Collyweston Stone Slate

Collyweston stone slate is a limestone roofing slate from the area around the village of Collyweston in Northamptonshire, England, near Stamford and the borders of Lincolnshire and Rutland. Despite the name it is not true slate but a thin-bedded limestone, considerably heavier than metamorphic slate. It is the traditional roof covering of many buildings in Northamptonshire, Cambridgeshire, Lincolnshire and Rutland.

Geology

The stone forms part of the Lower Lincolnshire Limestone Member of the Lincolnshire Limestone Formation, in the Inferior Oolite Group. It dates from the Bajocian stage of the Middle Jurassic, roughly 170 million years ago, and occurs in narrow beds mined from underground.

Frosting

Collyweston slate is famous for its traditional production method. Mined blocks were wetted, covered with earth and left outdoors so that winter frosts would expand the moisture in the stone and open its natural layers; traditionally the stone might be left out for up to three winters. Slaters then split the frosted blocks into thin slates and dressed them to shape.

Because natural frosting depends on cold winters, an artificial alternative was sought. In the late 1990s English Heritage (now Historic England), the Burghley Estate and Sheffield Hallam University developed an artificial frosting system that reproduces freeze-thaw cycles, and further trials to refine it were commissioned in 2012.

Revival

New Collyweston slates were produced in 2012 for re-roofing parts of Apethorpe Palace, and new slates have been used on Ufford church in Cambridgeshire and the Guildhall at High Wycombe. In 2015 planning permission was granted to reopen a Collyweston slate mine that had not been worked since the 1960s. Historic uses include Bodley's Court at King's College and Clare College, Cambridge.

Roof practice

Collyweston roofs were traditionally pegged over riven oak or chestnut battens and torched on the underside with lime mortar. Many have since been re-fixed with nails during repairs; on listed buildings, like-for-like pegging is generally expected. The Collyweston Stone Slaters Trust recommends cement-free lime mortar for repairs and advises that slates from barns or animal shelters may be unsuitable for houses because they can have absorbed salts and acids.