Voss roofing slates
 
Voss is the name for a type of roofing slate, a town properly known as Vossevangen, and a district located in Hordaland county, east of Bergen in Norway.
 
Slate has been used for roofing and walling stone in this region of Norway for centuries, and has been exported to the UK, Europe and even as far away as Japan. There is still a residual craft output of Voss slates, mainly for building restoration work in Norway. The quarries are located in the Upper Bergsdalsdekket, part of the geological Caledonian mountain formation which in Britain contains roof slate areas such as the pyrite-rich Ballachulish in Scotland.
 
The last known quarry of Voss Skiferbrud AS, established in 1895, closed in 2000 leaving a few self-employed slateworkers to continue the summer roof slate making.
 
The quarrying traditionally used explosives and cable saws to break the many known fissile slate beds into chunks small enough to work in the slate cabins. Here they are split and worked into the traditional fish-scale shape. Unlike most roof slate, Voss slates, which are harder and less fissile, were not holed but notched on each side and iron-nailed on the roofs - an obvious weak spot resulting in rust failure in the prevailing wet atmosphere in the fjord villages. These days galvanised nails are used.
 
The fish-scale slates or skiferhellene come in standard sizes of 8ins by 14ins (old Norwegian inches that is) which gives a coverage of 30.5 per square metre, and 10ins by 16ins (20.7psm). Non-standards are 7 x 13 (38psm), 6 x 12 (48psm) and 12 x 18 (15.2psm). They were not fish-scale to be entirely decorative, but trimming the relatively heavy slate down into the leaner fish-scale shape resulted in a considerable reduction in roof weight and hence a leaner wooden (normally softwood) roof structure. The slates were sorted for thickness, typically into three or four thicknesses, with the thickest slates at the bottom of the pitch and the thinnest at the top. The minimum recommended roof pitch is 22 degrees, presumably for the bigger sized slates.
 
The old Norwegian imperial measurement, tomme or tommemål, was roughly an inch, measured using the distance across the thumb at the root of the thumbnail. The derivation of the English word thumb is unknown, but perhaps it comes from the Vikings whose word for thumb and inch was almost identical.