The modern influx of new materials could lead to the loss of regional characteristics. However, one modern technology can be used - cements and concrete - while retaining local style as the materials can be made to match traditional systems.
Today, these materials are particularly widely used for the production of roofing systems that look similar to local materials such as slates, fired clay and terracotta. The versatility of these materials is useful in allowing local planning authorities to enforce standards that preserve the general character of an area while still conforming to modern standards of safety and convenience.
History of Cement and Concrete Roof Tiles
There has been a revolution in building efficiency and quality since the development of modern transport facilities. The wider range of materials and products that is available quickly and easily has led to buildings with improved insulation, ventilation and other properties. This in turn reduces heating and air-conditioning costs, which benefits the environment as well as making the building more cost-effective.
Not all of these widely available building materials are new, though. For example, the Myceneans were using pozzolanic cement technology around 3,000 BC. Cements were first used for making roof tiles until it could be mass-produced, in the 19th century, when the Kroher cement factory began to produce the first cement-based tiles in Bavaria.
The earliest tiles were cast in square shapes to be laid in a diamond pattern. 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 came to be preferred.
Over the years, new processes have emerged to increase the aesthetic appeal and durability of concrete tiles. The major breakthrough came in 1919 when the first extruded tiles were made in the UK. Spreading to Australia, South Africa and the rest of Europe, this process allowed tiles to be mass-produced, as many as 20 million tiles each year, instead of being virtually hand-made.
Improvements to Colorings, Coatings and Textures
Improvements in the colorings, coatings and surface textures of these commercially produced concrete and cement-based tiles make it possible for them to offer the look of the more traditional types of roof tiles, typically made from clay tiles or natural slates. A more impressive example of material mimicry is the concrete-based ‘cedar shake’-like tiles, which mimic the appearance of this very popular American roofing material but minus their flammability, which unfits them for houses in densely populated areas. Interlocking concrete tiles have been made using an extruded lightweight mortar, colored and textured to copy cedar shakes, easy to lay and weather-resistant.
Figure 1. A typical interlocking concrete roofing tile.
With the exception of materials such as thatch, it is now possible to imitate the appearance of most traditional roofing materials with concrete by controlling its processing to achieve such properties as the required surface texture and color while retaining its basic functionality – that of forming thin interlocking plates that will keep water out.
Figure 2. A traditional ‘cedar shake’ roof in north America made from concrete roof tiles.
Concrete Roof Tiles and Roofing Standards
Concrete tiles typically conform to national standards for specific properties such as water permeability, freeze-thaw performance and strength, by being made using a mortar mix with a sand grade known as ‘sharp sand’.
The Use of Pigments
Iron oxide pigments are added in exactly the right amounts to give the desired colors, providing the most cost-effective way of making terracotta-colored, black, brown and yellow tiles. It is important to avoid excessive pigment addition, since this is typically the most expensive component of the tile. Agglomeration of pigments must also be prevented, by dispersing individual pigment particles within the concrete mixture. Figure 3 shows how iron oxide pigment particles are dispersed in the mortar microstructure, being seen as high-density white specks.
Figure 3. Microstructure of a concrete roof tile. The white specks are pigment particles distributed throughout the system.
Efflorescence is a major hindrance to the achievement of bright colors. This refers to the formation of a powdery deposit on the surface of the tiles. This is hydrated calcium hydroxide, a chemical present in hardened Portland cement. It 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, severe efflorescence reduces the apparent brightness of the tiles and at worst could ruin the appearance of a roof.
Efflorescence can occur at any manufacturing stage or thereafter. Thus many measures are used to prevent its occurrence. The primary means of prevention lies in reducing the calcium hydroxide concentration in the hardened mortar, so that any residual efflorescence can be removed easily by acid washing.
Another common technique is to apply a protective polymer coating to protect the mortar from atmospheric carbon dioxide. Simple coatings can be formed using clear polymers or paints, but complex multi-layer coatings are also available. These impart a surface that makes these tiles virtually indistinguishable from locally available clay tiles. Such surface textures comprise 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, handmade clay tiles.
Life Expectancy and Design Criteria
Irrespective of the desired appearance, roof tiles must function impeccably and remain in good condition throughout their lifetime. This means that performance guarantees are often required for 60 years or even more. In other words, concrete roof tiles are expected to keep water out for many decades under a wide range of weather conditions.
To achieve this functional profile, sand-cement mortar is extruded onto a metal support pallet which is responsible for the detail on the underside of the tiles. The design of the tiles is crucial in their function: to minimize their total weight while retaining adequate tensile strength, remaining totally impermeable to water, and allowing durable installation to survive in situ even during the strongest storms.
Roof Tile Testing
Roof tile manufacturers carry out extensive durability testing to check that the tiles will last their expected lifetime. Tests to evaluate tile performance under natural weathering conditions in different climates are also performed. The tile criteria tested may include tile freeze-thaw properties, ultraviolet resistance, resistance to acid rain and resistance to algal growth. Laboratory testing helps elicit the different degradation mechanisms that may occur in different countries where the tiles are sold.
In addition, data is taken from the weathering test sites and reproduced in a wind tunnel to assess the performance of a whole roof made from the tiles, Figure 4. This is very important in fixing the standards for tile installation, ensuring that roof tiles or even whole roof sections will not be blown off by extreme winds and rain, thus guaranteeing the integrity of their roofing system.
Snow accumulation on concrete roof tiles should be considered as this may add to the already considerable weight of the roof, putting strain on the supports. Other potential problems with concrete tiles include moss and mildew accumulation in the spaces between tiles, which can be easily cleared off once a year using a pressure washer.
Important benefits of concrete roof tiles include:
- Extreme durability
- Wide range of colors, styles and shapes
- Insect resistance
- Increased crack resistance with newer polymer fiber-reinforced concrete tiles
- Lower installation costs compared to clay roof tiles
Figure 4. Wind tunnel testing of roof tiles, where a variety of wind/rain conditions can be simulated.
Future developments in concrete roof tiles will certainly include additional improvements to their appearance, to more closely resemble clay or slate tiles, or other traditional materials. In addition, perhaps an exciting new generation of concrete roofing materials will make its entrance, offering unique appearances as well as improved properties and roof structures.
Sources and Further Reading
This article was updated on 23rd May, 2019.