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Stone slates are vary greatly in geology and appearance. In selecting a stone you will need to decide first which is the most important factor. If it is a   conservation project authenticity may be paramount. In this case you should try to find out which quarry supplied the existing roof but, failing this, the geology of the slates can be established. Of course in some regions it may be obvious where the original slates came from - on the Isle of Purbeck for example - but  you shouldn't be too quick to jump to conclusions. In Northamptonshire it is easy to assume that a stone slate is a Collyweston, but there were several other  sources in the past so further research may be required. Ultimately however, Collyweston might be the only one available today and therefore the most suitable. Elsewhere more than one geological formation may have been exploited in the past. In the Cotswolds region several geological units have been quarried so, for authenticity and appearance reasons, some care is needed when making a choice. This brings us to the second level of choice - if stone from the original quarry isn't available the nearest geologically similar stone may be the next best option. Care is needed here because stones from the same geological unit may have different textures and colours in different regions so these factors also have to be taken into account. Very importantly they may also not be available in the same mix of sizes, may weather differently or support different vegetation. For all of these reasons it is wise to view a roof at least five years old where appearance is critical. Do not forget that plant growth also depends on location (latitude, height above sea level, proximity to the coast) and orientation (north or south facing roof slopes).
Substituting one stone for another may have durability implications. The stone slates which are traditionally used in a region have been through a long process of selection which eliminated those which did not last. So a stone slate which is perfectly satisfactory where it is quarried may not perform so well in a more severe climate. It is also unwise to mix calcareous (limestone) slates with sandstones. The calcium carbonate in the limestone can be dissolved by rain water and run down onto other slates where it recrystalises as the roof dries out. If it is absorbed into a sandstone slate the recrystalisation will eventually cause flaking or splitting.
As well as the geological influences the appearance of a traditional stone slate is affected by the method of manufacture. The treatment of the edges, for example, varies between regions or even within a region. They may be square or bevelled and the bevels are sometimes cut left and right handed on opposite sides so that they overlap on the roof. The edges of limestone slates sometimes spall after installation so that a blunt bevel gradually becomes more tapered. The use of stone saws during manufacture can also affect the look of a slate. Flat sawn surfaces and edges destroy the character of a roof, but slates made in this way can be successfully tooled after sawing to give a traditional appearance. This is a subtle issue and conservation officers will be able to give advice about what is acceptable and about local, traditional styles. 
The appearance of stone slates
Most quarries that make stone slates also make a full range of other stone products. Because the fissile layers are only part of the rock which is quarried stone slates cannot always be supplied off the shelf. Most companies will try to hold stocks to supply orders quickly. But it is wise to check as far in advance as possible especially, if you need special sizes or colours. 
In the end you may find that the stone slate you need is simply unavailable at present. But this may not mean that all is lost. There have been several examples where a small quarry has been reopened just to supply a single roof. Local Authority Mineral Planning and Building Conservation Departments usually support such initiatives and help may also be available from the Stone Roofing Association and English Heritage.
The table below shows some of the stone slates you may come across: some very much less common than others but all are important to the correct conservation of historic buildings and the local distinctiveness of their region. Advice on selection and use of stone slates is given in the free English Heritage Stone Slate Technical Advice Note. Your local conservation officer will also be able to provide help.
Limestones including sandy limestones
  • Dorset: Jurassic - Purbeck, Forest Marble
  • Somerset: Jurassic - Forest Marble, Lias (Hamstone)
  • Wiltshire Gloucestershire, Oxford: Jurassic - Purbeck, Corallian, Forest Marble, Hampen Marly Beds, Trougham & Taynton Stone (Stonesfield Slate), Fullers Earth, Chipping Norton Limestone. 
  • East Midlands - Northamptonshire, Lincolnshire, Cambridgeshire, Leicestershire: Jurassic - Blisworth Limestone, Upper Estuarine Limestone, Chipping Norton Limestone, Lincolnshire Limestone (Collyweston Slate), Northampton Sand, Lias.
  • North-east Yorkshire: Jurassic - Scarborough (Brandsby) Limestone
  • Nottinghamshire to South Shields, East Derbyshire: Permian - Magnesian Limestone
  • Sussex: Creataceous Wealden - Cyrena (Charlwood) Limestone
  • Sandstones including calcareous sandstones
  • Surrey, Sussex, West Kent: Cretaceous Wealden: Horsham stone
  • South Wales, Forest of Dean, Bristol: Carboniferous Upper Westphalian - Pennant Sandstone
  • Welsh Marches - Gloucestershire, Herefordshire, Worcestershire, Shropshire: Devonian Old Red Sandstone - Dittonian, Downtonian Hereford stone tiles; Silurian - Pridoli:Tilestones; Ordovician: Hoar Edge Grit, Cheney Longville Flags
  • South Pennines including East Cheshire, North Staffordshire and North Derbyshire Lancashire, Yorkshire, Durham, Northumberland: Carboniferous - Lower Westphalian - Coal Measures &  Namurian - Millstone Grit
  • Cumbria: Permian: New Red Sandstone - Penrith Sandstone & Carboniferous Namurian - Millstone Grit
  • Tayside, Caithness: Devonian - Old Red Sandstone
  • Dumfries and Galloway: Permian - New Red Sandstone
  • Igneous
  • Shropshire: Intrusive - Corndon Hill dolerite
  • Metamorphic
  • Strathclyde: Ross of Mull: Mica-schist
  • Limestone slates
  • Sandstone slates
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