Cotswold_Stone_Slates

Introduction

Natural stone roofing provides much of the  special character of many parts of the country, and this is especially so in the Cotswold Hills. Although, elsewhere in the country, many local traditions have been lost, in the Cotswolds this has not been the case. Stone slate roofing can still be found in the Borough along the Cotswold scarpement, from Snowshill in the north, to Great Witcombe in
the south. However, this long-established situation could be under threat unless the production of stone slates, and the craft of laying them, is studied and revived. Even the name seems to be disputed: often called stone slates they are clearly not a metamorphic slate (such as Welsh slate) and yet neither are they tiles, in the sense of a clay moulded object.

Types of Cotswold Stone Slate

There are, geologically, two stones from which Cotswold stone slates are made, both of which are oolitic limestones: ‘Forest Marble’ and ‘Stonesfield Slate’. The methods of producing slates from these stones are quite different, making the most of their individual properties. Forest Marble is split by hand very shortly after being extracted from near the surface of the ground, usually at a small quarry. At one Cotswold quarry where slates were, until very recently, made by this method, it was thought that they should only be split within a few days of being extracted, while they still
retained their natural moisture, or ‘quarry sap’. Such stone slates are called ‘presents’. This is the oldest method of producing Cotswold slates, probably dating from the Roman period.

Stonesfield Slate is the name generally given to frost-split stone slates, originally produced near the village of Stonesfield in West Oxfordshire. Because of its depth in the ground, the method of extraction of the slate was quite different to ‘presents’. The stone in rough block form called ‘pendle’ was hoisted to the surface from stone mines, and put out in nearby fields to become ‘frosted’. As the frost gradually split the stone along the thin natural bedding planes, these being a consequence of the geological formation of the material, slaters would work to assist the splitting process. The
resulting slate was much thinner and more regular than the rougher ‘presents’, and was highly prized for the most prestigious roofs, such as those of Oxford colleges. The thinner slates also afforded an opportunity for some sophistication, such as an angled dressing of the sides to provide a very slight overlap when they were laid.

Slate Sizes

The first and most obvious feature about Cotswold stone slates is that, as a material produced from fissile sedimentary rock, it is impossible to supply them in consistent sizes, unlike welsh slate, a metamorphic rock, the stone breaks naturally to provide far more smaller-sized slates than large ones. Over the years, a logical way of taking advantage of this geological accident has evolved; the practice of laying slates in diminishing courses. One of the main functions of a stone roof is to throw water well clear of the wall. Before the introduction of gutters it was even more important to project the eaves as much as possible. So the largest slates (normally about 600 mm long, but some up to 750 mm long) were fixed here in a double eaves course. These slates were given a special name ‘cussoms’, the next course being called ‘followers’. The smallest-sized slates (called ‘short cocks’) were the most common, available in large numbers and used at the highest part of the roofslope. They were only 150 mm long.

 

Cotswold_Stone_Slates_Details

Method of Fixing

Fixing of slates was achieved by one, or sometimes two, wooden pegs driven through a hole near the top of the slate. In the past, this hole was originally made by the slater finding, by feeling with his fingers, a thinner point in the slate and carefully, with a pointed hammer, breaking through to form an hour-glass shaped hole. Today, drills are used. Usually oak was cleft to form a peg and this was driven into the hole until it was firm, the pegged slates then being hung over the batten. Ridges were always made from a simple angled section of dressed stone. Sometimes, on the more elaborate buildings, mostly churches, a roll- topped stone ridge was used.  Stone slate roofs are very heavy (about 1000 kg per 10 sq m) and an effect of this weight is often a slight bend in the roof. However, this causes the slates to pack more closely together, and it is now thought that roofs were made deliberately to allow this bow, keeping out the rain and snow much more effectively.

 

Conservation and Replacement
There are many stories about the durability of Cotswold stone roofs, and recent concerns over the quality of the ‘presents’ now quarried, and their resistance to frost attack, the principal agent of decay in stone roofs. Historically, Cotswold stone roofs were well maintained, and kept free of moisture holding moss. Scraping roofs was a common maintenance task, although now it is mostly discontinued, partly on the grounds of expense. This is a false economy – many roofs now needing near to total replacement, where very few new slates would have been required if scraping
had been carried out. If a stone roof was scraped once a year it could last twice as long. Many roofs being replaced on the oldest buildings are thought to be original. So, with regular maintenance, a cotswold stone roof can easily last 100 years, and could last as long as 200 or 300 years. Where stone slates have decayed, the question of replacement arises. A common situation is to find that about 50% of the slates have perished, so new material has to be imported. While second-hand slate is commonly used, this practice has resulted in the stripping of many other old buildings, cannibalising their slates for the refurbishment market. Recently, serious attempts have been made to discourage this process, and to promote the use of new stone slates. Not only does such an
approach save many old buildings from having their roofs pillaged for supplies (occasionally through theft), but it also promotes the development of stone slate production.
Until recently, there were only two quarries in the Cotswolds producing stone slates. Today, the situation is greatly improved with five Cotswold quarries currently in production.

This Article “Cotswold Stone Slate Roofing” was kindly sourced from www.stoneroof.org.uk