Engineering Rome

The Roman Streets


The streets of Rome are arguably one of the most iconic parts of the city. Anyone who has come to Rome has noticed that these streets are very unusual. The stones are locally nicknamed “sampietrini” or “little Saint Peters” because of their initial use in the Piazza San Pietro, or what is known as St. Peter’s Square directly in front of St. Peter’s Basilica. Not only are the sampietrini noticeably difficult to walk on, but rolling suitcases becomes an unexpected challenge as the wheels get caught in the large gaps between them. After the realization that they probably brought the wrong pair of shoes, travelers will notice several different things as they take their time walking along these stones. First, they notice their dark, charcoal coloring. Then, they’ll see that these stones are placed in a very aesthetically pleasing pattern. some of which are particularly mesmerizing. Finally, if they’re really paying attention, they’ll notice the asphalt sitting between them. The stones did not always look this way, however. And, as the Italians like to say, there is a reason for everything. There is a long story about how the stones appear as they do today, and this story starts all the way back in Ancient Rome.

The history of Roman road building is an interesting one. As the city’s transportation changed from horse drawn carriages to motor vehicles, and their population changed from mostly citizens to mostly tourists, their roads needed to change as well. The aesthetic patterns and asphalt that travelers notice is not accidental. These changes were made to solve problems, but some solutions have consequently caused other issues. This paper dives into the reasons behind these changes and explores the effects of them. Topics including effective load distributing and drainage are discussed following a detailed history of how Romans built their first roads, which is where this story begins.

This analysis and photos in this paper come from an area in Rome called Campo de’ Fiori. It is located just east of the Tiber River and southeast of Vatican City. A large scale and small scale map of the area is shown below for reference (Figure 1).

Figure 1: Map of Campo de’ Fiori

The First Roman Roads – A Detailed Summary

Although the Romans did not invent paved roads, they were one of the first civilizations to use them extensively and develop a long-term plan. The over-arching concept for their road system and organization was for no major point in the empire to be isolated (Cosentino 2017). Their long, straight roads provided easy transportation of soldiers and goods. This road system was well developed and complex, with many parallels that can be seen between ancient roads and those of modern day. These similarities can be identified in many areas, including road nomenclature/ categorization, surveying and construction techniques, and the employment system.

The Romans had clear definitions for their different types of roads. The two major categories were called Viae and Strata, which could both be further divided into Actus, Iter, Semita, and Callis roads (Nibby). These names and definitions are summarized in Figure 2.

Figure 2: Summary of Roman roads. Source: Kellie Jaenicke

The way in which the ancient Romans name their roads can be paralleled to how modern day Italy uses similar terms. Common street descriptors such as Via, Corso, Largo, and Calle are all used to describe typical streets, main streets or avenues, wide or broad streets, and narrow pathways, respectively.

As it does today, construction of ancient roads began with surveying. Since the Romans were building before the invention of compasses, they used a sundial for navigation in order to set the road in the desired direction. Another commonly used surveying tool was a device called a groma. The groma was composed of two perpendicular sticks of equal length (the stelletta) perched on a vertical staff (the ferramento). Each end of the stelletta had a plumb line hanging from it. The stelletta and ferramento were connected by the rosto, which had a standardized length, and allowed the stelletta to rotate freely about a vertical axis; one parallel to the ferramento (Cornelius 2012). A visual of this description is shown in Figure 3.

Figure 3: Diagram of a typical groma. Source: Edilio Boccalieri

The groma was used for surveying straight and orthogonal lines (Cornelius 2019). With the help of the plumb lines, a groma was set vertically at each station and the stellettas could be aligned. Alignments could either be straight forward/backward, or 90 degrees to either side. Straight roads with orthogonal intersections allow for a neat, organized grid like street pattern, similar to how modern cities are laid out today.

Ancient Roman roads were built in three layers. The first of these was called the rudus, which consisted of a roadbed that was dug 30-60cm deep and was filled in part way with small stones and crushed brick for a solid foundation. The stones and brick were compacted with lime and other materials called pozzolans (Cartwright 2019). Pozzolans are materials that, when mixed with lime, have properties similar to that of cement. Having a solid foundation beneath a paved road is key to its longevity. If the only surface the paved road has to bear against is the soft soil, the road will quickly deform as heavy objects move across it. This method of providing a foundation for paved roads is used in modern construction, as well.

The second layer, the nucleus, consisted of finer gravel or sand. Finally, the third layer, called the agger or pavimentum, was a finished surface of stone rocks or slabs made of silica or limestone, since these materials were readily available nearby (Cartwright 2019). A visual of this layering is shown in Figure 4.

Figure 4: Layers of a typical ancient Roman road. Source: Cartwright 2019

Figure 5 shows the remains of the ancient Appia Antica. Small, newer sampietrini stones are placed along with the large, original stones. The sampietrini were most likely placed for maintenance and preservation of the modern day tourist attraction. Looking at the original stones, they are seen as weathered, with a seemingly random arrangement. The stones have moved and misplaced overtime, allowing for gaps large enough for weeds and grass to grow between them. However, when these roads were paved each stones was cut and shaped carefully to fit the ones surrounding it. This surface was once smooth, with gaps between stones smaller than the width of a knife (Cartwright 2019).

Figure 5: The ancient Appia Antica Road. Source: Kellie Jaenicke

The Romans even integrated a drainage system in the design of their roads. Stagnant water along these roads was prevented by setting the stones in the middle of the road slightly higher than the rest (Cartwright 2019). The rainwater would then flow down the sides of the road and drain into trenches that were dug along the side and were filled with small rocks and sand.

Most of the work building roads was performed by soldiers (Cornelius 2012). The empire knew that the soldiers would build these roads well since they had a large incentive to do so. More specialized workers were given job titles. Gromatici were the land surveyors who used the groma and sundials to set the road locations and paths. The agrimensori were the engineers who spotted the precise points where the road needed to pass through, and marked these points using poles. The libratores were responsible for digging, and, finally, there was a curator who was responsible for maintaining the roads within their specified area. The curators work mostly consisted of replacing loose stones (Cornelius 2012). Most of these jobs are seen in modern day road construction, but with different titles. Land surveyors are still used to set road paths and directions, contractors come to dig the soil where the road will lie, and the owner is generally responsible for the maintenance of the road.

The Cobblestones 1585 – Switching to the Sampietrini

The sampietrini were first used in 1585 but were not popularized until 1736. When the stones were becoming more popular, the Cooperative of the Selciaroli di Alfrenda assembled a group of skilled workers to place these stones around Rome (Cinelli 2019).

The Cooperative maintained the idea of having specialized workers. These workers included the miner, the ripper, the sbozzatore, the porter, and the paver. The miner prepared the mines and gathered material, the ripper created smaller stones out of larger ones taken from the mines, the sbozzatore created even smaller stones, the porter transported the finished stones to the paver, and the paver placed the stones in the roads (Cinelli 2019). They did this by sitting on the ground holding a mallet, and methodically placed the stones into the sand base. The pavers had a great deal of knowledge about how to place these stones. They had different tools and techniques that were used depending on the situation.

The first sampietrini stones were made of leucitite that was found in the Capo di Bove lava flow. This flow formed around 280,000 years ago by the Alban Hills, which are what remain of the ancient Latium Volcano (Selsi 2019). These stones are characterized by having white spots scattered throughout the material. Today, most of the original stones have been replaced with manufactured ones, which do not contain these white spots. Another defining sampietrini feature is the truncated pyramid shape that is embedded in the sand when it is placed (Figure 6).

Figure 6: Typical stone from Campo de’ Fiori. Source: Kellie Jaenicke

These stones were implemented due to their superior strength compared to the ancient stones. Typical basalt and limestone have a compressive strength of 100-300MPa and 30-250MPa, respectively, while the sampietrini stones have a compressive strength of 241-320MPa (Zoccali 2017). This increased strength was desired since the population of Rome was increasing, and the roads needed to be able to accommodate the influx of carriage traffic.

Sampietrini are produced in standard sizes. This was most likely due to efficiency and ease of production. Referring back to the ancient roads where each stone was carved specifically to fit to its surrounding stones, this process was incredibly time consuming and labor intensive. By implementing standard sizes, speed of production increases significantly and roads could be paved relatively quickly. Within theses standard sizes, the smallest stones are 6cm by 6cm while the largest ones are 12cm by 12cm. Larger stones have a typical depth of 18cm while smaller stones have a depth of 6cm (Zoccali 2017). a range of stone sizes is shown in Figure 7.

Figure 7: Display of different stone sizes. Source: Kellie Jaenicke

One major difference between the sampietrini pavements and the ancient roads is the foundations. As stated above, the foundation for the ancient stones was designed to be rigid and resist movement to the traffic above. The sampietrini stones are placed in sand and were originally designed to move with the earth. The irregular ground surface around the city made this type of foundation ideal, since the stones can undulate with the soil as heavy traffic moves across it.

Shapes and Changing Patterns

With increased pedestrian traffic due to population and tourism, and introducing heavy motorized vehicles it became more beneficial for the street pavers to place the stones in certain shapes (Cosentino). The most common shapes include the straight vertical, herringbone spines, tiered rainbows, and peacock tails. Some of these shapes are displayed in Figure 8. Different patterns are chosen based on the type and amount of traffic, as these patterns are efficient load distributors.

Figure 8: Comparison of different stone patterns. Source: Kellie Jaenicke

The straight vertical pattern meeting a diagonal pattern in the left side of Figure 8 was taken at Piazza Trilussa. This area receives large amounts of pedestrian traffic. The reason for the sudden change in pattern is not entirely clear, but one reason for it could have been give support the direction in which people typically walk.

The tiered rainbow in the right hand side of Figure 8 consists of adjacent columns of stacked arc patterns that form an diagonal pattern where they overlap. This is indicated by the red dots in the Figure.

This arrangement for the stones seems to make sense for heavy traffic areas. Arches are used widely in Roman construction because of their ability to distribute and control load patterns. The arc patterns for the stones are arranged so that the car reaches the keystone first (note the direction of the vehicles in the Figure). This orientation makes sense because the horizontal load from the forward moving car will be put in the correct direction for the arch to distribute. Figure 9 shows a visual of the load pattern for a layered arch design compared to a vertical stacked design. The stones are shown in blue, the external load from the tires is shown as a distributed load on top of the stones, and the load distribution is shown by the black arrows within the stones. In the arch design, the load from the vehicle can be set both vertically down to the stone beneath it, or horizontally to an adjacent stone. In this way, the load is being distributed among more stones in more directions, thus reducing the amount of load on each individual stone. In addition, the excess load, when it reaches the end of the arch, is concentrated and moved down in a stair-step fashion between the set of arches due to the diagonally stacked stones. In the vertical stacked example, the load from the car is distributed straight down the same few columns of stones. The load is unable to move horizontally since the stones are stacked directly on top of each other. The heavier, undistributed load puts more vertical pressure on stones, pushing them together resulting in a state of compression.

Efficient distribution of load is important for preventing two consequences: damage and large displacements of the stones. Although the stones are generally strong, it is important to acknowledge that these streets were not originally meant to carry 3000+ pounds per vehicle. If the load is not distributed, the increased compression could crush the stones and make maintenance even more difficult. In addition, the large load on fewer stones is likely to cause larger displacements, which will also increase the difficulty of road maintenance. Because the arch design minimizes both compressive forces and displacements, it appears to be a superior choice in high traffic areas.

Figure 9: Load distribution for different stone patterns. Source: Kellie Jaenicke

Adding Asphalt

As heavy traffic becomes a larger issue, more changes needed to be made to try and sustain the streets. Adding asphalt in the gaps between the stones is one of these major changes. The stones are placed in sand, and the lateral forces on the stones caused by the forward and backward motion of a vehicle were causing the stones to displace in large amounts. One of the advantages for using the stones in the first place was that they could move and accommodate the motion of the earth beneath them. However, this adaptability became more of a hindrance when heavier vehicles started to travel about them. Asphalt has been a common solution to this problem but has caused several other issues. The most evident of these is drainage.

To reiterate, one of the great advantages for using the sampietrini stones was that their drainage was very effective. Water could simply seep through the gaps between them and into the earth. However, when asphalt, an impermeable material, fills these gaps the water can no longer seep through. A tree tree located in Piazza della Quercia, near Campo de’ Fiori square, provides an example that this is true. As Tom Rankin explained, the tree, before the asphalt was placed, received its water from the rainwater that came through the stones. However, after the asphalt was placed the tree was no longer to receive adequate amounts of water. The tree died and was then replaced by another tree of the same type. With no other changes made to the system the tree, not surprisingly, died again. Once the reason for why the tree was dying became clear, the city input a seemingly extensive network of drains surrounding the tree for the water to reach it (Figure 10). The tree has not since died after these drains were placed.

Figure 10: Tree in Piazza della Quercia. Source: Kellie Jaenicke

Major consequences of placing asphalt between the stones appears when large amounts of rainfall hit the city. Since the ground is not able to absorb the rainwater, it quickly accumulates on the surface where it stays until it evaporates or runs into a nearby drain. However, the very irregular surface characterized by small mounds and valleys prevents the water from flowing in several spots (Figure 11). If the ground were permeable, these mounds and valleys would not be an issue, since the water could still flow into the earth. Now that this is not an option, the water stands still and forms large puddles.

Figure 11: Flooding during thunderstorm in Campo de’ Fiori. Source: Kellie Jaenicke

In addition, as more asphalt is placed over time and less rainwater seeps into the earth, the drains are required to catch more water and they start to overflow (Figure 12). One reason for this drain overflow could be that these drains were not originally designed to intake these quantities of water. Pavements and their respective sewer systems are typically built off experience and what amounts of water they are expected to receive. It is likely that these sewer systems were placed before asphalt was widely used. More water was able to permeate through the stones rather than the sewers. When asphalt is placed more water runs into the sewers rather than directly into the ground. As more asphalt is placed around the stones this cycle continues until the sewers can no longer keep up with the incoming amounts of water.

Figure 12: Sewer unable to drain water after heavy rain. Source: Kellie Jaenicke

Another consequence of adding asphalt that contributes to the inadequate drain capacity comes from another source. The sampietrini streets have many people walking along them at all hours of the day. Campo de’ Fiori is especially known for this because of its daily market. Each day, vendors set out their food products and clothing items, creating large amounts of trash. This trash, in combination with the trash produced by common people throwing their cigarette butts and other small pieces of garbage of the ground, gets swept away by the flowing rainwater and taken to the sewers where it collects (Figure 13). This amplifies the issue of overflowing drains. It is already difficult for the drains to intake the increased quantity of rainwater, and now that the rainwater carries garbage along with it the clogged drains can intake even less.

Figure 13: Clogged sewer drain. Source: Kellie Jaenicke


The Roman streets have changed a lot from when they were first placed about 2000 years ago. In several attempts to adapt to increasing populations and heavier vehicles, some issues have been resolved but many others have arisen. Standard sizes are a significant improvement from ancient times, and placing patterns achieve efficient load distributions for modern day vehicle traffic. However, the addition of asphalt has caused issues with local flooding and clogged drains.

The story of the Roman streets is not yet over; the city is actively removing more of the sampietrini stones and replacing these areas entirely with asphalt. Changes to the streets thus far have all been implemented because they are thought to be beneficial. However, unforeseen issues are bound to reveal themselves overtime, as has been shown. It is up to the city to combat these issues and form long term solutions.


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Cosentino, Gianluca. “Stone Road Pavements.” Stone Road Pavements, Sapienza Universita di Roma, Uniroma, 2017,

Nibby, Antonio. “Delle Vie Degli Antichi.” Wikisource, Wikisource, 14 Sept. 2015,

Robustini, Pasquale. “The Geology of Rome’s Sampietrini.” Pasqualerobustinicom,, 9 Aug. 2018,

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Zoccali, Pablo, et al. “Sampietrini Stone Pavements: Distress Analysis Using Pavement Condition Index Method .” Applied Sciences, Applied Sciences, 29 June 2017, file:///C:/Users/kelli/Downloads/applsci-07-00669%20(1).pdf.

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