Why “Incompatible Metals” and “Inert Catchment” Matter in Shed Building and Renovations
When builders put up roofs, gutters, and other parts of a building, they have to be careful about which metals they use. Using the wrong metal in the wrong place can lead to rust and damage over time—costing a lot of money to fix. Below are two important ideas that help explain why this happens and how to prevent it.
1. Incompatible Metals (Galvanic Corrosion)
What Is It?
Galvanic corrosion happens when two different metals—like copper and zinc-coated steel—touch each other and get wet (for example, from rain). The water acts kind of like a link or bridge between them, causing a tiny electric-like reaction that makes one metal wear out or rust more quickly.
Example: Think of it like a small battery forming when water connects the two metals. One metal (the “anode”) loses material (it corrodes), while the other metal (the “cathode”) doesn’t. That means the metal that corrodes could end up with holes or weak spots over time.
Why Does This Happen in Buildings?
- Mixed Metals: If a roof is made of steel and the screws are made of copper or even stainless steel, they will react with each other when it rains.
- Runoff: Water from a roof made of one metal can flow onto another metal in a gutter or downpipe, potentially speeding up corrosion.
How to Prevent It
- Use the Same Metal Where Possible: This helps avoid the reaction altogether.
- Separate Different Metals: If you really need to use different metals, placing a thin barrier (like a plastic flashing, washer, grommet or coating) between them can stop the electric-like reaction.
Check Product Guides: Building manufacturers often provide lists showing which materials can safely touch each other. There are resources available via Bluescope Steel and Bluescope Lysaght.
https://www.bluescopesteel.com.au/howto/avoid-incompatible-metals and https://lysaght.com/support-technical/support/design/material-compatibility
2. Inert Catchment
What Is It?
“Inert catchment” refers to a surface—like certain types of coated steel roofing—that doesn’t change the acidity (pH) of water running off it. So, if the rainwater was slightly acidic or contained salts, to begin with, it stays that way as it flows onto other materials. It doesn’t “neutralise” or tone down the water at all.
Why Does It Matter?
- Acidic Water: Rain is naturally a bit acidic, and when it stays acidic, it can damage materials that aren’t built to handle it (like some unpainted metals).
Damage Over Time: Guttering or piping made from sensitive metals might rust faster if it’s constantly hit with this unchanged water.
How to Deal With It
- Use Compatible Gutters and Downpipes: If you have a coated steel roof like Colorbond or Zincalume that keeps water acidic, choose gutters and pipes that can handle it. This means using the same coated steel.
- Protective Coatings: Painting or coating metal gutters can help protect them from acidic water.
- Pay Attention to Water Flow: Try to direct water away from areas where it might pool or stick around since long exposure makes corrosion worse.
In the end…
- Incompatible Metals (Galvanic Corrosion): Different metals touching in the presence of water can cause one of the metals to rust or corrode quickly.
- Inert Catchment: Some roofing materials don’t reduce the acidity of rainwater, which can make the water more damaging to other metal parts below.
Knowing about these two issues helps shed owners and builders understand why certain materials are chosen for roofs, screws, gutters, and pipes. By choosing metals that work together (or by separating them if they don’t), buildings can last longer without needing expensive repairs.
