Wooden centering (formwork) for diagonal ribs and transverse archesImage from John Fitchen's "Construction of Gothic Cathedrals"
The diagram suggests that the work of installing the vaults took place atop a platform supported by a gantry. Transverse arches and ribs, formed of stones cut precisely with radial coursing, are supported by rigid curved struts of braced timber. These elements were reusable. The diagram does not show the means of supporting the roughly shaped cubic stones (pendans) that formed the thin webs or serveries of the vault. This might be done with flexible boards, braced to form a curve, or with wicker. In some cases dirt was heaped atop a terraced wooden platform to support the vaults severies. At Amiens Cathedral, the high vaults were installed before the construction of the main roof.