History of Copper Roofs

Copper is one of the longest-lasting roofing metals. Exposed to air it forms a thin protective film that slowly turns from bright orange-brown to dark brown and finally to the familiar blue-green patina. Many historic copper roofs have lasted for centuries.

Early use

Histories of copper in architecture report copper roof shingles on the Lovamahapaya temple in Sri Lanka in the 3rd century BCE.

The Pantheon in Rome is sometimes listed as an early copper roof dated 27 BCE. That year belongs to the original temple begun by Agrippa, which was later destroyed by fire. The present Pantheon is a complete rebuilding completed under Hadrian in the early 2nd century CE (about 113-125 CE, possibly begun under Trajan; Britannica gives 118-128), and its dome was covered not with copper but with bronze tiles, described in some sources as gilded. The Byzantine emperor Constans II stripped them in 663 CE, and the dome was later covered with lead sheets.

Medieval and Renaissance Europe

• Hildesheim Cathedral, Germany, received a copper roof in 1280. Sources disagree on its fate: one says the roof survives, while another says it was destroyed by Second World War bombing.
• Kronborg Castle, Denmark, was roofed in copper in 1585; its tower copper was renovated in 2009.

How the patina forms

• Within about six months an oxide film becomes noticeable, evening out after about nine months.
• Over the first few years sulfide films darken the surface to brown and then to dull grey or black.
• The blue-green sulfate patina then develops: in about 7-9 years in marine climates, 15-25 years in industrial areas and 10-30 years in clean rural air. In very dry climates it may never form.

Once the patina is established, corrosion averages about 0.0001-0.0003 mm per year. On a 0.6 mm (0.024 in) sheet this means less than 5% loss in a century. Tests on 18th-century European copper roofs suggested they could, in theory, last a thousand years.

Joining methods

Copper is traditionally laid in pans joined by standing seams or batten (roll) seams, which allow the metal to expand and contract. These techniques are shared with lead and zinc work and are ancestors of modern standing seam roofing.

Today

Copper remains in use for heritage roofs, church spires, flashings and high-end architecture, often chosen specifically for the way it ages.