ROMAN ALUM. An alum extracted from the volcanic rock of the Solfaterra near Naples : it crystallizes in opaque cubes, and appears to contain more alu mina than the common octohedral alum. ROOF. In architecture, the upper most part of a building, containing the timber work, with its covering of slate, lead, tile, or other material. Carpenters, however, restrict their use of the word to the timber framing alone.
The inclination of the sides of a roof will, considering the species of covering to be the same in all, depend very much on the temperature of the country to which it is to be adapted. In the south ern and warm countries roofs do not re quire much elevation, while as we pro ceed northward they require a far greater pitch. In the warm, or, rather, hot cli mates, buildings require nothing more than a terrace for their covering ; but in the temperate climates, wherein the lati tude exceeds 42°, experience shows that the flat covering of a building'eannot be practised with any expectation of dura bility. The rains of hot climates are vio lent, while those of temperate climates are searching. In the more northern lati tudes, the moisture, the driving nature of the rain, and, in addition, the duration of the snow on the roofs, require, it is obvious, a more considerable inclination. Such materials as lead, copper, zinc, and the like, which, supposing them one piece, as, in fact, when used, they ought to he, are not fair examples from which to draw inferences in the theory whereof we speak ; for, if well executed, they must either of them be considered i one homogeneous piece : but in the case of tiles, whether of marble, stone, or clay, the case is far different. Without enter ing minutely into the details of this sub ject, we will merely observe that, sup posing the inclination of a roof to be zero at the equator, if we add to it an inclina tion of three degrees for every climate from the equator to the polar circle, each climate being taken at 2 42' 80", we ob tain results which show that the roofs and pediments of temples of antiquity must have been well studied in that useful point of view which regarded their dura bility and impenetrability by rain. The theory would give an inclination to roofs at Athens of 164°, and they are very nearly so inclined ; the temple of Miner va being 16°, and that of the temple of Eretheus 154°. In Rome, according to this theory, the inclination of a roof, and, consequently, pediment, should be 22° ; and experience finds it varying from 23° to 24I°. The advocates for the propriety of strictly copying Greek forms and de tails under the latitude of London will, if they have studied the esthetics of the art, find no little difficulty in establishing their doctrines after weighing this mat ter impartially. But our limits prevent farther observation : we will merely sub join a table conformable to the theory : A roof as respects its construction, in volves some knowledge of mathematics. Of the general principles on which its proper construction depends, we shall here subjoin some account. The obvious mode of covering a building, where a greater or lesser inclination of the sides of the roof is required by the climate, is to place two sloping rafters upon the wallS. If the walls be not of sufficient
weight, the thrust that will be thus ex erted on them by the tendency of the rafters to spread at their feet will throw the walls out of an upright, and the whole assemblage will be destroyed. By the laws of mechanics it is known that the horizontal thrust thus acting on the walls is proportional to the length of a line draivn at right angles to the rafter, inter secting a vertical line drawn from the apex, which it is manifest must increase as the roof becomes flatter. To counter act the thrust above mentioned, nothing more is necessary than to tie together the feet of the rafters by a tie beam. If the extent be not very great, the rafters may be kept from spreading by a minor tie, called a collar. Beyond certain lengths or spans, however, it will occur to the reader that a tie-beam will itself have a tendency to bend, or sag, as the workmen call it, in the middle ; and from this cir cumstance a fresh contrivance becomes necessary, this is called a king-post, or, more properly, king piece, inasmuch as it does riot perform the office of a post, but rather of a tie, for it ties up the beam to prevent its bending. If the rafters be so long as to be liable to bend, two pieces called struts, are introduced ; which, hav ing their footing against the sides of the king post, act as posts to support or strut up the rafters at their weakest point. The piece of framing thus eontrived is altogether called a truss. It is obvious that by means of the upper joints of the struts we obtain more points of support., or rather suspension ; and that but for the compressibility of the timber, there would be no limit to the space which a roof might be made to cover. This com pressibility takes place at those points where the fibres of the wood are pressed at right angles, or nearly so, with their di rection, and many ways are adopted for avoiding this inconvenience. The curb or mansard roof is one in which a story is obtained. Its principles are the same as those already mentioned, and do not here require farther notice. In the exe cution of roofs the expense of trussing every pair of rafters would be unnecessa ry, and the practice would also load the walls with a far greater weight than would be expedient : it is therefore the custom to place these principal parts of a roof at certain intervals, which, however, should never exceed ten feet. The raf ters which are actually trussed are called principal rafters ; and by the intervention of the purlene are made to bear the small er or common rafters, which are notched down on it. These common rafters are received by or pitch upon a plate called ; and the principal rafters, which fall on the tie-beam, are ultimately borne by the wall-plate.
When beams, in either roofs or floors, are so long that? they cannot be procured in one piece, two pieces to form the re quired length are scarfed together, by indenting them at their joints and bolt ing them together, of which practice twc modes are here subjoined.