Second Phase: c.1225-30 Robert de Luzarches and Thomas de Cormont

As soon as possible a covered aisle space in uno latere was provided for the parishioners of the recently-demolished church of S-Firmin the Confessor (mentioned in the 1236 charter) That the south nave aisle was constructed rapidly to provide this space is demonstrated by experimental forms (window design; higher tie beams) found in the angle of the south transept and nave. The south portal of the west façade was built at the same time (different form of the great relieving arch behind the gable).  The early completion of the western aisle of the south transept arm up to the aisle vault has left a great vertical suture just to the east of the buttress flanking the south transept portal. The later stones of the lower south transept façade butt up against this early work.