Slate – or Slates?

St Andrew’s Church is not covered with slate!

Aerial view of roof looking east (C. Wright)

The material called slate is a metamorphic rock.  It comes from a volcanic source, where it has been subjected to great heat and immense pressure.  This tends to form plates which can be split apart into thin sheets.  Typically, these are a dark blue-black colour and were used in school, in conjunction with another rock – chalk.

‘Slates’ is a term for any thin plates of stone, used for covering a roof.  In some areas, slate slates are common.  A builder will call them Blue Slates.

St Andrew’s is covered with Grey Slates – and these are made from sandstone.  This is a sedimentary rock, formed from successive compressed layers of sand and clay.  Just like slate, sandstone can be knapped into thin sheets (though not as thinly) and used for roofing and other purposes.

Knapping slates is a highly skilled task (believe me – I’ve tried!) and few, if any, quarries in Yorkshire can supply new slates.  Most re-roofing happens with reclaimed stone.  Indian quarries can still supply material but, as a Grade I listed building, I would hazard a guess that we’ll not be able to use that.

Chris Wright

Article from ‘The Bridge’ Summer 2017