The Great Mosque of Djenné is an exception to architecture's notion of the 'permanent building': here the structure is never finished and frozen but goes on living, reworked every year. Its material is neither stone nor fired brick but sun-dried mud brick — banco.

The walls are built of hand-formed bricks of mud and straw and coated with a thick banco render. Earthen building is defenceless against water; the rainy season erodes the surface every year. The solution turns the material's weakness into a ritual of maintenance: palm-wood beams projecting from the walls — toron — serve both as structural ties and as permanent scaffolding. Each year, in a festival the whole town joins, people climb onto these beams and replaster the mosque from top to bottom. The building is thus the shared, continuous work not of a single architect but of an entire community across generations.

The thick earthen walls also act as thermal mass: they absorb the day's heat and release it slowly at night; the windows are small and the interior kept cool — the passive climate solution of the Sahel. Although the present structure was rebuilt in 1907 with the same traditional technique, the typology and method reach back to the Middle Ages, to the Sudano-Sahelian architecture of the Mali Empire era. Djenné is the most magnificent example of an architecture whose material is 'weak' but whose system is sustainable — a building in continuous negotiation with earth, climate, and community.