Roman concrete vaults are praised for the impressive distances they were able to span, but one often forgets that the wooden structures on which the concrete was first laid is what determined the size of the final vaulted structure. Much of the technology for building large concrete vaults was based on woodworking techniques; this aspect of concrete vaulting has not received much attention, in part, because very little remains of these wooden structures. In this chapter, I examine the evidence that exists for the wooden centering and formwork and pose the questions: How were the centerings constructed? How were they lifted into place? How were they supported? How were they removed without damaging the work below? The sources used to answer these questions include the impressions of the boards left in the concrete, pieces of the actual wood (which are rare), ancient pictorial representations of wooden structures (albeit not centering structures), literary descriptions of wooden construction for bridges and siege towers, and comparisons of centering structures from later periods.

ASSEMBLING THE CENTERING

The construction of the most impressive Roman vaults was dependent on the builder's ability to erect large wooden centerings capable of taking the weight of the concrete. These wooden structures differed from wooden roof structures in that they did not require clear spans and had to take a much greater load with minimum deflection, but many of the joinery techniques and the structural principles were no doubt the same.