In these days of greater concern about climate change, and the use of energy which leads to it, some people will be surprised to learn that 50% of all energy use is caused directly by buildings. This means that addressing the issue of energy efficiency in buildings is now of paramount importance. Of the energy currently used by buildings, 90% is the energy consumed by the building and its occupants while the building is in use. Only 10% is the embodied energy, i.e. the energy used to manufacture or mine the building components, and the energy used during building. It is recognised that vast improvements can be made in building structures and insulation levels, possibly reducing energy use while occupied by around 75%. This would make the energy embodied in the fabric of the building of far greater significance.
So, where does the roof come in? The following is a brief list of the functions a roofing system can and should perform:
Shelter from the rain and rainwater collection for domestic use
Shade from the sun and UV protection
Skylights for daylighting deep within buildings
Surface for energy collection, solar hot water and photovoltaics
Roof form can be designed to minimise wind turbulence
Wind driven stack ventilation
Thermal and environmental barrier
Roof provides space for the most important insulation
Getting the design of the roof and the rest of the building envelope right will give the greatest benefits in terms of reduced energy use. But which roofing materials carry the least embodied energy? Which are better in terms of other pollution emissions and which are more likely to be recycled? Read on…..
First a little about the manufacturing processes of the various roof cladding materials.
Clay tiles are manufactured from kaolinite clay with various additives. The minerals are vitrified to bind the tiles at a high firing temperature in excess of 1100°C, the higher the temperature the longer the life of the tile. The high temperatures involved lead to a high embodied energy, see the table below.
Concrete tiles are made in the proportions of 1 : 4 Portland cement and aggregate. Mixing and chemical curing take place in temperature controlled conditions.
Natural slate was formed within the Earth’s crust many millions of years ago when mudstones were subjected to colossal heat and pressure, causing partial recrystalisation of the minerals during a process known as metamorphosis. These rocks are split along natural faults to produce the roofing slates.
Steel is produced from mined iron ore, coke and limestone melted together in a high temperature furnace. The coke turns to carbon monoxide reducing the iron oxide to iron. Approximately 25% scrap material is used in the final steel making process.
Aluminium is extracted from its bauxite ore by a highly energy intensive process, but outside the UK this process is often powered by hydroelectricity. Within the UK a large proportion of aluminium is from recycled feedstock, which gives the product a much lower embodied energy (see table).
Bitumen is the residual material produced after removal of all the volatile products from crude oil, such as petrol and diesel. Asphalt is manufactured from bitumen blended with limestone powder and fine limestone aggregate. Another source is lake asphalt, a naturally occurring material mainly imported from Trinidad. Asphalt tiles make up 75% of the US roofing market, but unfortunately these products have low durability.
Timber shingles are riven or sawn from suitable timber such as oak or western red cedar.
In the table below, the figures in the first two columns refer to a square metre of roof cladding material, thus making it easy to compare various materials.
Another important aspect of sustainability which is difficult to encapsulate in the above table is that of the pollution caused during manufacture and supply of the various materials. The Embodied Energy column gives a good indication of the relative quantities of carbon dioxide produced during manufacture, but says nothing of all the other more toxic pollutants. It is recommended that materials should be selected to perform the set task while minimising all impacts, including pollution.
Direct comparisons of the various pollution risks are difficult to make, but the following gives a rough indication of the kinds of pollutants involved.
Though the embodied energy is high, there are few toxicity issues involved in the manufacture of clay tiles. Concrete has a lower embodied energy, but greater toxicity in its emissions. The main pollution issues with steel roofing are the dioxins produced during smelting, and the pollutants produced in the manufacture of the usual PVC (Plastisol) coating. PVC manufacture involves considerable amounts of chlorine (a nerve gas) and results in the release of further dioxins when the steel is recycled. The main problems with aluminium are the very high energy costs when using virgin material, and the unnecessary use of polyester powder coatings to colour the sheets. Lead and copper both have a high degree of corrosion resistance, but their use for large areas of roofing can lead to a degree of contamination in the rainwater runoff. Copper is extracted from sulphide ores and the mining process is associated with pollution of waterways by heavy metals.
Bitumen and asphalt systems are renowned for their short product life. Due to their being very difficult to recycle, this leads to further use of precious oil reserves, unless an alternative replacement material is used. Both slate and shingles have no pollution implications apart from that involved in extraction and transport.
From the above it can be seen that the most sustainable options for roof cladding are timber shingles, closely followed by natural slate, as long as these materials are obtained from a local source (or at least within the country). After this, selection can be made on the basis of embodied energy, pollution and length of life. Consideration can also be given to recyclability, and it is likely that slates, concrete and clay tiles will be available second hand somewhere quite local.
Wooley, Kimmins, Harrison and Harrison, Green Building Handbook, E&FN Spon, 1997.
Most of the information in this article is based on ‘A Guide to Sustainable Roofing’ published by Redland Roofing Systems and researched for them by b consultants Ltd. The fact that Redland have published the guide says a lot about their concern for the environmental effects of the products they produce, especially when clay and concrete tiles are not shown by the guide to be the most sustainable option, except in terms of resources.