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The thickness of stone slates varies between geological sources and from quarry to quarry. The loading on a roof is also dependent on the range of sizes of a set of slates and the head lap used to lay the roof

Stone slates in common with other random slates are normally sold by weight with an indication of the area which can be typically covered by a given weight assuming a given head-lap - normally 75mm. To anyone unfamiliar with the system this is a confusing and cumbersome method of deciding how much slate they need. It is also a long standing problem. In 1695 the anonymous author of The Art of Building or an Introduction to Young Surveyors in Common Structures complained that 'their proportions in covering houses are uncertain, so I cannot assign a certain number, unless the builders shall prescribe a true proportion for the Slate'. 

It is difficult to be precise about coverage because there are many imponderables. Firstly the thickness of stone slates varies considerably. Then there is the mismatch between the number of slates of each length in a consignment and the width of the building. It is the roofers responsibility to make the mix of slates fit the shape of the roof but inevitably there will be too many slates of some lengths to complete a whole number of courses exactly with a consequent waste of slate and a reduction of coverage. Further, for technical reasons, the head lap must be altered whenever there is a change of slate length but the extent by which this reduces the coverage cannot be estimated without knowing how many courses there will be of each length range. 

MacDonald S et al 2003 English Heritage Transactions 9 p17.
The following weights in kilograms per square metre of roof give an idea of the loadings to expect. A square, the traditional measure of roof area, is 100 square feet. These values are indicative and it is very important to confirm them with your supplier.
  • Purbeck: I'm checking.
  • Forest Marble: 118 - 124 kg/sq m (1.1 - 1.25 tonnes per square) 
  • Fullers Earth - Eyeford Member: 137 kg/sq m (1.28 tonnes per square)
  • Cotswold pendle (frost split) slates Stonesfield, Taynton, Trougham:  86 - 95 kg/sq m (0.8 - 0.9 tonnes per square)
  • Collyweston (frost split): about 81 kg/sq m (0.75 tonnes per sq) 
  • Old Red Sandstone: 70 - 90 kg/sq m (1.29 - 1.66 tonnes per square)
  • Carboniferous (Pennine and Pennant) sandstones: 80 - 100 kg/ sq m (1.47 - 1.84 tonnes per square)
  • Horsham. Of  the Weight of this sort of Healing I have been informed by an observing Mechanick, that a Square of this kind of covering will weigh about 33 or 34 C.weight (Richard Neve 1726 The City And County Purchaser And Builder's Dictionary.) This is equal to about 180 kg/sq m (3.32 tonnes per square). This is presumably the loading of real double lap slating with, say, a three inch/75 mm head lap. For the system which is often used in Horsham today which approximates to single lap with metamorphic slate soakers you'll have to make your own adjustment.
  • Caithness Flags: 70 - 110 kg/sq m (1.47 - 2.03 tonnes per square)