What are random slates? The usual definition is roofing slates or stones in a range of lengths and widths.  In European Standard (EN12326-1) terminology they are slates with undefined dimensions. But that’s only half the story and misses some of their most important characteristics.

This is a roof of random slates

This isn’t. It’s a mixture of tally slates with some slate and a half slates to use with backers

Welsh slate (Cambrian age)              Cornish slate

                                 Collyweston limestone slate

Carboniferous sandstone                Swithland slate

                                        Collie Head slate Scotland

Historically random slates were made from roughly shaped blocks of stone just as the were obtained in the quarry.  If the blocks were very large they might be reduced in size by hitting them with large wooden mallets or by chiseling a grove along the surface and then hitting them.  This produced rivings like the Carboniferous sandstone from Alston quarry above.  They were then ‘squared’ by choosing one side to be the tail and dressing the two adjacent sides roughly perpendicular to form the long edges which will be vertical on the roof.  The long edges will usually be quite wavy or may both taper slightly towards the slate’s head.  Also they are not ‘squared’ all the way to the top and the head may be very rough or not dressed at all.  Instead the slates are left shouldered but this is not done so that the shoulder is a specific size or angle.  Rather the long edges are removed until the size of the shoulder(s) is small enough so that they would not allow leaks when laid on the roof. 

This is not a precise process because whether or not the shoulder would leak also depends on how the slate is arranged with its neighbours on the roof.  If the shoulder is too severe or the perpendicular joint in the overlying course is off-set too much towards the shoulder water will be able to get through.


Features of a random roof

Dressing the tail and sides of Collyweston slates

Most modern slates are made by diamond sawing the slate block and dressing to a rectangle.  In the European Standard for roofing slates there are tight controls on the straightness of the slate’s sides and deviations from a true rectangle. (There are no similar controls or a product standard for stone slates.)  However shouldering of rectangular slates is permitted provided “it does not adversely affect the performance of the roof with respect to wind uplift resistance and driving rain”.  So some manufacturers shoulder their slates to reduce the weight on the roof and to help the slates lay tightly with their neighbours.  The size of the shoulder for slates is typically no more than 25 % of the length and the width at each top corner which was the limit of the old BS680 Roofing Slates.

Some quarries will supply a mix of tally slates with some extra-wide slates included for use with backers.  This do not produce an authentic random roof.

Many stone slates are cut to a rectangle after splitting but are often deliberately shouldered. 

Typical shouldering of a modern Cumbrian slate

Each quarry would have its own size ranges and because these are a reflection of the geology of the slate or stone there is a regional element to random roofs.  Large sizes are common in the north of England (Pennine sandstones) and the south-west (Cornish rag slates), Sussex (Horsham stone), and in the north of Scotland (Caithness and Orkney flags). 

Smaller sizes have a very wide distribution covering all the slate (Scotland, Ireland, Wales, Cornwall, Swithland, Cumbria) and limestone regions (Purbeck, Cotswold and Magnesian limestone types and Collywestons). 

Schists, shales and sandstones were used in middling to small sizes.  Many examples can be seen in Scotland, Ireland and Wales and along the border with England.

Some quarries make up so called random mixes using stock sizes of tally slates plus some extra wide slates.  These don’t produce an authentic roof because usually there are too few widths.  Ideally the widths should be just as they arise as they are made but a full set in one inch steps is acceptable.

There is a further problem with widths.  When BS680 was published in 1944 it stipulated that the minimum widths of slates should be half the length.  This was based on the assumption that the slates would be centre nailed which, if the nail holes are one inch in from the sides, reduces the effective width by two inches.  But for top fixed slates, the effective width is the actual width of the slates so slates narrower than half their length are perfectly suitable for roofing.  BS5534 provides a method for calculating the minimum width for head fixed slates in relation to the slate length, roof pitch and the driving rain exposure of the building.  So when ordering top fixed random slates, to ensure an authentic appearance, narrower slates should be ordered.  This is not a problem with stone slates because they are still made in traditional sizes.

* Other sizes by arrangement

Until the nineteenth century all quarries made slates true mixture of random sizesThe mix would have covered a range of lengths and widths just as they arose in the quarry and would reflect the properties of the rock from which they were made.  So the size range would vary regionally and geologically and is what created regional roofing distinctiveness.  For some examples the range may even be diagnostic of a quarry. 

However as quarries and slate making technology developed, especially in north Wales, manufacturers started to supply slates in defined lengths and random slates in length ranges.  Randoms can be as long as 56 and as short as 4 inches. The sizes used by producers are the overall length but for top fixed slates the effective length is less because the dimension above the peg hole is ignored.  It’s usually about one inch.  This is important when calculating how many slates will be needed for a roof.  Producers will usually give an indication of the area of roof which can be covered with a tonne of slates for a given head lap but these values cannot be accurate because roofs vary so much.

Random roofs in Britain and Ireland

The colours of random slates are more variable than tally slates from the major producers.  Many show iron oxide staining and are known as rustics in the slate trade.  Usually the stains are the result of rusty water seeping through the slate beds but can also be caused by pyrite within the slates.

Collie Head rustic slate, Troup Head,

Gardenstown, Aberdeenshire >

Image courtesy of British Geological Survey GeoSenic >

Making truly random slates at Pans de Travassac in central France.

Les pans de Travassac >

Names of Yorkshire (Pennine) grey slates

Traditional names for stone slate sizes.

The numbers are not dimensions and the names in different regions are not equivalent in length.