Zinc Runoff From Steel Roofs in NZ: What the Testing Shows

Steel roofing is durable, practical, and widely used across New Zealand. But like many building materials, it also has an environmental impact that is easy to overlook. One of the most important is zinc runoff into stormwater systems.

Testing commissioned by European Gutters and carried out with support from Ideaslab in Timaru looked at two important questions: how much zinc is washing off common steel roof types, and whether filtration can meaningfully reduce that contamination before it reaches waterways. The results offer a useful insight into how different roof materials perform over time and why maintenance still matters.

Why Zinc Runoff Matters

Under the Resource Management Act 1991, contaminants cannot be discharged into the stormwater system unless that discharge is authorised. That matters because zinc is a recognised stormwater contaminant with known impacts on water quality, aquatic ecosystems, and the wider marine environment.

This is not a new concern. Auckland Council's 2013 Proposed Unitary Plan identified zinc runoff as a priority contaminant and proposed tighter limits around runoff from steel roofs. For property owners, builders, and asset managers, the key issue is straightforward: even when a roof is performing as intended, small amounts of metal can still wash off over time and accumulate in the environment.

What the EPSock Field Tests Found

One set of field tests looked at runoff collected directly from different steel roof types, then compared it with samples passed through the EPSock filtration system. The reported average zinc concentrations were:

• Unpainted galvanised: 30.4 ppm runoff, reduced to 0.6 ppm after filtration, a 98% improvement.
• Painted galvanised: 11.0 ppm runoff, reduced to 0.4 ppm, a 96% improvement.
• Unpainted Zincalume: 8.3 ppm runoff, reduced to 0.2 ppm, a 97% improvement.
• Painted Zincalume: 1.7 ppm runoff, reduced to 0.2 ppm, an 88% improvement.

The practical takeaway is clear: the highest zinc runoff came from unpainted galvanised roofing, while painted Zincalume performed significantly better. The filtration results also suggest that source-control products can substantially reduce the amount of zinc entering stormwater, particularly where roof wash-down or exterior cleaning is already being managed.

That aligns with long-standing industry understanding that poorly maintained galvanised roofs are among the largest contributors of zinc runoff.

What the Corrosion Testing on Pre-Painted Steel Showed

A second programme focused on pre-painted steel roofing, including commonly used COLORSTEEL® products. The goal was not simply to see whether these roofs corroded, but to develop an accelerated testing method that could capture runoff contaminants at measurable stages of the coating's life.

The testing combined UV exposure, water simulation, temperature effects, and controlled electrochemical cleaning cycles. In simple terms, that allowed the lab to observe when the protective coating and metallic layers started to lose performance and what metals were entering the runoff as that happened.

Four colours were tested through to coating failure, including Titania, Gull Grey, Ironsand, and Karaka Green. The reported findings showed a consistent pattern:

• Zinc was present in the early cycles.
• Iron levels rose more sharply once the protective zinc layer was depleted.
• Visible discolouration and rusting became more apparent in later cycles.
• Failure points tended to align broadly with the expected maintenance or recoating window of around 14 to 16 years.

In other words, pre-painted steel roofs generally release far less zinc than neglected galvanised roofs, but they are not impact-free. Over the life of the roof, runoff still occurs, and the cumulative effect can become significant across entire neighbourhoods, industrial areas, or large roof portfolios.

What This Means for Property Owners

For most homeowners and commercial building owners, the lesson is not that steel roofing should be avoided. Modern steel roofs remain one of the best roofing solutions available in New Zealand. The lesson is that roof condition, coating life, and runoff management all matter.

If you own or manage a steel roof, the key points are:

• Older galvanised roofs deserve close attention because they are more likely to generate elevated zinc runoff.
• Pre-painted steel performs better, but it still contributes contaminants gradually over time.
• Maintenance affects environmental performance as well as roof life. Poorly maintained roofs are generally more vulnerable to accelerated corrosion and higher contaminant release.
• Large roof areas multiply the effect. Even relatively low concentrations can add up over decades on commercial, industrial, and multi-unit sites.

Can Zinc Runoff Be Reduced?

There is no single fix, but several practical steps can help reduce long-term runoff risk:

• Keep roofs, gutters, and downpipes maintained so corrosion issues are identified early.
• Replace heavily deteriorated galvanised roofs before severe corrosion develops.
• Select appropriate roofing products and coating systems for the property's environment, especially in coastal or industrial zones.
• Consider source-control measures where runoff quality is a concern, particularly on larger buildings or sites with sensitive stormwater discharge conditions.
• Make sure exterior cleaning and maintenance work does not allow detergents or contaminants to enter stormwater untreated.

Filtration products such as EPSock are one example of source control. According to the reported product information, EPSock uses a geotextile sock filled with mineral filter media including black diatomaceous earth, activated carbon, and attapulgite. The system was developed to capture contaminants from roof runoff and cleaning discharge and is designed for both temporary and more permanent applications.

The Bigger Picture

The most useful conclusion from both testing programmes is that zinc runoff is not simply a theoretical issue. It is measurable, repeatable, and influenced by roof material, coating condition, and ongoing maintenance.

Poorly maintained galvanised roofs remain the highest-risk category, but pre-painted steel also contributes to zinc contamination over time. That means good asset management is not just about stopping leaks and extending roof life. It also plays a role in reducing the long-term burden on stormwater systems and waterways.

Need Advice on an Ageing Steel Roof?

If your steel roof is showing signs of corrosion, coating wear, blocked gutters, or recurring maintenance issues, a professional inspection is the best place to start. Edwards & Hardy can assess your roof's condition, explain your repair or replacement options, and help you plan the right maintenance strategy for the long term. Contact our team to discuss your roof.