From rock to roof – ADC

Chris Collins, AccuRoof

This part of Spain has the largest tectonic slate reserves in the world. Slate is also quarried in Wales and in Cumbria and Cornwall, and there are other sources outside Europe, including China and Brazil, but about 90 per cent of slate supplied in Europe comes from Spain. It’s high-quality slate, which includes alternatives to Welsh and Scottish slates that have been discontinued or are prohibitively expensive, including some slates that are National Parks approved. This part of Galicia has quarries and mines. You tend to find that different people take to different environments, and rarely move between the two. The quarry looks appealing in this weather, but it’s a tough place to work in winter. The factory obviously has more consistent levels of comfort but you have to deal with the noise by wearing relevant safety equipment.

Brian Greathead, Manalo & White

Once you start to understand the whole process, you start to view the entire landscape and its settlements in a completely different way. So for example when you drive down from the mine you see the pine trees that are used to make the crates for the slate. You see piles of bark which become the veneer that’s placed between individual crates. It’s an entire ecosystem that has evolved around slate.

Aled Williams, AccuRoof

The settlements in this part of Spain have built up around the slate industry, and the services needed to support the industry and its workforce. People work for a particular slate company for their entire adult life. This whole region has grown up around the slate industry, and they are extremely proud of what they do. It’s really important to keep coming over here and maintain the relationships. They’re not just going to sell their product to the highest bidder. It’s more than that. It’s integral to their culture. A whole way of life.

Brian Greathead, Manalo & White

I was surprised that so much of the process is still carried out by people rather than machines. When I asked the factory manager about it his response was, “I wish I only had human beings.” I asked if that was for sentimental reasons and he said “No. It’s strictly business. Machines don’t need holidays. They don’t need days off. They don’t get sick. But they also don’t know how to split slate. They can just about do the job if the slate is really clean and really pure. But when it comes to using their judgement, they can’t compete with human beings. If I could get rid of all the machines I would.”

Kaye Stout, Pollard Thomas Edwards

You imagine slates to be really fragile but then you see these women throwing them around and slapping them down onto the benches, then knocking them into the crates with these enormous mallets. It shows just how strong slate is. It’s also really strenuous manual labour. You think of quality control as being about paperwork and box-ticking and then you watch it in action and it’s a really physical job.

Gavin Eyles, CSK Architects

It’s mesmerising to watch the women sorting and grading the slate in the factory. You would assume that you could tell a bad slate from the way it looks, but there is also an acoustic dimension to the process of quality assurance. By tapping the slates and listening to the sounds they can check for faults that are impossible to see.

Chris Collins, AccuRoof

Slates sold for roofing in the UK are tested to British Standard BS EN 12326:2014 ‘Slate and stone for discontinuous roofing and external cladding. Specifications for slate and carbonate slate’. That means that the slate has been tested for three key characteristics: water absorption, thermal cycle, and sulphur dioxide exposure. We only supply the best performing slates, but the appearance varies from quarry to quarry, so if you want a consistency you need to specify exactly where the slate comes from and be confident about the traceability of the product. We track each step the material makes, from the quarry to installation. Every crate is labelled with the selection and number, together with the Declaration of Performance and CE Marking. Get those right and you’re halfway there.

Aled Williams, AccuRoof

Any quarry will sort and grade its slates into different grades. They are looking at thickness, flatness, evenness and any flaws. You’ll get a consistent appearance for any particular grade from a specific quarry. The degree of wastage varies from quarry to quarry, but, as an average, only 9% of extracted slate makes it to market. It sounds bad, but it’s a mark of how seriously we take quality control. Also, you have to think about what waste actually means. It’s an emotive word that implies something harmful, or something in the wrong place. But this is none of those things. We haven’t added anything to it. The entire process is chemical-waste-free. It’s a good, pure, unsullied, natural material sitting in the landscape it comes from. Any unused material goes back into the earth. The mined galleries are refilled with spoil from the new ones, and quarried areas are landscaped and restored.

Hannah Wilson, Piercy&Company

The amount of wastage is partly to do with our preconceptions around quality. It’s like fruit and vegetables in the supermarket. A lot of perfectly good produce never makes it to the shelves because it’s deemed to be the wrong shape or size. There’s an assumption that we need consistent aesthetic, but there would be a lot less waste if we just accepted that slate is a natural material and that a degree of inconsistency goes hand-in-hand with that. We get used to a certain visual aesthetic so it feels like a really wasteful product, but it could be less so if we want to accept it’s a natural material with the variation that implies. The fact that you can literally cut a tile with a pair of scissors to any size or shape you want makes me think we should be more playful about the way we use it. The cloud-shaped slates were beautiful. There’s no reason why you shouldn’t be able to order a particular sized tile and a template of any shape you want.

Miles Elliott, Groupwork

There must be a way of putting the discarded slate to productive use. Seeing how slate is extracted and processed for roofing has illuminated exciting possibilities for how that natural material could also be cut and used for structural purposes. Large slate blocks unsuitable for splitting into tiles were stacked on top of each other to create retaining walls in the local landscape, demonstrating the material’s huge load-bearing capacity and the creative opportunities for reducing and reusing industrial waste.

Aled Williams, AccuRoof

We are working towards a solution. In the meantime if someone wants to come along in a truck and take away the piles of unwanted slate they’re more than welcome. There are all kinds of things they could do with it. They could do bespoke small-scale projects. There is a cottage industry like this in the UK; it could also happen in Spain.

Aled Williams, AccuRoof

The British Standard Code of Practice (BS 5534) specifies two ways of installing slates: nails, which can be aluminium or copper, or hooks, which are stainless steel. In the UK, we tend to use nails. Copper nails are the strongest, but also the most expensive; but 92 per cent of slates on the Continent are fixed by hooks. Hooks allow you to go for a lower pitch, but I prefer nails. The nails are covered by the slate above, whereas the hook fixings are on show. To me, you see the hooks and then the slates, but using a thicker slate, the hook is visually absorbed into the chamfer of the slate. It’s a matter of personal choice. In extremely windy areas people use hooks and nails.

Laura Walton, Morrow + Lorraine

The hook system is really interesting from the point of view of disassembly and reuse. You can literally just dismantle the roof and use the slates again. There are no holes in the slates, which presumably makes them stronger. If there is an aesthetic issue, then maybe there’s a way to redesign the hooks to make them more of a feature, but actually I think there’s something very pure and appealing about the simplicity of the system and having the fixings so clearly on show.

Carl Gulland, John Pardey Architects

Slate is an incredible material for roofing: beautiful, versatile, very low water absorption, inert to environmental degradation and it comes with a material warranty of up to 100 years – travelling through Galicia we saw many slate roofs that must have easily been far older. But a 100 year warranty is of limited relevance to a housebuilder who only needs 25 years and can source ‘man-made’ slates at less than half the cost. It was interesting to hear that the planners in areas traditionally associated with slate – Wales, Scotland the South West – are far more insistent on real slate for it’s longevity and colour fastness. As a natural quarried material slate is undoubtedly less carbon-hungry than other roofing materials. It can also readily be reclaimed at the end of a building’s life to adorn a roof elsewhere. Perhaps because the environmental credentials of slate appear self-evident there has been limited focus on producing formal EPDs (Environmental Product Declarations) but as every aspect of a building’s impact is put under increasing scrutiny, empirical evidence for the credentials of slate would further support its specification.

Aled Williams, AccuRoof

The slate industry in Spain is currently working to collate data on the carbon footprint of their slate when supplied into the UK. It may not be the cheapest option but it’s going to last for hundreds of years and it’s always going to maintain its appearance. To an untrained eye, fibre- cement or concrete or artificial slate may look as good when they’re first installed but you’ll be able to tell the difference in a few years’ time. There are some parts of the UK where the planners insist on natural slate because it weathers better in extreme environments. But 90 per cent of slate in the UK goes to house builders, who may not be geared up to take the long-term view. The slate industry should probably be focusing its efforts on educating planning authorities about the lasting benefits of natural slate.