ROMAN URBAN TOPOGRAPHY in Britain and the western Empire

Ling: The mechanics of the building trade
Fig 35 Brick-faced wall at Pompeii, covered with mortar in preparation for marble veneer (Photo: author)
time of Nero onwards, the predominant technique. The
facing consisted of horizontal courses of flat, shallow
bricks, generally (in Italy at least) triangular in shape and
laid with one point inwards. Vertical joints alternated, as
in ashlar construction. At first the ‘bricks’ were roof-tiles
cut along the diagonal, but under Claudius they began to
be cut from true bricks fired to standard sizes (multiples or
fractions of the Roman foot). From then on, as builders
realized the advantages of the new technique, which was
simple to use, provided greater stability in the construction
process, and had a high resistance to heat, brick
production developed on a large scale, with many makers
of the 2nd and early 3rd centuries stamping their wares.
It should be stressed again that the role of these facings
was primarily to assist in the construction process.
Nonetheless, they also provided an attractive surface to
the finished wall (Vitruvius (ii.8.1) calls reticulate ‘charming’),
and their decorative potential was sometimes exploited
by the use of different colours to emphasize
specific areas or to form overall patterns. Even so they
were not always left visible. Some of the most careful
facings in reticulate and brickwork were originally concealed
beneath a bland and monotonous coat of stucco.
The role of the so-called relieving arches visible in the
facing of many walls is interesting. These are often merely
superficial and have no effect on the concrete behind; their
sole function lay in the statics of the reticulate or
brickwork before and during the setting of the core. Once
the concrete was hard they became redundant. There are
of course exceptions, as in the Pantheon, where the
well-known relieving arches are an integral part of the
structure, helping to transfer thrusts away from niches
and cavities in the interior of the walls and at the same time
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dividing the concrete mass into more manageable volumes
in the event of settling.
The building of vaults and domes was the most complex
operation in Roman concrete architecture, since it required
the preliminary construction of timber centering.
Where vaults were of limited span this could be lodged in
the spring of the vault, just as the centering for the
stone-built arches of the Pont du Gard rested on the
cornices and on projecting blocks built into the piers. But
Fig 36 The ‘Old Work’ at Wroxeter, showing petit
appareil with tile bonding-courses and putlog holes
passing right through the wall. Some of the holes
perhaps served for the construction of centering for
the vault, not simply for scaffolding (Photo: author)