_Narthex._--The narthex is covered with vaults, similar to those of the lower aisles of the nave. Each vaulted space is separated from the next by a segmental arch, six and a half feet wide with a span of twenty-six and a half feet, which abuts on the west wall of the nave, and the piers of the outer wall. The vault spaces vary from sixteen and a half feet in the middle to thirteen and a half feet towards the ends. The piers of the outer wall are connected together by arches above the window openings, and the spaces below the windows are filled up with thin ‘screen’ walls. The upper floor of the narthex is covered with a semicircular vault, intersected by the window arches between the piers of the outer wall. These piers are the continuation of those beneath, and have a width of six feet, and a depth of seven. They had to bear the thrust of the barrel vault of twenty-six and a half feet span: the buttresses previously mentioned, springing from the piers of the propylaeum, were subsequently added to strengthen them. In the exonarthex there are cross groins with arches between. The arches have a span of fourteen and a half feet and an abutment of seven feet. This seems to be of a later construction than the rest of the vaulting, and not improbably, as well as the piers, belongs to a reconstruction of this porch, undertaken to strengthen the west wall of the narthex. All the arches of the nave which stand on columns have iron ties; and to the three large openings of the gynaeceum at the west end of the nave there are wooden binders as well. In the lower rows of windows beneath the dome on both the north and south sides of the nave iron ties can be seen, which seem to stretch across the whole width of the large arches which support the dome. _Roofs._--All the exterior vaults are covered with lead about a quarter of an inch thick, which rests on a layer of wooden battens placed immediately upon the brick vaults. There are several passages and staircases for access to the roofs. Access to the exterior of the side aisles and narthex is gained by the staircases in the buttress piers: the stairs are supported on brick arches. In the north-east pier the stair space is only four feet eight inches by six feet seven inches, and in this are placed the flights of stairs two feet eight inches wide, with a space of fifteen inches between.[258] At the top of each flight spaces are hollowed out in the wall, which serve as landings from one flight to another.