Where have all the Scotch Slates gone?

Posted by on May 3, 2012 in Building Surveying

Where have all the Scotch Slates gone?

By Ian Macleod
Specification Manager for Sheffield Insulation Group (SIG), specialising in natural slate

Do you have a traditional building with a worn out Scotch slate roof laid to diminishing courses? Do you want to maintain the building’s traditional character either because it is Listed and you have to, or because you do not wish to downgrade its appearance and ultimately its value? If you answer yes to these questions your options are limited and becoming ever more limited.

You can try to re-use the slates. You will probably lose in excess of 50% in the process. Your roofer will need to find a supply of matching second hand slates to make up the difference and this is becoming increasingly difficult and the chances are that you will end up with a roof that is, frankly, a bit substandard because the smaller slates will be too small and a proportion of the slates will be not quite as good as you’d want them to be.

You could go for slates from the North of England Burlington slate company, either Burlington Blues or Westmorland Greens or perhaps their Brandy Crag grey slates. They are wonderful slates and the first two are widely used in Scotland but they are produced as larger, thinner double nailed slates that do not match the smaller, random width and generally thicker, single nailed Scotch slates. There are other cheaper options like Spanish and Chinese slate which are generally a far from satisfactory answer in terms of appearance and/or durability.

However the choice that is a really convincing visual match to the traditional Scotch slate is a new slate (if anything 500 million years old can be called “new”) now being produced by Greaves Portmadoc called SIGA 110. This slate meets a quality standard that you have no chance of achieving with second hand slate.

How have we got into this situation? The last remaining Scotch slate quarry in Scotland ceased production in 1955. It had been out-competed by the Welsh quarries and cheaper man made products. From then on there were no indigenous slates to re-roof buildings in Scotland. To retain the Scottish look it was necessary to strip slates from old buildings. So old buildings were cannibalised for the repair of Listed buildings: not a very satisfactory conservation equation and the wastage factor meant that two or three roofs were required for every re-roofing project.

Attempts to re-open a Scottish slate quarry have not, so far, been successful and the economics do not look promising.

“Scotch” slates as quarried in Scotland were of random width (between about 4″ and 12″ wide but mostly 6″ to 8″ however they came “off the block”. In length they varied from 10″ to 16″. They varied in thickness too. They were single head nailed with every three or four courses being double or “cheek” nailed. But actually the Welsh quarries often produced smaller slates as a byproduct of their main operation, sometimes all the same size, but suited to Scottish use. These might be described as “Small Scottish heavy slates”. Creating an even and durable Scotch slated roof with all the slates pleasingly arranged in diminishing courses is a skill that has not so far been codified and the British Standard 5534 is largely irrelevant and misleading.

The Portmadoc slate is a very good colour and texture match to the West Highland slates that we think of as being the classic Scotch slate as can be seen in the photo above. The cost of new slates is to an extent mitigated by the reduction in time taken to sort through, trim and size second hand slates. For the building owner the assurance that a reslated roof will endure for generations to come is also worth including in the cost equation.

GLM Case Study

St Serf’s Chapel Culross was in a dilapidated state after many years of neglect and vandalism. GLM set out to reuse the existing Aberfoyle-type slate. The existing slates turned out to be largely unusable. The Portmadoc slates were substituted and provided a very pleasing result. The roofer, A Thorburn Ltd, found that the new slates good to work with and was able to make a neat job of the curved chancel end.