One of the most significant pollutants of the Ōpāwaho Heathcote River is dissolved zinc the source of which is mostly stormwater running off our roofs.  You can help reduce the level of dissolved zinc in the river; just paint your roof.

All stormwater eventually enters a waterway and travels to the sea; that’s the way gravity works. When water runs off zinc-plated steel, it dissolves and removes minute amounts of zinc and this travels in the stormwater as it flows through the waterways.  This zinc dissolved in the stormwater is, unfortunately, toxic to macrobiotic life in the river.  Macrobiotic life is just a fancy name for the bugs and larvae that live in the gravels on the river bottom. Fish feed on the bugs and larvae.  Dissolved zinc reduces their ability to reproduce and to just survive. The greater the proliferation of macrobiotic life, the healthier the river.

It used to be that corrugated iron only came in one form – zinc-plated.  That is, 100% zinc plating on the iron. It’s the zinc that makes the iron silver and shiny and which prevents the iron from rusting.  Nowadays, they make a better zinc coating which has the brand name Zincalume®. This is made up of 43.5% zinc, 55% aluminium and 1.5% silicon. Zincalume® roofs shed up to 90% less dissolved zinc than the old-style zinc-plated steel.  Zincalume® is what we see particularly on large industrial buildings – it looks like a light grey metal.

Most residential buildings with steel roofs use a different finish called Colorsteel® which is Zincalume® with a couple of layers of paint over it.  It can also be fashioned into shapes that make it look like tiles. When new, stormwater running off this material contains very little, if any, dissolved zinc.

So, how do we reduce zinc in our waterways?  

If your roof and gutters are fairly new and/or made of Colorsteel®, you’re all good.  

But if your roof is showing signs of the paint wearing or flaking or if your roof is covered in Zincalume® or if your gutters are rusting, then you have two options to reduce the amount of dissolved zinc that your building is almost certainly adding to the local waterway: replace your roof cover with something other than Zincalume®, or just paint your roof!