Asbestos Cement is an extremely common roofing material for schools, apartment blocks and factories, as well as residential homes. Asbestos roofing sheets are made with asbestos fibres to reinforce thin rigid cement sheets. Made popular during the Second World War as a cheaper alternative to metal sheeting, asbestos cement sheeting was used extensively in the building industry until the the mid 1980's and in some case beyond.
Concerns about the health risks of asbestos have severely curtailed the use of asbestos in building materials worldwide since the 1990's.
Although no new asbestos roof sheeting is now being used, enormous quantities of existing asbestos roofing have now become a waiting ecological time-bomb.
Asbestos cement roofing surfaces undergo a weathering process after many years of exposure. A loose, friable surface layer develops and becomes colonised with dark coloured lichen, leading to the characteristic blotchy tone of asbestos roofs. The lichen attacks the cement causing exposure of the asbestos fibres. The surface becomes unstable, and the asbestos roof sheeting is weakened.
In addition, as Asbestos Cement roof sheets age and darken, the thermodynamic qualities that insulate the building spaces below the roof deteriorate. There is a substantial increase in roof cavity temperatures.
When exposed to weather and erosion elements, such as in roofs, the surface corrosion of asbestos cement can be a source of airborne toxic fibres. These invisible asbestos fibres are directly related to a number of life-threatening diseases including, asbestosis, pleural mesothelioma (lung) and peritoneal mesothelioma (abdomen).
Removing damaged asbestos roof sheets is possible under strict supervision and safety protocols. However, it would be expensive and impracticable on a very large scale when it comes to safe removal and disposal of large quantities of asbestos cement roofing sheets. Preventing the re-use of asbestos roof sheets that have been removed from roofs would also be very difficult.
The use of high-pressure water to clean asbestos roofs has been banned because of the likelihood that asbestos fibre would be released in this process and pollute the surrounds. Ordinary roof paint does not apply and stick well to the surface, and has a limited lifespan.
A more sensible approach is to replace only damaged roofs, and only the damaged sheet or sheets, and to maintain intact roofs through appropriate coating using sealing and specialist coating processes that limit the release of fibres without disturbing the asbestos cement surface. This is called asbestos encapsulation.
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