Final answer:
The nave of pilgrimage churches like Saint-Sernin at Toulouse and Saint James was covered by a vault, more specifically a masonry vaulted ceiling that was characteristic of Romanesque architecture.
Step-by-step explanation:
The nave of pilgrimage churches, such as Saint-Sernin at Toulouse and Saint James, were covered by a vault. This style of architecture was common in Romanesque churches, where the buildings needed to accommodate large numbers of pilgrims. Unlike the earlier timber roofs that were prone to fire, these churches featured masonry vaulted ceilings for better safety and stability. The nave of such a church typically had a stone or masonry vault, often in the form of a barrel or groin vault, which was necessary to support the heavy stone roofs and to span the wide naves of these grand buildings. In addition to their functional role, these vaults contributed to the grandeur and awe-inspiring atmosphere within the church.