As solar energy becomes more popular across South Africa, an unexpected issue is beginning to surface on many buildings: rust forming on metal roof sheets directly beneath solar panels.
At first glance, this can be mistaken for poor roof quality or aging materials. However, in most cases, the problem is caused by a combination of chemistry, water flow, and installation design – not the roof itself.
In this article, we unpack what’s really happening and how to prevent long-term damage.
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🔬 What Causes Rust Under Solar Panels?
The rust patterns typically appear as vertical streaks directly below panel edges or mounting points. This is not random, it’s a predictable and repeatable phenomenon driven by three main factors:
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⚡ 1. Galvanic Corrosion (Metal Reaction)
Solar panels are usually framed with aluminium, while most roofs are made from galvanized steel (zinc-coated).
When these two metals come into contact in the presence of moisture (rain, condensation, humidity), they form a galvanic cell – similar to a small battery.
👉 The result:
• The protective zinc layer on the roof begins to break down
• Corrosion accelerates in localized areas
• Rust starts forming on the steel beneath
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💧 2. Concentrated Water Runoff
Solar panels don’t allow water to fall evenly across the roof.
Instead:
• Water is channelled to specific drip points, especially at panel corners
• These areas experience repeated wetting and drying cycles
• Dirt, dust, and contaminants collect in these zones
👉 This creates highly aggressive micro-environments where coatings fail faster and corrosion begins.
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🧪 3. Contaminated Water (Electrolyte Effect)
Water running off panels often contains:
• Dust and airborne particles
• Bird droppings
• Industrial or coastal salts
This turns ordinary rainwater into a conductive electrolyte, which:
• Speeds up chemical reactions
• Intensifies corrosion rates
• Attacks protective coatings more aggressively
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⚠️ Why This Problem Is Increasing
With the rapid growth of solar installations, many roofs are now exposed to:
• Mixed metal systems (aluminium, steel, stainless steel)
• Poor or missing isolation between components
• Increased water concentration at specific points
👉 This means we are likely to see more widespread corrosion issues in the coming years.
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🛠️ How to Prevent Rust on Solar Roof Installations
The good news is that this issue can be effectively managed with the right approach.
✔️ 1. Protective Coating Systems
Applying a high-performance roof coating creates a barrier against moisture and chemical attack.
Look for coatings that offer:
• Excellent adhesion to galvanized steel
• UV resistance
• Flexibility and durability
• Corrosion resistance
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✔️ 2. Surface Treatment & Priming
Before coating:
• Treat any existing rust with a rust converter or inhibitor
• Apply a high-quality primer designed for metal roofs
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✔️ 3. Water Management
Where possible:
• Install drip deflectors or flashing beneath panel edges
• Reduce concentrated runoff zones
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✔️ 4. Proper Installation Practices
Ensure:
• Use of isolation materials (e.g., rubber pads, EPDM washers)
• Avoid direct metal-to-metal contact between dissimilar materials
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💡 A Smarter Approach to Roof Protection
At Aquagard Waterproofing Solutions, we’ve identified this as a growing issue across residential, commercial, and industrial roofs.
Rather than treating rust after it appears, we recommend a proactive protection strategy, especially for buildings with solar installations.
A properly designed coating system not only:
• Prevents corrosion
• Extends roof lifespan
• Improves overall performance
…but also protects your investment in solar infrastructure.
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📞 Need Help Assessing Your Roof?
If you’ve noticed rust forming beneath your solar panels, or want to prevent it before it starts, our team can assist with:
• Roof inspections
• Corrosion treatment
• High-performance coating systems
👉 Contact Aquagard Waterproofing Solutions today for expert advice and long-lasting protection.
