sod
A roof covering of living grass rooted in soil, including lawns over underground buildings.
Sod roofing is made from a dense layer of living grass growing from an underlayer of soil. It is both a primitive material, used by Vikings and other cultures in cold climates, and a modern "green" material, prized for its insulating properties, its absorption of rainwater and carbon dioxide, and its reduction of the "heat island" effect in cities where too many dark roofs cause the local environment to heat up in the summer.
Sod is also listed as the roofing material for underground buildings which lie under a lawn or parkland.
The living grass and its soil layer distinguish sod from thatch, which is made from cut plant stalks. The covering can lie over an underground building; it is not restricted to the visible sloping roof of an above-ground structure.
