Properly combining metals on the roof: How to prevent dangerous galvanic corrosion in zinc, copper, and aluminum
When roofing a metal roof, cladding facades, or planning roof drainage, a variety of materials are used today. Titanium zinc, copper, aluminum, or stainless steel – each material has its optical and technical advantages. But beware: anyone who blindly combines these metals on the construction site may be installing a ticking time bomb.
The keyword is: contact corrosion (or galvanic corrosion). If the wrong metals meet, they will corrode each other in a very short time. Pitting and expensive renovations are the result.
In this guide, we explain the infamous flow rule and show you how to combine metals on the roof without unpleasant surprises.
How does contact corrosion occur? (Brief & understandable)
Every metal has a so-called "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 (or water flows from one to the other) and moisture (such as rain or condensation) acts as an electrolyte, it happens: a tiny electrical current flows. The nobler metal attacks and decomposes the baser metal. The base metal virtually "sacrifices" itself.
The golden plumber's rule: Observe the flow rule!
On the roof, there is an unwritten law that every apprentice must master in their sleep: The flow rule.
💧 The flow rule states: Water must never flow from a nobler metal to a baser metal. In the direction of water flow, the nobler metal may therefore only come after the baser metal.
Why is this so dangerous?
Take copper and titanium zinc as an example. Copper is extremely noble, zinc is base. If it rains on a copper roof, microscopically small copper ions dissolve in the water. If this water then flows into a titanium zinc gutter, the copper ions deposit there. Tiny galvanic elements are formed – and the zinc of the gutter is literally eaten away. After a few years, you have fine pitting corrosion.
The direction of flow in practice:
- Allowed: Water flows from zinc to copper (base to noble = no problem).
- Forbidden: Water flows from copper to zinc (noble to base = destruction).
Which metals are compatible? The compatibility matrix
So that you don't have to ponder for a long time on the construction site, here is a quick overview of which combinations work directly in contact or in the direction of flow:
The savior in need: Stainless steel
As you can see in the table, stainless steel is the ultimate 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 plumbing tools or fasteners (such as clips) are often made of stainless steel or coated with special plastic inserts to prevent direct metal-on-metal contact.
3 professional tips for the construction site to prevent corrosion
- Use separating layers: If dissimilar metals (e.g., copper sheet on a galvanized steel substructure) must touch, use structured separating layers, bitumen sheets, or plastic strips as an insulator.
- Keep tools clean: If you have worked on copper with your seaming tool or sheet metal shears, thoroughly clean the jaws 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 raw aluminum is exposed and susceptible to contact with nobler metals.
Conclusion: Think first, then roof
The chemistry on the roof cannot be outsmarted. Anyone who ignores the flow rule will pay the price in the end. Always plan your material transitions from base to noble and, if in doubt, rely on separating layers or fasteners made of stainless steel.
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 on "Ask a question" at the bottom right and we'll clarify it directly on an equal footing!
