Verdigris

Verdigris strictly refers to copper acetate, a bright green compound formed when copper reacts with acetic acid (vinegar). It has been made deliberately for centuries as a pigment. Unlike roof patina, copper acetate is water soluble.

Verdigris versus patina

In everyday speech and in many roofing brochures, "verdigris" is used for the green coating on old copper roofs. Technical guidance points out that this is inaccurate. The patina that forms outdoors is made of basic copper salts, chiefly sulphates in urban and industrial air and carbonates in cleaner air, with chlorides contributing near the sea. These compounds are insoluble and form a stable protective layer, whereas true verdigris would wash away.

Dictionary and encyclopedic definitions are not fully consistent; some broader definitions include basic copper carbonates and sulphates under the verdigris name. For roofing purposes, the useful distinction is between natural, protective patina and corrosion products caused by acids or contamination.

Where acetate-type products may appear

Green, powdery or streaky deposits can form where copper is exposed to unusual chemistry, for example acidic run-off or organic acids. These deposits behave differently from a normal patina and may wash off or stain adjacent surfaces.

Practical notes

• A normal green roof patina is not damage and does not need removing.
• Artificially patinated copper uses controlled chemical treatments to imitate the natural colour; results can vary.
• Run-off from any weathering copper surface can stain light-coloured masonry and concrete below.

Colour without verdigris

A copper roof's colour sequence normally runs from salmon pink through russet and chocolate browns to blue-green or grey-green. In dry climates the green stage may never arrive, leaving the copper dark brown, which shows that the green colour depends on moisture and airborne compounds rather than on any acetate formation.