Properly combining metals on the roof: How to prevent dangerous galvanic corrosion in zinc, copper, and aluminum
*The image was AI-generated for illustrative purposes.
When installing a metal roof, cladding facades, or planning roof drainage, a wide variety of materials are used today. Titanium zinc, copper, aluminum, or stainless steel – each material has its optical and technical advantages. But beware: combining these metals blindly on the construction site can lead to a ticking time bomb.
The key phrase 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 at Roofify 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 electric current flows. The noble metal attacks the base metal and decomposes it. The base metal virtually "sacrifices" itself.
The golden plumber's rule: Observe the flow rule!
On the roof, there's an unwritten law that every apprentice must know by heart: The flow rule.
💧 The flow rule states: Water must never flow from a noble metal onto a base metal. In the direction of water flow, the noble metal must therefore only come after the base 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 surface, 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 will 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 on the construction site, here's a quick overview of which combinations work directly in contact or in the direction of flow:
The savior in an emergency: 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's 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 sheeting on a galvanized steel substructure) must touch, use structured separating layers, bitumen membranes, or plastic strips as an insulator.
- Keep tools clean: If you have just worked with copper using your seaming tool or your sheet metal shears, clean the jaws thoroughly before working on a zinc roof. Tiny metal shavings on the wrong sheet metal 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.
Video: Electrochemical Corrosion | Lehre To Go
Think first, then cover
The chemistry on the roof cannot be outsmarted. Those who ignore the flow rule will pay the price in the end. Always plan your material transitions from base to noble and, when in doubt, rely on separating layers or fasteners made of stainless steel.
Are you unsure about a material mix for your next project or are you looking for tools that protect coated sheets from scratches? Just use our new chat here in the shop – click on "Ask a question" in the bottom right corner and we'll clarify it directly!
