The double roof system of the typical Gothic great church — a stone vault surmounted by a timber roof — is both decorative and functional. The steep external roof provides the necessary weather proofing dictated by northern climates (shallow pitches were used for Greek temples); indeed the stone vault, perhaps cracked and in any case not waterproof, itself needs the protection of the outer roof (in Cyprus the Crusader churches hardly need this cover). However timber burns well, and one function of the stone vault is to provide a fire-resistant barrier between the outer roof and the church. There is thus a symbiotic action between the two coverings of the church; the timber roof protects both the stone vault and the church from the weather, and the stone vault protects the church from the potential fire hazard of the timber roof.

The stone vault, functionally installed in a great church, at once became integrated as an architectural element of the internal decorative scheme. Thus the simple quadripartite vault, lierne and tierceron vaults, net and star vaults, and the fan vault, were all developed as ‘solutions’ to the vaulting problem. As will be seen, these vaults, so different from each other visually, have structural actions very much in common.

The barrel vault

Figure 4.1(a) shows a straightforward ‘voussoir’ construction of a barrel vault — the resulting tunnel can clearly be extended to any desired length. This type of construction will need temporary formwork to support the masonry.