High-level gantry under the main vaulting
Image from John Fitchen's "Construction of Gothic Cathedrals"
This analytical drawing does not depict a moment in time since in reality the upper clerestory wall would be complete before the installation of the vaults. We see the gantry, or cantilevered brackets necessary to support a platform or terrace to facilitate the construction of the vaults. At Amiens the main vaults and flying buttresses were probably constructed simultaneously, soon after the completion of the clerestory wall, the nave in the 1240s and the choir c. 1260s. The vaults were sheltered under a provisional roof until the installation of the existing roof c. 1285-1305.