Zinc Runoff from Steel Roofs: What Property Owners Need to Know

Steel roofing is durable, practical, and widely used across New Zealand. But like many building materials, it also has an environmental impact. Zinc runoff from steel roofs is a recognised stormwater contaminant with known effects on water quality, aquatic ecosystems, and the wider marine environment.

Testing commissioned by European Gutters examined two important questions: how much zinc is washed from common steel roofing products, and whether filtration can meaningfully reduce that contamination before it reaches waterways. The results provide useful insights into how different steel roofing products perform and whether practical filtration measures can help reduce zinc entering the stormwater network.

Why Zinc Runoff Matters

The Auckland Council's 2013 Proposed Unitary Plan identified zinc runoff as a priority stormwater contaminant and proposed tighter controls on runoff from steel roofs. While these proposed controls were not ultimately adopted, the Plan highlighted zinc runoff as an important environmental issue requiring ongoing management.

The underlying issue is straightforward: even when a steel roof is performing as intended, small amounts of zinc can gradually wash from the surface during rainfall and accumulate in the environment over time.

What the EPSock Field Tests Found

The EPSock field tests collected runoff from different steel roof types before passing the water 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% reduction.
• Painted galvanised: 11.0 ppm runoff, reduced to 0.4 ppm, a 96% reduction.
• Unpainted Zincalume®: 8.3 ppm runoff, reduced to 0.2 ppm, a 97% reduction.
• Painted Zincalume®: 1.7 ppm runoff, reduced to 0.2 ppm, an 88% reduction.

While zinc runoff levels and filtration performance will vary depending on factors such as roof age, coating condition, rainfall intensity, environmental exposure including coastal or industrial locations, roof pitch, maintenance history, and other site-specific conditions, the field trial provides a useful indication of relative performance. The highest zinc runoff was recorded from unpainted galvanised roofing, while painted Zincalume® produced significantly lower runoff. The results also suggest that source-control filtration systems can significantly reduce the amount of dissolved zinc entering stormwater.

What This Means for Property Owners

For most homeowners and commercial building owners, the message is not that steel roofing should be avoided. Modern steel roofing remains one of the most durable and practical roofing solutions available in New Zealand. However, roof type, coating condition, and ongoing maintenance all influence long-term zinc runoff.

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

• Older galvanised roofs deserve closer attention because they are more likely to generate elevated zinc runoff.
• Pre-painted steel generally releases significantly lower levels of zinc than unpainted galvanised steel, although small amounts of zinc runoff can still occur over the life of the roof.
• Poorly maintained roofs are generally more susceptible to accelerated corrosion, which can shorten roof life and may increase the potential for zinc runoff.

Can Zinc Runoff Be Reduced?

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

• Keep roofs, gutters, and downpipes well maintained so corrosion issues are identified early.
• Replace heavily deteriorated galvanised roofs before severe corrosion develops.
• Select roofing products and coating systems appropriate for the property's environment, particularly in coastal or industrial locations.
• Consider source-control measures where runoff quality is a concern, particularly for larger buildings or sites with sensitive stormwater discharge requirements.
• Ensure 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 technology. 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 wastewater and is suitable for both temporary and more permanent applications.

The Bigger Picture

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 small amounts of zinc runoff over time. Good asset management is therefore not just about extending roof life and maintaining the appearance of the roof. It can also contribute to reducing long-term contaminant loads entering stormwater systems and, ultimately, New Zealand's waterways.

Need Advice on an Ageing Steel Roof?

If your steel roof or gutters are showing signs of corrosion, coating deterioration, or recurring maintenance issues, an inspection can help identify the most appropriate repair, maintenance, or replacement strategy. Edwards & Hardy can assess the condition of your roof, explain the available options, and help you develop a long-term maintenance plan that supports both roof performance and service life. Contact our team to discuss your roof.