Archaeologists working on sites that include Roman Villas have found stone slates which are almost identical to those used today for the roofs of so many Cotswold houses. Unlike the factory tile, the stone slate is hand made. There are two types of limestone from which the slates are made, each type requiring a different method of production. One method is to extract the stone from near the surface of the ground and then, in the next few days, split it while it still contains natural moisture. This is the oldest method of obtaining Cotswold slate and was used by the Romans.
The other method is to mine blocks of limestone at considerable depth in the ground. The rough blocks are hoisted to the surface and laid out to become exposed to rain and then later to frost. The frost gradually splits the stone along its natural grain with the slaters assisting in the splitting process. The result is a slate that is much thinner and more regular than the earlier method. This is the slate seen on our finest buildings and was used extensively during the building of the Oxford colleges.
During their manufacture the nature of the stone means that it is impossible to supply them in consistent sizes, smaller slates are far more numerous than larger slates. Before the use of gutters became generally used, the roof was required to throw the rain water well clear of the walls, so to get maximum projection the largest stone slates were used along the eaves. This row or course is called 'cussoms', the next course above being called 'followers'. As the roof extends to the ridge so ever smaller courses are used, a useful way to accommodate all the different sizes of slate.
To prepare a building, wooden battens, or laths are fixed along the roof from which the slates will be hung. The distance between each batten decreases half an inch with each row, from the bottom upwards. This is because each row of slates is half an inch shorter than the one below it. The slates come from the quarry in batches by length, and there are thirty standard lengths, each with its own name. Starting from the top, the smallest are 'short pricks', then 'middle pricks', 'long pricks', 'short cuttings', 'middle cuttings', 'long cuttings'. Then 'monities', 'becks', 'bachelors', nines', wibbuts', and so on in longs, middles or shorts down to the 'follows' and 'cussoms' or 'eaves'. The slant of a stone roof is only 48º to 55º, against 65º for thatch. The slate will have a hole or perhaps two holes cut through near the top. The old way to cut the hole was for the slater to feel with his fingers for the thinnest point in the stone and then using a pointed hammer, carefully break through. Today drills are used. An oak peg is lightly driven into the hole until firm and then the pegged slate is hung over the batten. Stand in the Market Hall on Chipping Campden High Street and look up to the underside of the roof to see the slaters art at work. Something so simple and yet so skilful as never failed to amaze me. The finished roof is capped by a simple angled section of dressed stone. By looking carefully at the line of the roof, you will see that in the best examples it is not level but tilts up at the ends. It is not that the rest or the roof is sinking but that the end few slates are packed up to make the outer edge of each slate fit down tight so that the wind cannot get under it and blow it off. However, during building, the roof is intended to bow slightly under its immense load and this causes the slates to pack even more closely together offering an even more effective defence against the rain and snow.
Frost is the main cause of decay in slates, so if a roof is regularly scraped to remove the moisture bearing moss and with just the occasional replacement of a few slates then it could last as long as 300 years.
John James Hissey, when traveling in the Cotswolds in 1908 wrote:
The Cotswold type of house appeals to me on account of its honest construction, lastingness and simplicity. It is both built and roofed with stone; its walls are delightfully thick, so that the interior is warm in winter and cool in summer; its roof of thin split stones, sized down from the top to the eaves (the smallest being at the top), makes the loveliest covering possible to imagine, for these stone slates form a mosaic of many greys, ranging from cool to warm; nor are they laid with machine-like, monotonous regularity as are the blue slates or red tiles of a modern building, and their rough surfaces encourage the growth of gold and silver lichen, further enhancing their charm. The old builders understood the importance of a roof, and they took pains to make it beautiful, and they made it high pitched, the better to throw off the rain and the snow. A roof emphasises the shelter that a home gives to a man.