The construction of curb roofs is usually quite simple, the common method being about as indicated in Fig. 83. The wall plate may be either just below the attic joists, as shown, or it may be above the joists, but is seldom more than 2 ½ feet above the joists, as it is desirable that the dormer windows shall come above the plate. If the plate is above the attic floor joists, the latter should be anchored to the wall if of brick, or well spiked if of frame, and the wall plate bolted to the wall, or well spiked to the top of the studding if of frame, as the plate is depended upon to resist the outward thrust of the lower rafters.
Where the two sets of rafters meet at the curb, a plate or purlin, at least 4x6 inches, should be placed, and the upper end of the lower rafters cut so that the latter will support the plate ; this plate should receive the upper rafters, and, if the height of attic story will permit, the ceiling joist also.
If the attic ceiling comes below the curb plate the rafters may be supported by a purlin, as shown at B. These purlins should be tied together across the roof at least once in 14 feet. A roof framed as shown in Fig. 83, and without any cross partitions, would be likely to collapse under a severe gale or from a heavy weight of snow on one side only; hence, if the roof space is not divided in length by cross partitions, bracing should be employed. As a general thing such roofs are further stiffened by dwarf partitions, as shown by the dotted lines.
The construction of mansard roofs will depend, in a great measure, upon the outline. Fig. 84 shows the construction of two of the most common shapes. In both of these drawings the weight of the roof is carried directly to the wall plate, the 2x4 uprights, in Section C, being considered merely as a partition or furring, which could be entirely omitted if desired. The curve of the roof should always be formed on 2-inch furring pieces built out from straight rafters, which should take the weight of the roof and always rest on the wall plate. The wall plates should not be more than 2½ feet above the floor joist, and should be so secured that they cannot spread.
Mansard roofs are generally accompanied by heavy projecting cornices, which, if of wood, may be formed as shown in Fig. 84. The cornice at B is supported by plank " lookouts " built into the wall and secured by spikes driven through the plate. These lookouts may be from 2 to 3 feet apart.