Sustainability: Waste Reduction

By Graham Hepburn

Metal roofs have long been recognised as one of the best ways to keep buildings weather-tight but most people probably don’t realise they are a shining example of waste reduction.
From manufacture to installation, metal roofing has come a long way  – not only is it made more  efficiently but the process is cleaner with waste minimised at every turn.

The fact that steel and other roofing metals such as aluminium, copper and zinc can be recycled repeatedly with no loss of performance gives them an endless life cycle and means less energy is wasted converting raw materials into new metal products.
This constant recycling means that steel especially is not ending up in landfill like other waste or demolition building materials. The recovery rate of steel from buildings is estimated at 85% and a recent report on commercial construction waste found that more than 90% of steel was recycled.

Scrap metal is a valuable commodity to New Zealand’s two steelmakers, particularly Pacific Steel, a division of Fletcher Building, which makes all its steel from scrap.
New Zealand Steel, which makes 620,000 tonnes of steel a year at Glenbrook, has an average preconsumer recycled content of about 12% in its products. New Zealand Steel manufactures coil and sheet for use in building cladding and other industries. The coil may be metal coated with zinc – commonly known as galvanized steel – or a combination of aluminium/zinc alloy to produce ZINCALUME® steel. Since the introduction of ZINCALUME® steel nearly two decades ago nearly all new steel roofs use it as their substrate. ZINCALUME® steel uses less raw materials (Aluminium/Zinc Alloy – 150 gms/m2 ) than Galvanised (Zinc – 450 gms/ m2) whilst providing better durability.

Scrap metal is not only used to feed the kilns but also to control the temperature generated by the chemical reactions in the furnace. While recycling steel reduces the amount of materials being dumped in landfills, the process also saves an enormous amount of energy: recycled steel can be made by using as little as 25 per cent of the energy it takes to make virgin steel and that doesn’t take into account the knock-on effects of reductions in mining, transportation, and greenhouse gas emissions.
The American Institute of Architects estimates that each tonne of recycled steel saves 1100 kg of iron ore, 600 kg of coal, and 50 kg of limestone. The AIA also states that every kilogram of steel produced from recycled sources
rather than raw materials saves 12.5 MJ of energy, 86% less emissions to air are produced; 40% less water is used; and 97% less mining waste is created. Thus the relatively high embodied energy in steel made from virgin materials is significantly reduced globally by the universal high percentage use of scrap.
One of the problems with steel production used to be the piles of slag generated by the process but rather than slag being a nuisance “byproduct” it’s now looked on as a “coproduct” that is treated and then widely used for drainage, filtering and roading.
Water usage also used to be an issue in the steel making process but these days Glenbrook recycles the 1 million tonnes a day it uses so that only 1 % of cleaned wastewater is discharged. Technological advances have meant even the waste gases from the kilns used in the steel making process at Glenbrook are recycled in a cogeneration plant that produces up to 70 per cent of the electricity used on site.
It’s not just the steel-making process and the base coating ( ZINCALUME® steel ) that has become more efficient, but also the paint systems used to produce COLORSTEEL® and ColorCote® have too. They have far less by-products such as solvents as improved paint technologies have allowed the wide use of water borne paint systems that not only have less waste and are better for the environment but also last longer.
Metal roofing technology has become much more advanced over the decades, meaning that less steel is needed to make metal tiles or longrun roof material used in both residential and commercial applications.