Copper Run-Off and Staining

As copper weathers, rain slowly dissolves small amounts of copper compounds from its surface. That run-off has two practical effects: it can leave blue-green stains on light-coloured materials, and it can attack less noble metals that it flows onto.

Staining

Copper run-off is well known for staining marble, limestone, stucco, mortar, brick, stone and concrete beneath copper roofs, gutters, downpipes and flashings. Stains are most visible where water concentrates, such as below drip edges, scuppers and the ends of valleys. Planning water paths, using gutters and downpipes, and keeping drip edges clear of walls reduces staining. Some owners use clear coatings on copper to slow weathering, though these need periodic renewal.

Corrosion of other metals

Dissolved copper deposits on more active metals and sets up tiny galvanic cells. Water from copper should not drain onto:

• aluminium roofing, gutters or flashings;
• zinc or titanium-zinc;
• galvanized steel or 55% aluminium-zinc coated steel such as Galvalume.

Metal roofing codes of practice rate both copper run-off onto these metals and direct contact with copper as unsuitable. Where copper and other metals must coexist, designers route copper run-off away or separate the metals with membranes and bituminous coatings.

Compatible companions

Copper alloys such as bronze and brass and 300-series stainless steel are the usual fastener and accessory metals with copper, according to copper industry specifications.

Rates of release

Weathering of copper is slow. New Zealand guidance indicates corrosion rates of roughly 2 to 3 micrometres per year at first, falling over time as the patina matures. Even small amounts of dissolved copper are enough to stain light surfaces and attack zinc or aluminium over many years.