-y-Bugail. But machines are making their way into the Welsh//Slate factory.

There is an automatic splitting line and seven automatic dressers working alongside the people. All slates produced, whether by machine or man, are carefully checked and graded prior to packing so the quality is consistent for the customer, but whereas a man (they are all men) can produce 800-1,100 slates a day, the machine, which was installed in September 2013, can produce as many as 3,000-3,500 on a good day – a good day being when nothing, such as an inconsistency in the slate, interrupts production.

All roofing slate offcuts and rejects, from man or machine, are crushed and become part of the aggregates production business that comprises the largest volume of material arising from the operation.

It is usually about half the rock that comes out of a dimension stone quarry that ends up as a finished masonry product. With slate, it is about 5%. In years gone by most of the rest ended up discarded down the mountain sides, producing tips that, ironically, are now protected as industrial monuments and which serve as bunds to hide today’s quarrying activities.

These days, half of the million tonnes of rock that Welsh//Slate takes from Penrhyn each year is crushed, which, thanks to a decision by the European Union this year, is now once again exempt from Aggregate Tax. The other half is overburden that remains in the quarry as backfill.

Just about all the stone that leaves the quarry is turned into sellable product, although only a little over 20,000 tonnes of it ends up as roofing, 40% of which is exported. Another 5,000 tonnes goes into other architectural products.

Chris Allwood, the Managing Director of Welsh//Slate, describes the operation now as “right sized”. He says in the past there might have been a tendency to be over ambitious. “Now we do what our resources can accommodate and supply stone to masons and stone processors for them to do their bit. That, for me, is really the essence of it: realising what you’re good at.”

As far as a contribution to revenue is concerned, 65% of Welsh//Slate’s £21million turnover comes from roofing, 30% from aggregate and the other 5% from various architectural products. Pre-tax profit is about 10%.

Chris Allwood says his role since taking over from Alan Smith in 2012 has been to continue Alan’s good work. He would always give priority to producing the highest value products – the roofing and architectural stone.

As Chris says: “Our ethos is to produce the best quality product we can from the rock extracted. Our best product is roofing slate. We would never give priority to the production of low value construction products. It’s not good stewardship of a very scarce resource.” Even with the crushed products, priority is always given to higher value garden products, pathways and parking lots before it is sold for fill.

Inevitably, perhaps, the people working at Welsh//Slate have seen the introduction of more machinery in the factory as a threat, but Chris says: “The proof of the pudding is in the eating – our workforce hasn’t significantly changed.”

The machinery is still being evaluated, but it is commonly used in Spain, which produces most of the roofing slate used in the UK. The Spanish slate is not as hard as the Welsh slate, which makes automating there easier. In Wales, the machines have had to be adapted and the resulting hybrids are expensive.

Nevertheless, Chris says the option of investing in more machinery is under constant review. “You have to address things like dust and repetitive strain injury, so you have to embrace automation. But even with the machines you need someone with splitting skills who will understand how to process the stone; who knows by eye what will happen when you make the hammer blow.”

Welsh//Slate is a significant employer in this part of rural Wales, with approximately 200 people working at the two quarries and in the associated factories and office. The majority are in Penrhyn, with around 30 at Cwt-y-Bugail extracting the stone and running a line producing worktops, flooring, cills and other bespoke items.

Because the company is a major employer and contributes significantly to the economy of the area, as well as producing a world famous product that is iconically Welsh, it has received considerable support from politicians of all parties in Wales, from local councillors up to the Welsh Assembly. They have all enjoyed photo opportunities at Penrhyn and supported the quarry against the imposition of the aggregates levy.

“It’s very encouraging to know you have that support,” says Chris, whose background since leaving university has been in quarrying, with Steetley, Tarmac and Aggregate Industries, before moving on to steel fabrication with Kingspan and Arcelor Mittal.

Chris: “When Alan was moving on I was interviewed for the job at Welsh//Slate and accepted within about 10 minutes.”

It is not just politicians who like Welsh//Slate, either. It also enjoys the support of many local people – and not just those who work there. The company has an established liaison group with the local population to maintain an active dialogue with them. And in part of the quarry, it has allowed another company to establish what is advertised as the longest zipwire ride in Europe, which attracts a lot of visitors to the area.

The current planning permission for Penrhyn runs until 2032 and, says Chris: “I think its important that we proceed as the best neighbours we can be, so if we wish to continue quarrying after that date we can proceed with our application with full understanding in all quarters.”

Chris emphasises that slate is a traditional part of Welsh life. So much so that there are moves being made to turn Welsh slate into a UNESCO World Heritage Site, as was reported in this magazine in October.

Welsh//Slate was initially cautious about the idea. “It makes you think it’s over; that it’s from a bygone age,” says Chris. “I was at pains to make sure everyone knew slate in Wales still had a life and made a great contribution to the economy.”

And it is not just the skills to produce the slate that are important but also the skills to lay it on a roof, because if those skills disappear Welsh//Slate will not be able to sell its roofing.

“It’s key we have a working quarry within the World Heritage Site to perpetuate these skills and retain an industry to be proud of,” says Chris, who is starting to believe that the UNESCO listing might even contribute to that aim.