The arch form is that which is usually adopted for the construction of floors (diagram A), the spandrels of the same being, however, in some cases filled in with the material, so as to form a horizontal floor (diagram B). These arches, when thoroughly set, exert no thrust upon the outer walls, and, in fact, from their slight rise and thoroughly homogeneous character, they possess as much the nature of a beam or landing as that of an arch. For this reason their use is, in many cases, advantageous, where that of brick arches would be altogether inadmissible. The arches are supported at the points where they abut upon the walls by projecting courses of brickwork, and at intermediate points by rolled iron joists or rivetted girders. They have a minimum rise in the soffit of one inch to every foot of width, and they are turned in this proportion up to spans of 10 or 12 feet. Corridors and cottage floors are formed in this manner without the introduction of any joist or beam whatever (diagram C); the soffits of the arches, after the removal of the centres, simply require to be finished with the last or setting coat of plastering. In cellars or other basement offices no extra coat whatever is necessary.
