Zinc Is Good
Zinc and roofing go hand in hand and have done so since the 1800's. Roofing products incorporating zinc have been used to roof the historic buildings of Paris to the garden shed in your back yard and for very good reason. Zinc is truly an amazing material. Zinc has the ability to protect less noble materials from rusting, such as the steel base on galvanised steel or where it's used in a pure zinc sheet application it protects itself.
Zinc sheet used in roofing in its purest form, allows a patina (zinc hydroxycarbonate) to form which is insoluble in rainwater, and thus significantly reduces the corrosion rate. The durability of Zinc can be affected by some acid pollutants, the main one being sulphur dioxide (S02). During the 1970's Europe acknowledged S02 pollution of the atmosphere as a major envionmrental problem and have taken the required steps to significantly reduce it. The reduction in the corrosion rate of Zinc roofing has been staggering up to one third. Fortunately in New Zealand, with the exception of geothermal areas, S02 is not a significant pollutant.
Pure zinc roofs in Europe currently have a life expectancy of up to hundred years with little or no maintenance.
Not only does zinc make sense to use in roofing it is also an essential element for all living organisms. Humans are unable to synthesise their requirement of zinc and need to consume zinc - up to 15mg per day for men to meet the World Health Organisation recommendations. Unfortunately in many developing countries there is a deficiency of zinc in the diet. As a result the World Health Organisation ranks this as the 5th health risk factor for developing countries and attributes 800,000 deaths worldwide to zinc deficiency.
Recently there has been information issued by the ARC over concerns of zinc build up in Auckland harbours. The ARC is concerned that the level of zinc, at selected sites within the Manukau and Waitemata, is higher than natural background conditions and is increasing. For every organism there is a range of optimum zinc concentrations. The ARC stated in the July 05 Storm publication we don't yet know the concentration of zinc at which cockles, for example, will start to disappear The ARC also acknowledge that the two common methods for measuring these effects suffer from large uncertainties and are working with NIWA to develop an Ecosystem Health Model to better understand the link between sediment contamination and the effects on animals.
In 2003 the ARC commissioned Kingett Mitchell to look at zinc run-off from roofing. Kingett Mitchell determined that the main source of zinc run-off, from roofing, was coming from unpainted galvanised roofs. In 2004, the MRM commissioned Tonkin and Taylor to conduct a similar study to understand the amount of zinc run-off from various types of metal based roofing. Both studies concluded, not surprisingly, that the highest level of zinc run-off comes from unpainted galvanised roofs (100% zinc over a steel base often referred to as galvanised iron). At the other end of the scale, also no surprise, was painted zinc/ aluminium coated steel. The big difference in the results was with unpainted zinc/ aluminium coated steel (43.5% zinc and 55% aluminium) where the Tonkin & Taylor result was significantly lower (ZINCALUME® is the most common brand of this material in New Zealand). The MRM study used controlled test rigs with over 500 samples taken over a period of 8 months. All the run-off samples were tested in an internationally accredited New Zealand laboratory.
In contrast, the ARC report involved only 5 samples. Even so the difference was surprising and on investigation it was discovered that one of the ARC samples was taken from a contaminated roof. The roof had cement spilt on it, which due to its high alkaline content reacts with the aluminium and accelerates zinc run-off. However, on the basis of these results the ARC issued a draft policy on roof run-off, without any public consultation and without following any of the formal processes required by the Resource Management Act. They concluded that painted roofs, due to their low zinc run-off, didn't require any water treatment, but that galvanised iron (their term) and unpainted ZINCALUME® did. The reason given for including ZINCALUME® with galvanised iron was due to the wide range of results - influenced by a contaminated roof!
The below results from the MRM test data clearly show that unpainted zinc/aluminium is more in line with results from painted roofs rather than unpainted galvanised roofing.
building owners, it has and will increasingly have a dramatic impact on the amount of zinc run-off from roofs. Remember it is not known at what level zinc in waterways goes from being a positive to a negative and in ARC's TP217 it indicates the biggest issue for Auckland harbours is actually sedimentation build up -Sediment run-off from land to sea is an
increasing threat to the Auckland Region, not only to inter-tidal flats, but to the sub-tidal coastal realm as well There is also the question of whether the zinc is bio available or not. All run-off from roofing is bio available, but this allows it to quickly form non-bio available zn-complexes with soils/sediments and is also diluted by large volumes of harbour water. Zinc from other sources is significant, such as car tyres that generate zinc oxide, which may directly accumulate as sediment in harbours/estuaries.
Zinc run-off from roofing is just one component in a broader urbanisation problem and with the positive environmental changes made over the last decade in metal roofing, it should be supported by the ARC not targeted, as the below graphs indicate. Based on ARC data, Auckland City stormwater catchment carries about 12800kg/year of zinc of which galvanised based roofs contribute 8200kg/year. In Figure 2, Scenario 1 shows the reduction in zinc from roofs if they were all replaced with ZINCALUME® Scenario 2 shows a more realistic result where the
galvanised roofs are replaced using a ratio of 2:1 for pre-painted ZINCALUME® to unpainted ZINCALUME® being the replacement material. This ratio reflects the portions sold in each product group in the current market. With no change in current practices zinc in the water ways, over time, can be reduced by 20 times - now that's GOOD.