Another change was necessitated by an event which took place on St. Paulinus' Day, October 10th, of the year 1323. For on that day a calamity such as had never before happened befell the church. The celebration of Mass at an altar of the Blessed Virgin was just over, a great multitude of people, men and women, still being in the church, when two of the Norman piers of the main arcade on the south side fell outwards one after the other with a great crash, and about the space of an hour afterwards the wooden roof of the nave which had been supported by these columns also fell; the piers themselves had crushed the south wall of the aisle and the cloisters, so that a complete wreck was made of the south-eastern part of the church westward of the tower. But this disaster was accompanied by a great marvel, for though many persons were standing close by, not one was injured; and a still more wonderful thing is recorded: the monk whose duty it was to guard the shrine of St. Amphibalus, which at that time stood in the nave, had been celebrating at the altar--he had finished even to the washing of the sacred vessels--when he saw the columns fall; he withdrew a little from the altar and received no harm. Some of the wreckage fell on the shrine of St. Amphibalus, and though the marble pillars supporting the canopy were broken, yet the chest which contained his relics suffered no harm. This wonderful preservation of life and limb and shrine was naturally attributed to the intervention of the blessed martyr St. Amphibalus.
Abbot Hugh of Eversden began to rebuild this ruined part of the church, and this accounts for the five bays of the nave arcading westward of the rood-screen being in fourteenth-century style. He did not live to finish all this work, but it was carried on by his successor, Richard of Wallingford (1326-1335), and finished by the next Abbot, Michael of Mentmore, about 1345. The present rood-screen, which probably took the place of a previously existing one of Norman date, was built in 1360 by Thomas de la Mare. No further change of importance was made until the time of John of Wheathampstead, who was Abbot from 1420 to 1440, and again from 1451 to 1464. He left his marks in various parts of the Abbey, and for the most part his work was bad: he did almost as much to injure the Abbey as the nineteenth-century restorers who swept away much of his work have done. He rebuilt all the upper part of the west front, and inserted Perpendicular windows at each end of the transept; he turned the high-pitched roofs of nave and transepts into flat ones, and lowered the slope of the roofs of the aisles. His object in doing this was to be able to use the old beams again whose ends were decayed, and which were shortened by cutting off the unsound parts. The result of this was that the Norman triforium arches on the north side were thrown open to the sky; these he filled with Perpendicular tracery, converting them into windows. The tracery still remains, although the new roof has the same slope as the original one, and the triforium is now again inclosed beneath it. He also pulled down the wooden octagon on the central tower. His chantry on the south side of the high altar was probably erected soon after his death.