When Grecian and Roman architecture was first introduced into this country from Italy, roofs began to be made lower, and the rafters were three-fourths of the breadth of the building : this was called true pitch, and subsequently the square seems to have been considered as the true pitch. In these several gradations of changes, the material for the covering has been supposed to be impervious stone or slates ; and the roofs themselves to be those which cover ordinary dwellings ; for, after the Italian architecture began to prevail in the last century, platform roofs, and those of a pediment pitch, were introduced in many sumptuous man sions and public edifices ; hut the mate rial employed for covering was lead. At the present day, when good slates are to be had in abundance, we can execute roofs to the Grecian declivity; but with regard to the general practice, the pitch of the roof depends on the style of ar chitecture introduced in the buildings ; the proportion of the pitch, in ordinary dwellings, is between one-third and one fourth part of the span ; mansions and public buildings are executed in every style that has prevailed in different times and among different people ; and the proportion of the roof, as well as other parts, are rigidly adhered to ; this con sequently produces a great diversity in the heights.
There are some advantages in high pitched roofs ; they discharge the rain with greater rapidity ; snow continues to lie a much shorter time on their surface, and they are less liable to be stripped by heavy winds.
Low roofs require large slates, and the utmost care in the execution ; but they have, however, this advantage, that they are much cheaper, since they require shorter timbers, and consequently much smaller scantling; besides, they have less pressure on the walls. The roof is one of the principal ties to a building, when executed with judgment, as it hinds the exterior walls together. There are a va riety of forms in the vertical section of roofs, besides the simple and customary one above mentioned. The figure of the roof depends on two or more vertical and horizontal sections. A span, or pent roof, is that which stands upon walls of a quadrangular plan, and of which the transverse vertical section ;s every where a triangle throughout its length, and slopes from two opposite sides. A hipt, or Italian roof is that, the sides of which incline alike to the horizon, and termi nate either in a point, line, or raised plat form. Vitruvius calls a hipt roof, which from a rectangular plan, a testudi nated roof, or simply a testudo. When the plan of a roof is a parallelogram, and when the vertical section across the two opposite walls, which have not a greater span than that across the other two walls, consists of four sloping sides on the out: side, each two forming an exterior an gle, the roof is called a curb or mansard roof, whether there are gables on the other two sides of the building, or the different sides of the roof, equally in clined, all around, upon each respective wall.
Figures of roofs which rise from square, rectangular, and polygonal plans, forming only exterior angles on the outside, and which terminate in a point over the cen tre of the plan, are denominated from the base on which they rise, and from a vertical section passing through the apex perpendicular to any one of the sides of the base and to the horizon; that is, a roof standing upon a square pentagonal, or oc tagonal plan, having a triangular vertical section, is called a square pentagonal or octagonal pyramidal roof; when such a roof is said to be polygonal, the epithet only applies to the figure of the base. An octangular roof is one whose base is an octagon, whatever be the form of the vertical section. All roofs, the horizon tal sections of which are similar figures, either polygons as above described, or circles or ellipses, and the vertical sec tions of which are segments of convex curves, such as of circles, ellipses, para bolas, &c. are called domes ; hence a square dome is one that rises from a square plan; an octangular dome, from an octangular plan ; a circular dome from a circular plan ; and an elliptic dome from an elliptical plan. Domes upon cir cular plans are called cupolas. A circular or elliptical roof, the vertical section of which consists of two similar and equal concave curves meeting in the apex, is called a trumpet mouthed roof. When the roof is circular or elliptical, and the vertical section an isosceles triangle, the apex of which is that of the roof, the roof is simply called a conical or conoidal roof. When the vertical section of a circu lar dome is a parabola, hyperbola, or el lipsis, the dome is then called a parabo loidal dome, a hyperboloidal dome, or ellipsoidal dome, these epithets com prehending both the base of the figure and vertical section. All figures of roofs, which insist on the foregoing bases, whatever be the form of their verti cal sections, are called by the general name of pavilion roofs, as they only cover one simple building. Frdm the intersec tions of two or more simple roofs of the same or of different kinds, a multitude of complex figures will be formed : the plans of some of these are denominated by letters of the alphabet, as an ell roof is one which rests upon a plan in the form of the letter L a tee roof upon a plan in the form of the letter T; and an aitch roof upon a plan formed like the letter II ; but when two common roofs, having their ridges parallel to each other, and a side of the one either joins one of the other, or these two sides intersect each other, and thereby leave a gutter above the roof; then the roof which is thus com pounded of the two simple roofs is call. ed an em roof, as the vertical section is in the form of the letter. : or rather an in verted W as M : this is an instance where the roof is denominated by the vertical section, and not by the plan. All roofs whatever are said to be truncated, whe ther they terminate in a plane or raised platform, or have a void at the top, bound ed by a level curb.