Nave Roof: 1300-1305
The main structural unit is formed of a triangular ferme with reinforced rafters (arbelétriers), a horizontal tie beam (entrait) at the base and kingpost (poinçon) at the center. Set at 90 degrees to the surface of the roof and anchored in the kingpost are great diagonal braces (contrefixes). These units are set at intervals of 3.7m or 12 feet.
In the intervals between the main units are 5 secondary rafters gapped at one foot. They do not have tie beams, kingposts or braces and serve to support the surface of the roof.
The longitudinal section shows thh horizontal beams (pannes) that support the roof surface and bracing of the main rafters in the form of a Saint Andrew's cross.
The 4 revolutionary features of the Amiens roof in comparison to earlier roofs:
- The reduced dimensions of the lumber
- Alternation of major structural units formed of great triangles (fermes) and smaller rafters.
- The great diagonally-placed forked braces (contrefixes) between the king post and the roof.
- Perfection of assembly.
This light-weight roof was conceived soon after the 1284 partial collapse of the upper choir at nearby Beauvais Cathedral: the new features in the design may reflect the infusion of ideas from England (Chichester Cathedral).