History of Green Roofs

A green roof is a roof deliberately covered with soil or growing medium and plants. The idea is old, but the modern engineered green roof, with root barriers, drainage layers and lightweight substrates, is a late-20th-century development led by Germany.

Legends and early examples

The Hanging Gardens of Babylon are often cited as the first roof garden, but their existence is disputed and no Babylonian text mentions them. Better documented are traditional sod roofs of Scandinavia, used on most rural houses in Norway from the Viking Age and Middle Ages until they were replaced by tile and corrugated iron; sources date their decline from the 18th to the late 19th century. Sod roofs are also common on traditional farm buildings in Iceland, and Kashmiri houses traditionally used burze pash roofs of birch topped with soil for winter insulation.

Early modern roof gardens

• 1914: the Moos water treatment plant roof in Switzerland, cited as one of Europe's oldest green roofs.
• 1936: the Rockefeller Center roof gardens in New York.
• 1938: the Kensington Roof Gardens in London.

The German movement

The modern green roof trend began in Germany in the 1960s. In the 1970s, stormwater problems pushed cities towards lightweight planted roofs, and by the 1980s the technology was widely known. The research society FLL was founded in 1975, and its green roof guidelines, most recently revised in 2011, became a reference across Europe. By the early 2000s many German cities required green roofs on certain buildings, and an estimated 10% of German roofs are now green.

Policy elsewhere

• Linz, Austria: has paid developers to install green roofs since 1983.
• Switzerland: federal rules since the late 1990s.
• Toronto: a by-law of May 2009 required green roofs on certain new buildings.
• Washington, D.C.: green roofing grew from 2.3 million sq ft (about 214,000 m²) in 2016 to 5.1 million sq ft (about 474,000 m²) by 2020.
• Denver: an initiative took effect in January 2018.

Extensive and intensive

Extensive roofs have 2-12.7 cm (about 1-5 in) of substrate and weigh about 50-120 kg/m² (10-25 lb/sq ft). Intensive roofs are deeper, support a wider range of plants and weigh about 390-730 kg/m² (80-150 lb/sq ft). Research summarised in 2023 found green roofs reduce summer roof surface temperatures substantially and cut stormwater runoff.