The form of curb roof known as the "mansard" roof is supposed to have been invented by Francois Mansard, a distinguished French architect of the seventeenth century. This shape of roof afterward became very common in France, and is therefore often referred to in this country as the "French" roof. Curb roofs were very commonly used on the early Colonial buildings of this country, but they were quite different in their proportions from the French type. The Colonial curb roofs also received the name of "gambrel" roofs.
By means of a mansard or curb roof the same amount of available space may be obtained more cheaply than by means of a gable roof, and the outward thrust on the walls is not as great with a mansard roof as with a pitch roof.
On the other hand, mansard roofs, where they stop on top of the wall, have the disadvantage that the gutter is not so easily freed from snow, and such roofs are also considered more inflammable than roofs of one-half or less pitch.
The New York and Chicago building laws require that roofs whose inclination with a horizontal is greater than 6o° shall be of fireproof construction, except (in New York) on dwellings less than 35 feet in height.
The curb roof is frequently used in this country on dwellings, and particularly on those in the Colonial style. The mansard roof was the prevailing type of roof on pretensious dwellings thirty years ago, but is now little used except on city dwellings built in blocks, and on public buildings, for which it seems to be particularly appropriate.
As regards the proportion or pitch of mansard roofs there appears to be no recognized rule.
Rivington's "Notes on Building Construction," Part II., gives the method shown in Fig. 80 for determining the outline of the roof. A semicircle is described on a horizontal line connecting the wall plates, and is divided into five equal parts. The chords, 0 1 and 4 5, give the lower lines of the roof, and the upper lines are obtained by drawing chords from 1 and 4 to the center of the semicircle.
If the angles of the roof lay in the line of a parabola, instead of a semicircle, the beams would, theoretically, be in equilibrium; that is, the outward thrust of the upper rafters would be just balanced by the inward pressure of the lower ones, but as any inequality in the loading, or the least wind pressure, would immediately disturb the equilibrium, this consideration is not of much practical importance.
As a matter of fact, the outline of both mansard and curb roofs is generally determined by the consideration of external effect, although the pitch of the upper portion will depend somewhat upon the roof covering, a rise of 6 inches to the foot being the least that should be given to roofs that are to be covered with slate or shingles. For curb roofs the outlines shown in Fig. 81 may be considered as representative of modern American practice. Diagram A is the outline of one end of the Governor Brooks house, at Medford, Mass., which was built in 1764, and is fairly typical of the roof outlines of that period. Diagrams B and D are taken from modern Colonial houses having two stories below the roof, and diagram C gives the outline of a one-story house designed by Mr. W. R. Emerson, who has been particularly successful with this style of architecture. Diagram E shows the outline of the wings of the Garden City Hotel, Garden City, L. I., designed by Messrs. McKim, Mead & White, architects.
The numbers shown in connection with the roof lines indicate the rise of the roof in one foot of horizontal projection.
Diagrams A, B, C and D, Fig. 82, and section C, Fig. 84, are representative outlines of mansard roofs as used on dwellings in this country thirty years ago, and diagram E is typical of the modern mansard roof, the curved outlines being now seldom used.
City houses built in blocks, when surmounted by a mansard or steep-pitch roof, generally terminate in an ornamental cresting, as shown at E, the main roof being covered with tin or gravel roofing and pitching toward the rear of the house. Very frequently a balustrade or parapet is placed above the wall cornice and in front of the base of the roof. Mansard and curb roofs are almost invariably-pierced by dormers.