By Brian Zabala
Photos are by the author unless otherwise stated.
Much of what we know of ancient Rome is largely due to the traces of their civilization that have survived until today. During my time in Rome, I noticed that these traces were embedded at every level of the built environment. From the generous use of columns borrowed from ancient structures, to the continuing relevance of public buildings adapted for modern use, there are many instances of Roman construction enduring in the present.
However, what interested me the most was the mystery surrounding ancient construction. How exactly were these enduring examples of Roman structures built? What logistical challenges were involved during construction? And who was responsible for maintaining them? While we may never know the exact details, their are many clues available that allow us to make several assumptions about the construction process.
In this article, I will examine how public infrastructure, such as aqueducts, temples, baths, and other public works, were built. Specifically, I will explain how evidence of ancient Roman infrastructure construction as described by surviving contract documents, the evolution of material usage, and the methods for construction of remaining structures show that the construction process has changed little since then.
Contracting documents regulated the work and payment to contractors.
Like in modern times, contracting provided the legal, economic, and practical means for construction in ancient Rome. The construction and maintenance of public buildings was the responsibility of the offices of multiple elected officials. Most notably, these included the aediles, who were responsible for enforcing the maintenance of public buildings, and censors, who were responsible for managing and financing maintenance and construction of public works, among other responsibilities (Britannica). This public infrastructure included walls, temples, roads, theaters, bridges, and other structures. Construction and maintenance was dedicated to contractors who bid on contracts initiated by the censor, who would award the contract to the lowest bidder. The censor was also responsible for managing public funding of these projects, and had the ability to contract tax collection to contractors as well. The terms of each contract were published by the censor describing the “rights and duties” of the contractor (Puteoli).
Figure 2 below shows the lex pareti faciendo, which is a contract dated to 105 B.C. for the construction of a wall near the market of Puteoli, about 250 kilometers south of Rome. The first column of the contract begins by calling for the contractor to “provide bondsmen and register their estates as securities,” suggesting that they are being instructed to insure any potential losses during construction (Puteoli). The contract also details the location, material, and dimensions of the wall, stating that the contractor “shall make it 6 ft. wide, 7 ft. high,” and that there will be “2 cross-beams made of fir-wood and 1/2 ft. thick each way” (Puteoli). This continues into the second and third columns, which describe the legal obligations of the contractor, the date that work is set to begin, and the method of payment for the work.
Figure 1: The lex parieti faciendo, a contract for wall construction around the Macellum of the Roman colony Puteoli in 105 BC near modern Pozzuoli in Naples (National Archaeological Museum of Naples).
Officially, public funds collected through taxes were appropriated by the Roman Senate to provide funding for public projects. However, this was not always the case, as individuals, such as the emperors during the imperial period, provided their own money toward the construction of public buildings to win social and political influence.
Current contracting methods are structured similarly.
Generally, an owner or their representative will award contracts to contractors. On government or public projects, the design-bid-build format is widely used, with contracts usually awarded to the lowest bidding contractor. However, some projects may work as design-build, meaning work is contracted to a single firm that manages both the design and construction. While the idea of contracting is not new, some key differences include the legal terms and consequences of contracting work, and the responsibilities of the contractor.
For more information on current project delivery methods in construction, this article provides a more in-depth overview of the types of contracting available and modern considerations when contracting construction work. The next section will consider how materials shaped the construction process over time as new uses, methods, and production capabilities were introduced.
The evolution of material use over time transformed the scale and efficiency of construction.
Roman society greatly depended on materials available nearby, as they provided the most easily attainable sources of stone, wood, and clay. Some public projects, such as the Pont du Gard aqueduct in southern France, sourced its limestone exclusively from a limestone quarry created during construction about 600 meters away (Adams, 1994). However, the ways in which these materials were used not only had a significant effect on what could be built, but also how they were built. The introduction of new materials, such as concrete, made possible the construction of new architectural elements, such as domes, as seen in the Parthenon in Figure 2 below.
Figure 2: The interior of the Pantheon dome with the oculus at the center
However, as Roman society grew, so did its need for critical infrastructure to support a growing population. Some argue that the changes in material use over time were a response by industry in order to build faster and more efficiently. The following sections discuss these changes in more detail.
Material use dependent on skilled labor limited construction.
Early on, cut stone was most prevalent in the construction of public buildings. However, the rate at which stone could be extracted from the mines and quarries served as a “limiting factor” in the speed of construction (Wilson, 2006). Construction would have likely benefitted from using materials close to the site in order to offset the time it took to produce these materials through lower transport times. Stone such as tuff, made of compressed volcanic ash, is widely available close to the city of Rome. It was used extensively in construction in the form of large blocks and bricks that were mined in and around the city. Figure 3 below provides an illustration of ancient tuff mining at Grotta Oscura, about 15 kilometers north of central Rome.
Figure 3: Illustration of mining galleries in tuff at Grotta Oscura (Adams, 1994)
Tuff was not only widely available, but also soft enough to collect with small cutting tools. The rough shaping of the blocks was conducted in the mines, where the blocks were transported out to the site by road or by river. Stonemasons were required on site to chisel the stones to the correct size and assist in the placement of blocks of tuff during construction (Blagg, 1976). However, this took a considerable amount of time, as early structures were primarily gravity-supported, requiring precise placement of each part of the structure.
Other stones, such as travertine, were often mined near rivers such as the Aniene, where large blocks could be rotated onto wooden barges in trenches while the river was blocked and divided into various sections. Once the barrier between the river and the trenches were removed, they began to fill and the barges were allowed to float. Afterwards, the remaining barriers to the flow of the river were also removed, which lowered the water level upstream and in the trenches. The flow then pushed the barges out into the river. As the Aniene flows into Tiber, this process allowed transport of quarried travertine directly to central Rome. Figure 4 below shows a modern travertine quarry in Tivoli, a region that has several quarries about 20 kilometers east of Rome.
Figure 4: The quarry of Fratelli Pacifici S.p.A. northeast of central Rome
The industrialization and standardization of material production allowed for innovation in construction.
Over time, this process was superseded by the increasing use of brick, which could be produced at a large scale at particular sizes. This standardization allowed for multiple brick suppliers to contribute to a single project, which increased the speed at which construction could be completed (Wilson, 2006). Mortar was required with brick use, and was primarily produced with lime and gypsum (Delaine, 2021). During construction involving significant uses of brick, it was common to find kilns on site to provide mortar for building. Figure 5 below show an example illustration of a lime kiln as described by Cato.
Figure 5: Illustration of a lime kiln cross-section (Adams, 1994)
The use of bricks also allowed for the use of more unskilled labor during construction, making a larger number of workers available for each task at a time. Skilled stonemasons were no longer a determining factor in the speed of construction. While these methods were not a new discovery, the scale of Roman material production was unmatched until about the 14th or 15th centuries (Wilson 2006). The next section will introduce some of the methods used in the construction of major elements used in Roman structures.
The modular nature of Roman infrastructure provided organized methods for its construction.
Several basic, but repeating elements served as the foundation elements of Roman structures. These include walls, arches, and columns. The figures below illustrate some examples of these basic elements in Roman construction. The Baths of Caracalla are one example of Roman wall building in public infrastructure that is evident today. However, much of what is left would not have been visible except for during construction, as the remaining eroded concrete cores were faced with brick, which was then faced with more decorative stone, such as marble. Figure 6 shows some of the remnants of this decorative stone veneer, which often displayed reliefs carved into the frieze. In Figure 7, a reinforced example of the arches within the baths are shown, while Figure 8 shows examples of columns used on the outer walls of the Colosseum.
Over time, existing construction methods constantly evolved, often in response to geographic, economic, and technological challenges and discoveries. New developments, such as the addition of pozzolana to concrete, increased the speed and types of uses available for concrete construction, such as for underwater structures. The use of fired clay as a durable and replicable building material offered a substantial improvement over unfired clay, and was used extensively in later periods. In Figure 9 below, a Roman relief from the National Museum in Rome provides a symbolic representation of what construction may have looked like.
Figure 9: A Roman relief showing various work performed during construction (Museo Nazionale Romano)
Among all ruins of ancient Roman structures, the walls are often the last visible remains. Figure 10 below shows a wall painting of Roman builders that gives some insight into how they were built. This particular wall appears to be made of brick, and the workers along the top of the wall are holding trowels to presumably spread mortar for placement of the next layer. The two workers on the lower left are transporting small pallets of brick for use by the bricklayers. At this height, scaffolding is used, similar to how walls are constructed today.
Figure 10: Wall painting showing a scene of builders at work from the Hypogeum of Trebius Justus along the Via Latina in Rome (Museo Nazionale Romano)
Earlier wall construction used opus incertum, which consisted of blocks of tuff placed outside an inner concrete core. This was superseded by opus reticulatum, which is a more uniform method of facing that uses tuff chiseled into bricks, which are angled and shaped as triangles to promote bonding with the concrete fill. Ultimately, opus testaceum became the most prevalent method of wall construction due to the development of industrialized brick production, as discussed in the previous sections. This method is simlar to opus reticulatum, but instead, standard size clay bricks are used. The addition of bonded tiles at vertical intervals up the wall appears to be in response to the growing height of structures built within this method, which required curing of the previous concrete layers to avoid settling. Figure 11 below shows some examples of these techniques.
Figure 11: Various Roman wall building techniques, including opus incertum (1), opus reticulatum (2) with stone block “quoins” (3), and opus testaceum (4) with bonded tiles layered across the concrete core (5). Foundation walls were cast in wooden formwork (6) (David Darling).
Evidence for the use of opus testaceum at the Baths of Caracalla is prevalent. It is considered the second largest Roman public bath, only smaller than the Baths of Diocletian. Large wall sections required a more robust method for wall construction in order to support the significant vertical and lateral loads imposed by the wide barrel vaulted ceilings. In the overview of the Baths of Caracalla in Figure 12 below, much of the remaining concrete and interior is exposed.
Figure 12: Overview of the Baths of Caracalla
Closer inspection of the walls is shown in Figure 13 below, which allows some assumptions to be made about how they were constructed and in what order. The size of the holes in the brick and concrete facing suggest that large timbers were placed during construction to support another level above the ground floor. Smaller holes seem to indicate that they were used to anchor either the scaffolding or the decorative stone facing, which was secured to the brick with bronze ties.
Figure 13: Putlog holes above the floor level at the Baths of Caracalla, suggesting that timbers were placed here to support a second floor
The illustration in Figure 14 below gives an example of the scaffolding during the construction of the Baths of Caracalla.
Figure 14: Illustration of the possible scaffold system used during construction of the Baths of Caracalla (DeLaine, 1997)
While walls constituted the primary structural support of each building, arches provided a practical means of supporting the functional and aesthetic features, including entrances, windows, halls, bridges, and arcades. Figure 15 below shows an example of arch construction. Prior to the placement of the voussoirs, or wedge-shaped stones, wooden supports, called centering, are used to hold them in place until the keystone completes the arch, which is held in place in compression.
Figure 15: An example of wooden centering used to support an arch prior to completion (Britannica)
One example of public infrastructure that relied heavily on arches were the aqueducts. While much of what is visible today include the elevated arcades of aqueducts such as the Aqua Claudia, the majority of Roman aqueducts were constructed underground. In order to transport water from the source about 45 kilometers east of Rome, tunnels were dug through the foothills along the Aniene to maintain a steady grade. Figure 16 below shows a surviving arcade segment of the Aqua Claudia at the Parco degli aquedotti in Rome.
Figure 16: Overview of an elevated section of the Aqua Claudia
Figure 17 below provides a closer view of the arcade section of the Aqua Claudia. Reinforced brick has been used to provide structural support at each end of this remaining section. Since the arches produce lateral loading on the columns, an unintended consequence of removing other sections of the arcade can lead to further collapse of the remaining ones.
Figure 17: The Aqua Claudia in the Parco degli acquedotti in Rome
The arcade sections constructed for earlier aqueducts primarily used stacked stone blocks placed by heavy lifting equipment, such as wooden cranes, pulleys, and levers. Stones were fitted in place by skilled workers, while centering, scaffolding, and other temporary and wooden structures were reused as each section was completed (Wilson, 2006).
Columns were used for more than just structural purposes. They were also used as decoration, and in many cases, were used as monuments with various themes, such as the Column of Marcus Aurelius in Rome shown in Figure 18 below.
Figure 18: The Column of Marcus Aurelius in the Piazza Colonna in Rome
While early columns were chiseled from single blocks of stone, the majority were composed of several smaller stone disks that were stacked in the shape of a column. This is the case with the monument columns, such as the Column of Marcus Aurelius, and Trajan’s Column. Other columns are structural columns, such as those shown in the portico of the Temple of Antoninus and Faustina in Figure 19 below.
Figure 19: Overview of the Temple of Antoninus and Faustina in the Roman Forum
Figure 20 below shows the columns along the portico of the Temple of Antoninus and Faustina in closer detail.
Figure 20: The Temple of Antoninus and Faustina in the Roman Forum, with the Church of San Lorenzo in Miranda constructed inside.
Where there are no official records, it is often left to imagination when considering what construction looked like in ancient Rome. However, after closer examination, it becomes clear that there was a legal and practical order when constructing public buildings. The changing use of materials and the means through which they were used in construction are reflected in the walls, arches, and columns that are inherent in each Roman structure.
While there are vast voids in our understanding of construction in ancient Rome, traces of history along with modern interpretation provide for a clearer picture of the construction process in antiquity. Through drawings, ruins, and historical reference, it becomes more evident that despite the vast difference in time between then and now, some aspects of ancient construction are not unlike those that exist for construction today.
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Blagg, T. F. C. (1976). Tools and Techniques of the Roman Stonemason in Britain. Britannia, 7, 152–172. https://doi.org/10.2307/525771
Britannica, T. Editors of Encyclopaedia (2020, May 15). publican. Encyclopedia Britannica. https://www.britannica.com/topic/publican
DeLaine, J. Production, transport and on-site organisation of Roman mortars and plasters. Archaeol Anthropol Sci 13, 195 (2021). https://doi.org/10.1007/s12520-021-01401-5
DeLaine, J. (1997). The Baths of Caracalla: A study in the design, construction, and economics of large scale building projects in imperial Rome. Journal of Roman Archaeology, Supplementary Series Number 25, Portsmouth, RI.
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Wilson, A. (2006). The Economic Impact of Technological Advances in the Roman Construction Industry Actions. Innovazione tecnica e progresso economico.