The Romans also employed flat roofs and ceilings. In certain of the baths so much iron has been found amid the debris, that it is supposed the roofs were constructed with a framework of this material, fitted together with T joints. Otherwise the ceilings were made of crossed beams, dividing the space into _coffers_. The exterior of the roofs was covered with a sheathing of terra-cotta tiles or, as in the original roof of the Pantheon, of bronze gilded plates, which now are replaced by lead. =Vault and Wall Decoration.=--Sheathing was also applied to the exterior and interior of the whole structure, forming, as it were, a garment of decoration. In the case of vaulting, the interior decoration was composed of stucco coffering; square, hexagonal, or octagonal panels, inclosed within raised framework that was arranged in a geometrical pattern. Sometimes the coffering was replaced by mosaics; which were of two kinds. 1. _Opus tessellatum_ formed of tesseræ or cubes of marble or glass, arranged in patterned designs that often included figures. 2. _Opus sectile_, in which the tesseræ were cut into various shapes, to form the pattern, as in marquetrie. A rich kind, made of red and green porphyry, was distinguished as _Opus Alexandrinum_. At other times the vaulting and walls were covered with hard plaster, wrought to a fine surface, which was polished and frequently embellished with mural painting. The walls were also overlaid with slabs of coloured marble, in the selection and treatment of which the Romans took a notable pride. Further, both the exterior and the interior walls were relieved with carved decoration, which took the form of pilasters, arches, mouldings, and panels, encrusted with arabesques. These and the other embellishments could be so easily applied to the concrete shell, that Roman decoration had a tendency to become profuse and over-elaborated. Whereas in Hellenic architecture every decorative detail was an intrinsic part of the structure, Roman decoration was something added after the structure was completed. It was, in effect, like clothing, fitted to the form of the body, and varying in design and sumptuousness according to the taste and purse of the wearer. Since architecture generally was an expression of pomp, pride, and power, it was inevitable that the richness of decoration should frequently run to extravagance. To the lay-student, at least, the actual forms of Roman architecture are of less interest than the uses to which they were put. For the Roman genius was displayed in practicalness; in the resourcefulness with which it extended the scope of architecture to serve the necessities and ideals of life. Hence the temple-form has ceased to occupy the chief attention; the truly monumental character of Roman architecture is distributed over a variety of achievements of magnificence and utility.