Swahili Coral-Stone House

The coral-stone house is the characteristic urban building of the Swahili coast of East Africa, from Kenya's Lamu archipelago and Mombasa to Zanzibar and Kilwa in Tanzania. Its walls are coral rag or coral blocks set in lime, and its roofs and upper floors are thick layers of coral and lime laid over closely spaced mangrove-pole joists. Two of its towns, Lamu Old Town in Kenya and the Stone Town of Zanzibar, are UNESCO World Heritage Sites.

History

Swahili trading towns began building in stone in the late first millennium. At Shanga, in the Lamu archipelago, stone building started in the early to mid-10th century. At Kilwa Kisiwani in Tanzania, the Great Mosque's 12th-century prayer hall was fully roofed on coral and later timber pillars; a vaulted southern extension followed in 1294-1302 and a large dome in the early 14th century, showing that coastal builders could roof in vaults and domes as well as flat slabs. At Gedi in Kenya (12th to 16th centuries), mosque roofs rested on timber beams carried by square stone pillars. Lamu has been inhabited continuously for more than 700 years, and most of Zanzibar's Stone Town dates from its 19th-century trading boom.

Construction of the roof

In Lamu houses, walls are uncoursed coral blocks in lime mortar. Roofs and floors are thick coral laid on wooden joists spaced about 30 cm (12 in) on centre. The joists are mangrove poles, harvested from coastal mangrove forests and still sold on the Lamu waterfront for roofing. Scholars have suggested that the mangrove pole sets the module of the house: its practical span limits each room, which is why rooms are often only about 3 m (10 ft) wide.

Lime was made by burning coral and ageing it to a fine powder; mortar for structural work was strengthened with broken seashell. In Zanzibar, builders combined coral rag, lime, mangrove poles and coconut palm material in multi-storey buildings.

The flat roof was used as living and working space: a roof terrace over the outer living room served for drying laundry, and a small cooking penthouse, often under a thatched shelter (kidara cha meko), kept smoke away from the fine lime plaster below.

Maintenance and decline

Coral stone is friable and needs frequent maintenance, and many historic houses are in poor repair: in Zanzibar's Stone Town about 80% of its 1,709 buildings were reported to be deteriorating. Traditional building knowledge is also at risk from the growing use of brick and cement and of imported building technologies, which has prompted documentation projects among Lamu's masons and master builders. In Zanzibar, conservation of the historic town is governed by the Stone Town Conservation and Development Act of 1994.

Regional terms

• kidara cha meko (Swahili): thatched cooking shelter.
• ua (Swahili): courtyard.
• mtatato: a room bridging the street, carried on mangrove beams about 4 m (13 ft) above street level.