What are the different types of roof tile surface finishes?
Choosing a roof tile with the right surface finish is crucial to the finished appearance of the roof and how it will look in the future. It is important for roofing contractors to understand why manufacturers use certain surface finish treatments on clay and concrete tiles, and how this affects the aesthetics and performance of the product in the short and long term.
Clay tiles
The beauty of natural clay tiles lies in the way which they retain the natural colour of the clay material before maturing to a darker, richer shade. The degree to which this occurs and the rapidity with which any changes takes place depends upon two factors. The first is the nature of the tile itself, and the second is the environment in which it is laid.
Machine made tiles are extremely dense and durable. Accumulations of extraneous material are slow to take hold and the appearance of the tile is slow to change. The colour of the tile will not fade under any circumstances and the slow change that does take place will be a steady darkening of colour.
Due to the nature of the clay tile production process, slight colour variations are to be expected and actually in many cases enhance the look of the finished roof. We advise not to mix tiles from different batches on the roof and to achieve the best effect tiles should be taken from several pallets and mixed at random.
Some clay tile colours are achieved by applying an “engobe” to the surface of the tile prior to firing in the kiln. An engobe is a ceramic slip coating consisting of 45-90% clay, 10-40% Frits (glass) and 1-15% manganese oxides. The glass content produces a strong, vitrified scratch-resistant surface, whilst the manganese oxides give the required colour. The clay content means that when the tile is fired, the engobe and the tile ‘fuse’ together and become one, meaning that the coloured coating is as durable as the tile.
Natural Red
Mixed Tone
Engobe
Noble Engobe
Glazed
Concrete tiles
The beauty of natural clay tiles lies in the way which they retain the natural colour of the clay material before maturing to a darker, richer shade. The degree to which this occurs and the rapidity with which any changes takes place depends upon two factors. The first is the nature of the tile itself, and the second is the environment in which it is laid.
Machine made tiles are extremely dense and durable. Accumulations of extraneous material are slow to take hold and the appearance of the tile is slow to change. The colour of the tile will not fade under any circumstances and the slow change that does take place will be a steady darkening of colour.
Due to the nature of the clay tile production process, slight colour variations are to be expected and actually in many cases enhance the look of the finished roof. We advise not to mix tiles from different batches on the roof and to achieve the best effect tiles should be taken from several pallets and mixed at random.
Some clay tile colours are achieved by applying an “engobe” to the surface of the tile prior to firing in the kiln. An engobe is a ceramic slip coating consisting of 45-90% clay, 10-40% Frits (glass) and 1-15% manganese oxides. The glass content produces a strong, vitrified scratch-resistant surface, whilst the manganese oxides give the required colour. The clay content means that when the tile is fired, the engobe and the tile ‘fuse’ together and become one, meaning that the coloured coating is as durable as the tile.
Efflorescence and lime blows
Concrete products and mortar consist of sand, gravel, cement and water. The cement is produced by burning together, among other things, alumina and lime. Being natural raw materials, the composition of these substances vary, depending on their origin.
Although quite rare, efflorescence is a naturally occurring phenomenon caused by rainwater, condensation or dew penetrating into the pores of concrete. This carries calcium hydroxide, otherwise known as lime, to the outer surface. The water then evaporates, leaving a white film or bloom.
Lime naturally occurs in the cement which is used in the manufacture of all concrete products including roof tiles. Since the lime content of the concrete can vary and the weather conditions obviously differ, the level of efflorescence can also fluctuate considerably.
In the case of mortar bedding at ridges and hips, rainwater may wash the lime down the roof and deposit it on the roof tiles. Once deposited on the surface of the roof tile, the calcium hydroxide reacts with carbon dioxide in the air and becomes an insoluble calcium carbonate.
The process soon ceases as the outer pores of the mortar are closed by the insoluble carbonate. The same chemical process which brings the lime to the surface of a tile carries on, enabling it to be degraded and washed away by the rain, so that eventually the efflorescence disappears by itself – usually in a matter of months. Once the lime has disappeared from the surface of the tiles it rarely re-occurs. Rainwater is slightly acidic, therefore long term weathering will eventually remove the efflorescence, but it is difficult to predict how long this will take. If it is considered necessary to remove the efflorescence without waiting for natural weathering to take its course, then a suitable proprietary hydrochloric acid solution can be applied to the affected area.
Occasionally clay tiles may show small white-centred chips on their surface. These occur 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 expansion can usually be prevented by the manufacturer submerging the tiles in water.
Manufacturers make 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 cannot 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.
Conclusion
In summary, manufacturers take measures to make sure that their clay and concrete roof tiles have a protective and aesthetically pleasing surface finish so a new roof will look fantastic as soon as it is installed and for many years to come.
To see a true representation of a tile’s colour and surface finish, it is best to order a sample when possible. Manufacturers try their very best to recreate the exact colours of roof tiles in their brochures but can only be as accurate as the printing process allows.
