The Romans only developed fired clay bricks under the Empire, but had previously used "mud brick", dried only by the sun and therefore much weaker and only suitable for smaller buildings. Development began under Augustus, using techniques developed by the Greeks, who had been using fired bricks much longer, and the earliest dated building in Rome to make use of fired brick is the Theatre of Marcellus, completed in 13 BC. The process of drying bricks in a kiln made it so these bricks would not have cracks in them when they dried. The "mud brick" took a very long time to dry and limited brick creation to certain seasons. The fire dried brick allowed the brick production to increase significantly, which created a mass production of bricks in Rome.
Roman brick was almost invariably of a lesser height than modern brick, but was made in a variety of different shapes and sizes. Shapes included square, rectangular, triangular and round, and the largest bricks found have measured over three feet in length. Ancient Roman bricks had a general size of 1½ by 1 Roman foot, but common variations up to 15 inches existed. Other brick sizes in Ancient Rome included 24" x 12" x 4", and 15" x 8" x 10". Ancient Roman bricks found in France measured 8" x 8" x 3". The Constantine Basilica in Trier is constructed from Roman bricks 15" square by 1½" thick. There is often little obvious difference (particularly when only fragments survive) between Roman bricks used for walls on the one hand, and tiles used for roofing or flooring on the other, and so archaeologists sometimes prefer to employ the generic term Ceramic Building Material (or CBM).
The city walls of Constantinople
, showing several courses of brickwork
The Romans perfected brick-making during the first century of their empire and used it ubiquitously, in public and private construction alike. The mass production of Roman bricks led to an increase in public building projects. Over time the public and private relationship diminished as the brick business turned into an imperial monopoly. The Romans took their brickmaking skills everywhere they went, introducing the craft to the local populations. The Roman legions operated mobile kilns and introduced bricks to many parts of the empire. The bricks became time records and geographical pinpoints to where the Roman military was operating. Roman bricks are often stamped with the mark of the legion that supervised their production. Roman brick was used to construct famous architecture such as the Red Basilica in Pergamon, Domus Tiberiana in Rome, and the Basilica of Maxentius in Rome. The use of bricks in southern and western Germany, for example, can be traced back to traditions already described by the Roman architect Vitruvius, although he probably refers to mud brick. In the British Isles, the introduction of Roman brick by the Ancient Romans was followed by a 600–700 year gap in major brick production.
When building in masonry, the Romans often interspersed the stonework at set intervals with thin courses of bricks, sometimes known as "bonding tiles". This practice gave the structure added stability. It also had a secondary aesthetic effect of creating a polychromatic appearance.
In the 1530s, the English antiquary John Leland successfully identified Roman bricks (albeit under the misleading designation of "Briton brykes") at several geographically dispersed sites, distinguishing them by size and shape from their medieval and modern counterparts. This has been described as one of the earliest exercises in archaeological typology.
After the Western Roman Empire fell in the 5th century many of the commercial stone quarries in Europe were abandoned. This led to a consistent pattern of reuse of Roman building materials throughout the next several hundred years. Like much of the Roman stone, Roman bricks were gathered for reuse throughout this period. For example, in the 10th century the abbots of St. Albans gathered enough Roman brick during this time period to have their own stockpile of the building material.
When brick production resumed in earnest on the British Isles, the 1½" to 2" height of the Roman-style brick gradually increased during the early Medieval period. Brick from the Ancient Roman Empire was commonly reused in medieval Europe as well as in later periods. This reuse can be found across the former Roman Empire. In Great Britain, where construction materials are less plentiful, Roman structures were quarried for their stone and brick and it was commonly reused. Examples of this type of reuse in Great Britain can be found in Anglo-Saxon churches at Brixworth, Corbridge, St. Martin's, Canterbury, and St Nicholas', Leicester, and also in St Albans Abbey church (now St Albans Cathedral).