Clay Tile Roofing
Compared to other roofing materials, clay tiles have a number of advantages. Clay tile have small pores and can therefore absorb and store considerably more water than other materials. On the other hand clay dries very quickly. Clay tiles roofs will be dry again shortly after a rainfall, hence growth of moss and lichens is rare.
Clay tiles are a natural product and therefore available only in a limited selection of colours. During the manufacturing process the clay tiles are fired at temperatures of up to 1000 degrees Celsius, resulting in hard and robust tiles. The manufacturing process uses comparatively large amounts of energy due to high firing temperature, however due to their durability, clay tiles are overall still energy efficient.
Clay does not weather and lasts more than 50 years, exceeding the durability of concrete and asphalt shingles which last only 15 to 25 years on average.
Glazed clay tiles are visually appealing and their appearance will not change for decades. The glaze inhibits the growth of greenish films, moss and lichens.
Green roofs have a number of advantages over conventional roofs.
- look attractive
- are valuable biotopes in the city
- improve air quality
- retain rain water to some extent
Green roofs also have advantages with regard to building engineering:
- balancing temperature fluctuations (cooling in summer, warming in winter)
- prolonging the life of the roof
The roof pitch is crucial for building a green roof. A slope between 5 and 15% is ideal, since it does not require any building alterations to prevent the substrate from sliding off, and no special drainage to prevent stagnant moisture.
Roof cladding and prevention of root penetration
To permanently protect the roof structure beneath the green roof from exposure to weather a drainage layer and a barrier against root penetration must be installed. A simple asphalt layer is not sufficient, since it can be penetrated by some types of roots. Additional protection must be added with asphalt compatible plastic foils. Special PE-foils or polyolefin covered fabrics are suitable protective layers. Those materials are ecologically less problematic than commonly used PVC. However, they cannot or only with difficulty (PE foil) be connected at the abutting surfaces and must be installed with generous overlapping.
The substrate serves as root penetration layer for the vegetation, as a nutrient and water store, and provides water drainage. To achieve this, top soil is mixed with light mineral matter (pumice, lava, broken expanded clay, expanded slate or recycled porous bricks). On slanted roofs without drainage the percentage of top soil should decrease from top to bottom, so that the drainage effect of the substrate increases towards the edge of the roof. It should be noted that the top soil for extensive green roof systems should not be too nutrient rich. When the clay/silt percentage is over 20%, sand should be added to decrease the nutrient content. The thickness of the substrate depends on the vegetation and the slope of the roof. The substrate on an extensive green roof system with a slope of 5% is usually 15 cm thick.
Roofs pose an extreme environment for vegetation. The plants are often exposed to high winds, and depending on the thickness of the substrate and direction of the roof they may also suffer stress from dryness. To most efficiently use the structural and ecological advantages of a green roof the vegetation should be as thick as possible. This can be achieved in the most cost effective and easiest way with a mixture of wild grasses and herbs, with a substrate thickness of 12 – 15cm. Moss or sedum should be used if weight is an issue and allows only for a thin layer of substrate (3-8cm). Some sedum varieties are resistant to dryness and can survive for some time on a completely dry substrate.
Combining Photovoltaic Systems and Green Roofs
Photovoltaic systems and green roofs can easily be combined.
By installing a photovoltaic system on a green roof the efficiency of the system can be increased up to 50% during the summer.
When an operating temperature of 25 degrees Celsius is exceeded the efficiency of a solar system is reduced by 0.5 per cent with each additional degree. In contrast to brick, gravel or metal roofs, plants reflect little sun light, so that the photovoltaic system does not heat up as much. The difference in temperature between green roofs and regular roofs can be up to 40 degrees, which equals an increase in efficiency of 20 per cent. Unlike a conventional installation that requires penetration of the roof structure, installation on the vegetation of a green roof does not cause damage to the roof cladding.
In addition to the positive effects for the generation of electricity, the vegetation on a green roof provides a habitat for small animals, protects against weathering, binds dust, and filters air pollutants. Green roofs act as a buffer to protect the house from heat and cold and are therefore an excellent addition to an energy efficiency upgrade.