How to correctly combine metals on a roof: Preventing dangerous contact corrosion between zinc, copper, and aluminum
*The image was AI-generated for illustrative purposes.
Anyone installing a metal roof, cladding facades or planning roof drainage can now choose from a wide range of materials. Titanium zinc, copper, aluminum or stainless steel – each material has its own visual and technical advantages. But beware: blindly combining these metals on a job site can, under certain circumstances, create a ticking time bomb.
The term to know is contact corrosion (or galvanic corrosion). When the wrong metals come into contact, they can corrode each other in no time. Pitting and costly repairs are the result.
In this guide, we at Roofify explain the notorious flow rule and show you how to combine metals on a roof without any nasty surprises.
How does contact corrosion occur? (Quick & easy to understand)
Every metal has what is known as an “electrochemical potential.” Metals are classified as less noble (such as zinc or aluminum) or noble (such as copper or stainless steel).
When two different metals are connected directly (or water flows from one to the other) and moisture (such as rain or condensation) acts as an electrolyte between them, this is what happens: a tiny electric current flows. The more noble metal attacks and breaks down the less noble metal. The less noble metal essentially “sacrifices” itself.
The golden rule for roofers: Follow the flow rule!
There’s an unwritten rule on roofs that every apprentice must know by heart: the flow rule.
💧 The flow rule states: Water must never flow from a more noble metal onto a less noble metal. In the direction of water flow, the more noble metal must therefore come after the less noble metal.
Why is this so dangerous?
Take copper and titanium zinc, for example. Copper is extremely noble; zinc is less noble. When rain falls on a copper roof, microscopic copper ions dissolve into the water. If this water then flows into a titanium zinc gutter, the copper ions settle there. Tiny galvanic cells form – and the zinc in the gutter is literally eaten away. Within just a few years, you’ll see extensive pitting.
Flow direction in practice:
- Allowed: Water flows from zinc to copper (less noble to noble = no problem).
- Prohibited: Water flows from copper to zinc (noble to less noble = damage).
Which metals are compatible? The compatibility matrix
So you don’t have to spend ages figuring it out on site, here’s a quick overview of which combinations work in direct contact or in the direction of water flow:
The lifesaver: stainless steel
As you can see in the table, stainless steel is the ultimate wildcard on the roof. It is so inert and stable that it can be combined directly with almost any material. That’s why high-quality sheet metal tools and fasteners (such as clips) are often made of stainless steel or coated with special plastic inserts to prevent direct metal-to-metal contact.
3 pro tips for preventing corrosion on site
- Use separating layers: If dissimilar metals (e.g., copper sheet on a galvanized steel substructure) need to touch, use structured separation layers, bitumen sheets or plastic strips as insulation.
- Keep your tools clean: If you’ve used your seam closer or metal shears on copper, clean the jaws thoroughly before working on a zinc roof. Even tiny metal shavings on the wrong sheet are enough to start corrosion!
- Pay attention to the coating: Aluminum is often well protected by its anodized layer or powder coating. But as soon as you cut or scratch the sheet, the bare aluminum is exposed and vulnerable to contact with more noble metals.
Video: Electrochemical corrosion | Lehre To Go
Think before you roof
You can’t outsmart chemistry on the roof. Ignore the flow rule and you’ll pay for it in the end. Always plan material transitions from less noble to more noble and, when in doubt, use separation layers or stainless steel fasteners.
Unsure about a mix of materials for your next project, or looking for tools that protect coated sheet metal from scratches? Just use our new chat here in the shop – click “Ask a question” in the bottom right and we’ll talk it through with you!
