Origins Roof tiles Pretoria Cement-Based Roofing Products, Pretoria South Africa

Origins of Cement-Based Roofing Products

Cements featured in construction in ancient times, but were not used for making roof tiles until mass production of the materials started in the 19th century. The first cement-based tiles were made in 1844 at the Kroher cement factory in Staudach, Bavaria. Until 1920 they were normally cast as square products, which were laid on the roof in a diamond pattern. In the early days, pigmented coatings were applied to the surface of the tiles, but efflorescence and poor colour durability rapidly spoiled the appearance. As a result, uncoated tiles of plain cement were preferred and quickly became popular.

Extruded Concrete Roofing Tiles
Over the years, concrete tiles and their production processes have evolved to give the products more aesthetic appeal. The major breakthrough came in 1919 when the first extruded tiles were made in the UK. This new technology was soon commercialised in Australia, South Africa and the rest of Europe, allowing tiles to be mass-produced instead of being virtually hand-made. The new factories could make as many as 20 million tiles each year.

Improvements to Colourings, Coatings and Textures
Other improvements have followed including improved colourings, coatings and surface textures. These are now making it possible for concrete roof tiles to offer the look of more traditional roofing tiles, figure 1. Modern concrete tiles typically have the appearance of clay tiles or natural slates.

Concrete Roof Tiles and Other Roofing Materials
With the exception of materials such as thatch, it is now possible to imitate most traditional roofing materials with concrete. Control of the concrete’s properties and processing is vital to achieve the required surface texture and colour without compromising its functionality - that of thin interlocking plates designed to keep out water. The tiles must also conform to national standards for specific properties, including water permeability, freeze-thaw performance and strength. To achieve these performance requirements, concrete roof tiles are typically made using a mortar mix with a sand grading known as ‘sharp sand’. Iron oxide pigments are added to give the desired colours, providing the most cost effective way of making terracotta, black, brown and yellow tiles.

The Use of Pigments
It is important to achieve the desired colour using the minimum amount of pigment, as the pigment is usually the most expensive component in a concrete roof tile. The pigment must therefore be used efficiently, and so it is vital to ensure that it does not form agglomerates and that individual pigment particles are dispersed as widely as possible in the concrete mixture. A dispersion of iron oxide pigment particles - the high density white specs distributed throughout the mortar microstructure

A major enemy of bright colours is excessive efflorescence, the formation of a powdery deposit on the tiles. This arises from hydrated calcium hydroxide, which is present in hardened Portland cement. Calcium hydroxide seeps through the pores of the concrete to the surface of the tile, where it is carbonated by atmospheric carbon dioxide and forms calcium carbonate. A light dusting of calcium carbonate is not harmful, as it will be washed off by rain, leaving a pristine surface. However, a severe case of efflorescence will reduce the apparent brightness of the tiles and at worst could ruin the appearance of a roof.

Reducing Efflorescence
Efflorescence can occur not only in service on the roof but also during the manufacturing process or in packaging. Roof tile manufacturers therefore use a number of measures to eliminate its occurrence. It is a complex area, but most manufacturers simply aim to reduce the quantity of calcium hydroxide present in the hardened mortar, thereby reducing the risk of surface carbonation. If any efflorescence does then form, it can be easily removed in the production process by an acid washing treatment.

Another widespread technique is to protect the mortar from atmospheric carbon dioxide by applying a polymeric coating. Simple coatings can be formed using clear polymers or paints, but complex multi-layer coatings can also be produced that give a surface virtually indistinguishable from locally available clay tiles. Such surface texture is another important parameter in the manufacture of concrete roof tiles - for example, there is a market in the UK for flat concrete tiles that look like weathered, hand made clay tiles.

Life Expectancy and Design Criteria
Irrespective of the desired appearance, the functional performance of roof tiles is essential an must be maintained throughout a tile’s life. Guarantees of 60 years or longer are often demanded, and so concrete roof tiles must keep water out for many decades while withstanding a wide rang of weather conditions. Tiles are made by extruding the sand cement mortar onto a metal support pallet that gives the tiles their underside detail. The top surface of the tile is normally smooth, the profile being defined by the shape of the pallet and the extruder itself. This design is critical to maintaining the properties of the tile. Tiles are designed to minimise total weight while retaining enough tensile strength to withstand all handling stress The design must ensure that the tiles remain totally impermeable to water, and also has to take into account the need for adequate fixing, to ensure that the roof remains in place during event of the strongest storms.

Roof Tile Testing
Roof tile manufacturers carry out extensive durability testing to check the tiles will survive their long guarantees. Selected sites are used for natural weathering tests, the sites being chosen for evaluating tile performance in a range of different climatic conditions. Performance criteria including freeze-thaw properties, ultraviolet resistance, resistance to acid rain and resistance to algae growth. Laboratory testing is also canted out to explore the various degradation mechanisms that may occur in different countries where the tiles are sold.

Weathering Tests and Roof Tile Fixing
In addition, data is taken from the weathering test sites to provide information on local weather conditions. These conditions can then be reproduced in a wind tunnel to assess the performance of a whole roof made from the tiles, figure 4. This is very important as it allows the fixing of the tiles to be evaluated. Manufacturers can ensure that extreme winds and rain will not lead to roof tiles or even whole roof sections being blown off, and so guarantee the integrity of their roofing system.

The Future
Future developments in concrete roof tiles will certainly include additional improvements to traditional appearances. Concrete tiles will be made to look even more similar to clay tiles, slates and perhaps other traditional materials. Nevertheless, concrete tiles have been around for some time now. Perhaps it is time for the development of an exciting new generation of concrete roofing materials that offer their own unique appearance as well as improved properties and roof structures.

TopTiles Roof Tile

TOPTILES are manufactured using state of the art modernized technology in accordance with the requirements of the SABS 542-1990 standard specification being tested on a regular basis by the department. In conjunction with the roof tiles, TOPTILES manufacture concrete ridges suitable for both new and refurbishment projects that are designed to meet the requirements of the Building Regulations.

Roof tiles, Pretoria South Africa