ROMAN URBAN TOPOGRAPHY in Britain and the western Empire

Ling: The mechanics of the building trade
Fig 37 Detail of the city walls of Silchester, showing bonding-courses of flat stones and a vertical ‘gang join’ at the left (Photo:
M Fulford)
with large vaults, and inevitably with domes, such as that
of the Pantheon, the centering had to be built up from
floor level and, as one can see from reconstruction
drawings, would itself have been a structural tour de force.
Despite the great surviving triumphs of Roman vault
construction, there were obviously many failures, and it is
striking to observe a certain caution, if not actual diffidence,
on the part of builders throughout Roman times.
Down to the end of the 1st century BC the rubble caementa
which formed the aggregate in the core of the vault were
laid radially like the voussoirs of an arch, as if the builders
distrusted the strength of their material. Only later did
they begin to build up their vaults over the centering in
horizontal layers, just as they did walls. When groined
vaults appeared at the time of Nero’s Golden House in
Rome, the angles were first carried out in brick, and the
intervening spaces then filled with concrete. The architect
may have felt that he was reinforcing a point of weakness,
but in fact brick ribs tend to create a line of potential
cleavage. There are many examples of vaults or domes in
which ribs and concrete have come apart, the one perhaps
still standing, the other collapsed. Brick ribs were also
used to divide up barrel vaults, presumably so that the
work could be completed in manageable sections and the
same centering moved from one to the next; once again the
result was frequently a crack (or worse) between the brick
and the concrete. More success was achieved by careful
selection of the materials for the caementa. For example, in
the Pantheon they were graded from the heaviest and
strongest in the lower part of the dome to the lightest
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(pumice) in the area round the central oculus. At a later
period, for example in the Circus of Maxentius outside
Rome, vaults were lightened by the incorporation of
empty amphorae,’ and later still (chiefly in the Christian
period) by the use of purpose-made terracotta tubes.
To return finally to Britain. Here concrete walls were
comparatively weak, and on the whole they are more
accurately described as mortared rubble. Although brickwork
is not unknown, facings were generally in rubble or
small squared blocks (‘petit appareil’) with frequent
bonding-courses of tiles or flat stones, often two or three
courses thick. Good examples of the former can be seen in
the ‘Old Work’ in the Roman baths at Wroxeter (Fig 36),
and of the latter in the town walls at Silchester (Fig 37),
where one can also discern the vertical joints between
sections of walling carried out by different work gangs. As
was the case with ashlar construction, the aggregate and
facing-stone were regularly those which could be most
easily obtained. Thus at Colchester the city walls were
built with the locally-available septarian nodules, while in
the chalk country, as at Verulamium, the normal facing
material was flint. But while British builders were certainly
affected by regional factors, it would be wrong to think
of them apart from their continental counterparts. Britain
was merely one part of a much larger cultural world and
most of the building practices found here followed those
developments in the mainstream of Roman architecture
which have been the subject of this paper.