Frequently asked questions: clay tiles

Cassius slate roof tiles
Useful answers to the most popular questions about clay tiles

Q. My clay tiles appear to have a sprayed on finish. Why is this and will this weather off? A. The beauty of natural red clay tiles lies in the way in which they retain the natural colour of the clay material, whilst maturing to a darker, richer shade. Colours such as Flanders, Tuscan and Antique are created by applying an 'engobe' to the surface of the tile prior to firing in the kiln. This is a ceramic material, as is the tile itself. Therefore when the tile is fired the engobe and the tile 'fuse' together and become one. This means that the surface colours of the tiles are permanent will not fade under any circumstances and the slow change that does take place will be a steady darkening of the red colour.   It should be noted that Wienerberger's Sandtoft brand surface-coated clay tiles easily pass the European Frost Resistance test, BS EN 539-2: 1998, with no damage to the tile or its surface.

Q. My clay tiles have small white-centred chips on their surface. What is causing this and will it affect the durability of the tiles? A. The small pits that are occasionally visible on the surface of clay products are created when pockets of lime immediately below the surface expand, causing the surface above the lime to be pushed up or 'blown'. This expansion takes place as soon as the tiles leave the kiln as the tiles absorb moisture from the atmosphere. Lime occurs naturally in most clays and can usually be neutralised, ie prevented from expanding, by submerging the tiles in water.   Wienerberger makes every effort to prevent lime blows, although occasionally the process can still occur before the tiles have been fully soaked. The expansion action of the lime only occurs immediately after the tiles leave the kiln. The process stops once the tiles have absorbed moisture and can not re-occur. Therefore there is no risk of further 'pitting' to the tile surface after the tiles have been laid on the roof. It is a common misconception that clay products can be attacked by frost action due to irregularities within the surface finish but there is no possibility that the small pits, or 'lime blows', will affect the future durability of the tiles.   All our clay tiles pass the European Standard test for frost resistance, BS EN 539-2: 1998. Under the terms of the European Standard for clay roofing tiles, BS EN 1304: 1998, small pits or chips 7 mm or less in size in the surface of the tiles are not regarded as faults.

Q. My new clay pantile roof looks wonderful but I am concerned about the gaps between the tiles. Will my roof leak? A. Clay pantiles have been around for about 400 years now and have long proved themselves to be an extremely efficient form of roof covering. In early methods of clay production there was very little control during firing over the final shape of a product. This meant that the design of a roof tile was such that it had to be weathertight whatever its final shape - the shape of a traditional pantile is a wonderful testimony to early roof tile designers. Not only do they keep the water out but the gaps around the tiles minimise the suction effect of the wind, helping to prevent wind damage. Because modern clay tiles are pressed, rather than extruded in the way that concrete tiles are, far more weatherproofing features, such as top interlocks and weather barriers, can be designed into the surface of the tiles.

Need help? Contact Sandtoft Technical Services on 0844 9395 999


Related articles