Roman Architecture

Construction.

Walls were built in two ways, either of ordi nary masonry or of concrete (faced or unfaced). While there are several examples of early stone walling without courses (cy clopean and polygonal) especially in some of the towns, e.g., Norba, Praeneste, near Rome, most of the stone walls existing are built of squared blocks laid in regular courses as headers and stretchers (opus quadratum). The earliest of these walls are of tufa. Later come those of peperino and travertine. The blocks of stone in these walls are fairly large, 2 ft. x 4 ft. or more, and were often held together by iron cramps fixed in lead.

Concrete walls, except below ground, were always faced. They are divided into types according to the kind of facing used. (a) Opus quadratum, i.e., ordinary stone walling, is used as a facing for concrete, especially for important public buildings under the earlier empire, e.g., exterior of the Colosseum. (b) Opus incertum is the most common facing for ordinary concrete walls of the 2nd and 1st centuries B.C. The face of the concrete is studded with 3 in. to 4 in. irregularly shaped pieces of stone, usually tuf a. (c) Opus reticulatum came into vogue in the 1st century B.C. and remained in use until the time of Hadrian. The construction is like that of opus incertum but the pieces of stone were pyramid shaped with square bases set diagonally and wedged into the concrete wall. Quoins 9 in. x 3 in. of the same material or of brick were used at the angles. (d) Brick and tile faced concrete (so called opus testaceum) is by far the most common material for walling under the empire. Triangular tiles were used with their points turned into the concrete and their long sides showing, thus giving the appearance of a wall built of thin bricks. Bonding courses of bipedales were employed at intervals of 2 or 3 feet. (e) Mixed brick and stone facing (so called opus mixtum) was popular under the later empire and especially under Diocletian.

The stone arch occurs frequently in Italy from the middle of the 2nd century B.C. onwards, usually in city gates, bridges and aqueducts. The discovery of concrete, however, enormously facilitated the spread of arch construction. Concrete arches were faced with stone or tile voussoirs, and with the latter bipedales were used at every 6th or 7th voussoir.

The vaults used by the Romans were simpler geometrical forms, i.e., the barrel vault, the intersecting (groined) barrel vault and the segmental vault. By the 1st century B.C. quite extensive systems of barrel vaulting were employed as in the substructions of the Tabularium in Rome, the temple of Hercules at Tivoli, etc.

The later vaults were built up on brick rings about 2 in. apart, joined by brick bonders, forming rectangular compartments which were filled with concrete. Additional layers of concrete were laid above. When set the concrete vault exerted no thrust. The surfaces of the vaults were tile faced or covered with stucco. A fine example of Roman vaulting is the basilica of Maxentius.

The construction of domes naturally follows that of vaults. Here again the fact that the concrete dome was a dead weight without thrust was of the greatest importance in simplifying the problem.. Tie ribs of brick were used and sometimes relieving arches as in the case of the Pantheon where the facing bricks are laid horizontally. At the crown of the dome was a brick ring.

The Orders.

There are five Orders of Roman Architecture, Tuscan, Doric, Ionic, Corinthian and Composite, Tuscan and Composite being modifications of Doric and Corinthian respec tively. The rules followed by the Roman builders were elastic; few examples are of the same proportion and there was much licence allowed in execution. It has, however, been usually sup posed that some system in which form and detail were definitely standardized was essential for the construction of Roman build ings, built as they were at a high speed by ordinary workmen. Marcus Vitruvius Pollio, the Roman writer on architecture, would at any rate have us believe that imperial architecture was brought under some such rules. But the greater part of Roman architec ture is later than Vitruvius, an architect and engineer who lived, and wrote in the time of Augustus. His book (De Architectura) is our great authority for the earlier Roman building and con struction, but was primarily a handbook for architects and is based for the most part on the works of Greeks of the late 4th century B.C. and the Hellenistic period. The Renaissance exalted Vitruvius to the supreme authority on classical building; but it is none the less certain that there were no hard and fast rules, certain general proportions only being observed. It would be impossible here to treat of the details of the Roman Orders (see ORDER). In general the proportion is slenderer than that of the corresponding Greek Order, and there is a tendency towards greater elaboration combined with a decline in the quality of the execution. Columns are often unfluted, but the faces of the entablature left plain in Greek work are covered with decoration.