Applied exterior finishes

Ceramic tiles are now classified in BS EN 14411 and fall into 2 main categories according to their method of manufacture.

• Extruded tiles, which are shaped in the plastic state in an extruder and then cut into tiles of predetermined thickness and size.
• Dry-pressed tiles, which are formed of powder or small grains and shaped in moulds under high pressure before firing.

Ceramic tiles are further subdivided according to their water absorption (see BS EN 14411).

In themselves, ceramic tiles are virtually impervious, although the finish necessitates a larger proportion of joints than other materials, with the result that the weak point in the system is often the quality of the joint grouting. Tiles are generally very durable, so long as first quality materials were selected and used. If the quality was not specified during construction, there is a risk that second quality or 'commercial - run of kiln' tiles were used. (Wall technology, Volume G: Applied finishes, Special Publication SP 87/G, CIRIA, London, 1992.) These tiles may not be of the same consistent quality.

Most external tiles are glazed, that is they are finished with an impervious glassy layer that can be either matt or glossy in appearance. Tiles that have water absorption of less than 3% are described as vitrified. Porous body tiles are manufactured, but are not usually installed in external situations.

Ceramic tiles are inflexible. They cannot accommodate movements in the background and crack or spall if subjected to stress arising from expansion and contraction of the background or pressure arising from salt growth or frost action. Movement joints in the structure must be carried through to the outside face of tiling, and flexible joints are preferable along the line of dissimilar background materials where differential movements are possible.

Crazing of glazed tiles can occur, particularly where the tiles are incorrectly fixed, as a result of expansion, frost and soluble salts. Vitrified tiles do not usually deteriorate as a result of crazing, but in areas that are very exposed, and where the background material has a high sulphate content, failures can occur. Industrial areas with high atmospheric sulphur content can degrade pointing and grouting materials.

Panels of tesserae and ceramic tile finishes could be fixed in a variety of ways using either tile adhesive or sand and cement.

• In areas where a smooth uniform background was available, thin bed adhesives could be used (3mm thickness) but there was a risk of voids forming, which could collect water and freeze, disrupting individual tiles. Further, if the bed thickness were exceeded there was a serious risk that excessive shrinkage stresses could be created with consequent cracking of the surface finish.
• If the background was less accurately formed, thick bed adhesives could be used (3mm+), but again care was needed to avoid voids.

In both cases, the late fixing of tiles onto adhesive could also create adhesion failures that may not become manifest for several years.

Problems have been encountered with nylon backed tesserae panels used in external situations due to rotting of the nylon or difficulties with the adhesive spread on the back of the tiles. Therefore these types of tiles, when used in external situations, must be treated with caution.

The results of adhesive failure can be spotted easily and invariably by numbers of individual tiles scattered around the ground at the base of a building. Although individual tiles are light, when they fall from a great height the effect can be dangerous.