Combining metals on the roof correctly: How to prevent dangerous galvanic corrosion with zinc, copper, and aluminum
*The image was AI-generated for illustrative purposes.
Anyone who installs a metal roof, clads facades, or plans roof drainage today has access to a wide range of materials. Titanium zinc, copper, aluminum, or stainless steel - each material has its own visual and technical advantages. But beware: Anyone who blindly combines these metals on the construction site may inadvertently create a ticking time bomb.
The key term is contact corrosion (or galvanic corrosion). When the wrong metals come into contact, they can destroy each other within a very short time. Pitting corrosion and expensive repairs are the result.
In this guide, we at Roofify explain the notorious flow rule and show you how to combine metals on the roof without unpleasant surprises.
How does contact corrosion occur? (Short & easy to understand)
Every metal has what is known as an “electrochemical potential.” A distinction is made between base metals (such as zinc or aluminum) and noble metals (such as copper or stainless steel).
When two different metals are directly connected to each other (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 nobler metal attacks the less noble metal and decomposes it. The less noble metal essentially “sacrifices” itself.
The golden rule for sheet-metal workers: Follow the flow rule!
There is an unwritten law on the roof that every apprentice must know by heart: the flow rule.
💧 The flow rule states: Water must never flow from a nobler metal onto a less noble metal. In the direction of water flow, the nobler metal may therefore only come after the less noble metal.
Why is this so dangerous?
Take copper and titanium zinc as an example. Copper is extremely noble, while zinc is less noble. When it rains on a copper roof surface, microscopic copper ions dissolve in the water. If this water then flows into a titanium zinc gutter, the copper ions deposit there. Tiny galvanic cells form - and the zinc in the gutter is literally eaten away. After just a few years, you will have severe pitting corrosion.
The direction of flow in practice:
- Permitted: Water flows from zinc to copper (less noble to noble = no problem).
- Prohibited: Water flows from copper to zinc (noble to less noble = destruction).
Which metals are compatible? The compatibility matrix
So you do not have to spend a long time wondering on the construction site, here is 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 absolute joker on the roof. It is so inert and stable that it can be combined directly with almost all materials. That is why high-quality sheet-metal tools or fasteners (such as clips) are often made of stainless steel or coated with special plastic inserts to prevent direct metal-to-metal contact.
3 professional tips for preventing corrosion on the construction site
- Use separation layers: If dissimilar metals (e.g. copper sheet on a substructure made of galvanized steel) must touch, use structured separation layers, bitumen membranes, or plastic strips as an insulator.
- Keep your tools clean: If you have first worked on copper with your seam closer or your sheet-metal shears, clean the jaws thoroughly before working on a zinc roof. 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. However, as soon as you cut or scratch the sheet, the bare aluminum is exposed and susceptible to contact with nobler metals.
Video: Electrochemical corrosion | Lehre To Go
Think first, then install
You cannot outsmart the chemistry on the roof. Anyone who ignores the flow rule will pay the price in the end. Always plan your material transitions from less noble to nobler and, when in doubt, use separation layers or stainless-steel connecting elements.
Are you unsure about a material mix for your next project or looking for tools that protect coated sheets from scratches? Simply use our new chat here in the shop - click “Ask a question” in the bottom right and we will clarify it directly with you!
