The roofing of the tomb chambers is of particular interest, because evidence for the roofs of Sumerian buildings is rare. Stout timbers, reed or palm frond matting, and a sealing of clay would have created a sturdy roof for a house, strong enough to hold the weight of a person. The same system could have been used for larger buildings, if interior columns divided the span of a room into manageable dimensions. In certain cases a more elaborate roofing of mud bricks was attempted. In the Royal Tombs of Ur, the chambers were vaulted or, rarely, domed with brick or limestone rubble, using the technique of corbelling (see below). Valuable evidence for vaulting techniques has come from excavations conducted in the 1960s at the second millennium BC site of Tell al Rimah, in north-west Iraq; well-preserved mud brick arches and vaults, some in the pitched-brick technique, are essential components of a large temple of the early second millennium BC.
The progression to the true arch and domical vault (= the dome) is one of the important architectural developments in the ancient Mediterranean and Near East and will be examined later in
This book. The early techniques just mentioned, corbelling and pitched-brick, merit explanation. But first, the distinction between an arch and a vault needs to be appreciated: an arch is a twodimensional span, covering a doorway or window, whereas a vault is three-dimensional, covering a room. The principles of arch construction can often be applied to vaults.
In a corbelled arch, on each of the two sides each successive block projects further inward until finally the two sides touch at the top (Figure 2.18d). If left by itself, the corbelled arch will eventually collapse: the weight pressing down toward the empty center of the arched space is not sufficiently counterbalanced by the weight of one brick on top of another. To solve this problem, a counterweight needs to be placed on the outside, to press the outer edges of the bricks downwards, to direct pressure toward the brick just below. If well incorporated into a sturdy wall, a corbelled arch could stand. In contrast, vaults made in the corbelling technique never stand alone, without counterweight, unless the space they cover is small (as in a small room of a house). Good-sized corbelled vaults are underground, with a packing of earth around and above the structure to provide the necessary counterpressure.
In the true arch as distinct from the corbelled, stones are specially cut in wedge shapes to fit into one continuous curve (Figure 2.18a). The form and placement of the keystone, the wedge
Figure 2.18 Diagram: (a) true arch; (b) barrel vault; (c) groin vault; and (d) corbelled arch
At the top of the arch, illustrates how the pressure from each stone is not directed exclusively downwards, but also to the side. The vertical struts that support the arch need to be reinforced in order not to buckle outwards, but the arch itself should not collapse. As with corbelling, the principles of true arch construction can be extended to three-dimensional forms, the vault (two are important in Mediterranean antiquity, the barrel vault and the groin vault. Figures 2.18b and c, respectively) and the dome (a hemispherical vault).
The pitched-brick technique of roofing falls somewhere between the above two methods (Figure 2.19a—b). The bricks are not specially cut into wedge shapes, nor are they placed flat one on top of the other. Instead, each successive brick is tilted slightly in order to form a curved line. The extra space at the top is filled with fragments. Although much more fragile than a true dome, such a structure can stand on its own.
Figure 2.19 Diagram: The pitched brick vault: (a) view from below; and (b) in cross section
World History
