RIBA Core Curriculum
- Building conservation and heritage
- Design, construction and technology
- General Awareness
The history of English architecture is intertwined with the use of clay tiles with varying periods of popularity. This CPD looks at the roots of traditional heritage roofing and then moves forward to consider the modern challenges associated with heritage roofing and product selection. The course then highlights how designers can recognise potential issues and minimise the risk of damage associated with heritage roofing.
In this section
- Making roof tiles
- Types of materials and traditional areas
The use of clay for roofing was first introduced by the Romans, and the methods used today by true handmade clay manufacturers would still be instantly recognisable by them.
The major differences are the preparation of the clay prior to making and the firing of the made product.
After the departure of the Romans the use of clay declined until the medieval era in the thirteenth century.
A durable and safer alternative to thatch was sought and King John introduced bylaws to replace thatch in London with tiles of clay and slate.
The size of the plain clay tile was standardised in the fifteenth century by Edward IV and remains today at 10.5 x 6.5 inches (265 x 165mm).
It was not until the nineteenth century that peg tiles reverted to the Roman nib design. Traditional nibless peg tiles are still used, mainly in conservation sensitive areas of the southeast.
Between the two world wars a tremendous number of clay tiles were imported by barge into this country from Belgium and France. Many of these bright red clay tiles were inferior and under fired.
After the Second World War a failure of these under fired tiles as well as the legacy of roof repairs left by the blitz, laid the foundation for an unprecedented demand for sales of tiles.
Enter the Era of the concrete tile
In the early sixties the demands of new construction work for speed and economy led to the design of low pitch concrete roof tiles with relevant cost savings on brickwork and timber.
New towns like Basildon and Harlow, built to cope with London overspill, had the government of the day talking about the real possibility of 300,000 house starts a year.
A big refurbishment programme in the early seventies was prompted by the failure of slates due to nail sickness on the ribbon housing developments built in and around London and other major towns and cities at the turn of the century.
The Ronan Point disaster in East London, highlighted not only the structural unreliability of some concrete high rise developments, but the previously unknown social pressures, on those living in tower blocks.
Apart from the social implications, people genuinely wanted to see their houses looking like houses. After a spate of inner city tower blocks and industrialised housing, low-rise development became increasingly popular.
Shift towards conservation
Demand for handmade materials was static up until the late seventies/early eighties, but around this time emphasis on conservation and heritage led to a change in attitudes to building.
The restoration and preservation of older housing stock, rather than the replacement of old for new meant that natural building materials such as slates and tiles were required as authentic replacements to match existing roof coverings.
Many of these houses were fine old period buildings of historic significance, built with steeper pitched roofs of forty degrees or more using traditional handmade Plain tiles.
History has a habit of repeating itself and in the last few years the percentage of apartments has exceeded 50% of all new build, especially in the South East.
Making roof tiles
The manufacture of true hand made tiles begins with the careful selection of the clays. The rich reds of the Wealden clays give Keymer Tiles, the oldest known manufacturer of the clay roof tiles, their deep natural colour.
The natural colours of other clays such as Staffordshire blue and Cambridge gault are evident in their respective regions.
The various coloured clays are layered into a kerf and weathered for at least a year.
The clay is put through a milling process which involves a wet pan, two sets of high speed rolls, a double shaft mixer, an extruder and finally a wire cutter which produces a tile size piece of wet clay ready for the master tile maker.
The craftsmen use a process that has been passed down through generations for more that 400 years. The clay batt is sanded then pressed into a mould and the excess clay is wired off.
The sand acts as a lubricant enabling the tile to be released from the mould as well as giving the tile the correct sand finish and also carries the pigment to colour the tile.
A punch is brought down which forms the nibs and nail holes. The punch also impresses from a strike plate the company name, the mould used and the maker’s identification mark.
Clay reacts to pressure and in effect has a “memory” so that each tile reflects the individual maker’s movement when it passes through the ensuing drying and firing processes, making each tile unique.
A master tile maker will produce 1,500 standard size tiles each day, but less then half that number if making special tiles such as unders and overs, which require the clay to be cut to shape and formed over a wooden mould.
The tiles are placed on individual curved wooden trays, which give them the camber or curve during the drying process. They remain on the trays for 3 to 4 days during which time they are dried in drying chambers then held in a ‘warm store’ where they harden enough to be handled, they will also shrink by 7 per cent during this process.
The dried tiles are checked for quality before being set onto a kiln car. The tiles are set into ceramic U cassettes (44 tiles/cassette) which are placed onto the deck of the kiln cars. Each kiln car holds 3,696 tiles and there are six kiln cars in the kiln.
The temperature in the kiln is raised to 1050 degrees centigrade where it is held on “Soak”, it is then allowed to cool. This process takes 4 days during which the tile shrinks by a further 2%. When cool enough to be handled each individual tile is checked for quality by drawers before being placed into a crate ready to be transported to the customer.
Types of materials and traditional manufacturing areas
There are numerous manufacturers of roof tiles, ranging from the small one-man fittings shop to the multi national high volume producer.
The current pitched roof market is roughly made up of 66% concrete, 8% clay and 26% other materials such as natural and artificial slates and stone. This compares to most Continental European markets where clay is over 50%.
The pitched roof has remained popular because it sheds the worst of the inclement British weather quickly. Nothing is more popular that the ubiquitous plain tile.
The use of handmade clay plain tiles is principally found where the need to match the surroundings is paramount. This need is generally dictated by Planning Authorities constraints, which are most strictly laid down in conservation areas or where buildings are listed as being of historic interest.
The greatest concentration of demand is in areas where plain clay tiles were the traditional roofing material in the 18th century and before. In Cornwall, Wales and Scotland roofs were mostly slate, while down the east coast, especially in East Anglia, clay tiles but mostly pantiles were used.
South eastern and northern counties of England are the prime areas for the use of plain tiles and present demand is predominantly in a rough square bounded by Kent, Somerset, Cheshire and Lincolnshire plus a small pocket in North Lancashire.
Clay tiles are now produced in more than 50 colours. From deep reds, browns and oranges, the plum-coloured hues of heather, the muted blues of the Staffordshire tiles, and the buff coloured tiles of the Cambridge Mix, they range to almost black tiles, reminiscent of the “smut” glazed tiles characteristic of Norfork.
True natural burnt clay colours span a range from pure red to a flashed effect mixture of reds, browns and blues to a pure Staffordshire blue. Variations can be obtained by controlling the kiln atmosphere during firing. The iron content which occurs naturally in the clay undergoes a chemical change, to produce this wide spectrum of natural colours.
Whatever the style of a building, traditional or modern, clay roof tiles offer flexibility, choice and an option that will weather the storm and look beautiful for future years. If you have any queries regarding suitability, fixing pattern, minimum pitch etc the best person to contact is the manufacturer.
You’ve reached the end of the CPD. To make sure you’ve taken on board the key learnings of this course, please fill out the quick multiple choice Q&A below. This will certify that you have completed the CPD and provide you with an email certificate, which, if the course is accredited, you can share with RIBA.