A Guide to the Repair of Historic Roofs - Dublin City Council

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ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFS3. Cladding Materials and MethodsSlateTHE FORMATION OF SLATEThe word ‘slate’ comes from the Middle English ‘slat’ or‘sclate.’ It is related to the French word ‘esclater’, whichmeans to break into pieces and refers to the cleavingcharacteristics of the rock.Slate is a naturally formed metamorphic rockextracted or mined from discovered seams within theground. These seams were formed millions of yearsago by great pressure exerted on mud layers,containing key clay minerals, lying on the base ofshallow seas. During massive earth movements, areasof the sea bed were uplifted into mountains and themud, now covered by sedimentary shale, wassubjected to tremendous heat and directionalpressures causing the minerals present in the mudlayers to crystallise into rock layers. The re-orientationof clay minerals in parallel planes to the forces ofgreatest pressure is described as ‘fissility’, and theability to split along these planes is called ‘cleavage’. Itis this particular formation process that gives slate itschief characteristics and differentiates the materialfrom the thicker sandstone flags also used in Ireland asa roof covering.SOURCES OF IRISH SLATE‘……. slates are to be got in most places, on reasonableterms; all along the eastern sea coast, slate from Walescan be procured – and in the interior, both north andsouth, valuable quarries are accessible. All along the lineof the Shannon, and the channels connected with it, theKillaloe slates are procurable – and perhaps, not evenWales itself can produce a more lasting, or manageableslate than the Killaloe quarries can produce…’The Dublin Penny Journal (1833)Regional Irish slate possesses a wide range of coloursand textures, from the coarse and heavy Clare slate tothe smooth and light appearance of Valentia Islandpurple slate from County Kerry. Because of thesecharacteristics, as well as its rarity, Irish slate is particularlyworthy of identification, protection, and retention.Reports and published descriptions dating from thenineteenth century identify the location, operationand output of Irish slate quarries. The survey work ofthe Geological Survey of Ireland (GSI), compiled justafter the first Ordnance Survey, is an importantarchive. To date, 217 references to slate locations in 19counties have been established by the GSI. SamuelLewis’s Topographical Dictionary of Ireland (1837)identified and described the quality and location of atleast 122 sites for quarrying slate. County Cork wasconsidered to be the most significant producer ofslate, followed by counties Kerry, Tipperary, Clare,Wexford, Waterford, Kilkenny and Wicklow, with smallpockets identified in counties Mayo and Donegal.View of a worked slate quarry at Roshine, County Donegal27
ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSA selection of slate, both imported and native, indicating the wide variety of colour, texture and size available1 2 3 456111416 177 8 9 1012 13 15 18 191. Ahenny, counties Kilkenny/Tipperary2. Chinese Grey3. Delabore, Cornwall (double fixed historic sample)4. Glaslacken, County Wexford5. Penrhyn Heather, Wales (modern ‘Blue Bangors’)6. Weathering Green, Vermont, USA11. Canadian Heather Plum, Trinity Bay, Newfoundland, Canada12. Stone Quality Slate13. Westmorland Blue/grey, Cumbria14. Campo Grey, Spain15. Glaslacken, County Wexford16. Permanent Green, Vermont, USA7. Welsh Grey (Cwt-y-Bugail and Greaves Quarries, Wales)8. Stone Quality Slate (oak pegged with single head nailed)9. Corbally/Garrybeg, County Clare17. Broadford, County Clare18. Westmorland Green, Cumbria19. Delabole, Cornwall10. Killaloe, County Clare28
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A Guide to the Repair of Historic Roofs - Dublin City Council

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