The lower slope may almost be considered a part of the wall, and at the point where the slope changes there is a secondary plate from which the upper slope starts, as shown at A in Fig. 1S5. The secondary plate may be utilized as a support for the ends of the ceiling joists B, which should also be securely spiked to the rafters, as shown in the figure. The rafters C forming the upper slope, must be cut over the plate A, and firmly spiked to it, while at the top they rest against a ridge board D. The rafters E, ing the lower slope, are cut out at the top so as to form a seat for the plate A, and must be very securely fastened at the bottom to the main wall plate F. It is an excellent plan to have the floor joists G spiked to the lower rafters, so as to act like tie beams across the building and to counteract the outward thrust of the rafters. Sometimes these floor joists are dropped below the wall plate F, and are supported on a ledger board notched into the wall studding I. This construction is not so good as that shown in the figure, because the joist is not so effective as a tie across the building. If it is employed the floor joist must be securely nailed to the wall studding I, and they must not in any case be dropped more than 2 or 3 feet below the plate. The plate must always be firmly nailed to each stud to prevent it from being forced outward as it receives the thrust from the rafters E.
A good rule for determining the point at which to place the secondary plate, and for determining the general shape of the roof, is illustrated in Fig. 186. Let the points A and B represent the main plates on each side of the building. Draw a line A B between them and bisect this line at C. With C as a center and C A as a radius describe the semicircle A D F F B. At any distance G above A B draw a line D F parallel to A B, cutting the semicircle at the points D and F. Also bisect the arc at E. Then by joining the points A D F F and B by straight lines as shown, we will have the outline of a gambrel roof. The proportions of the roof may be varied by varying the distance G.
Gambrel roofs are not very strong unless they are stiffened by cross partitions in the attic stories, and these should be provided whenever it is possible. No gambrel roof, unless it is well braced, should be used on a building which is exposed to high winds, or which is likely to receive a heavy weight of snow.
Mansard Roof. A mansard roof is framed in very much the same way as is a gambrel roof, as may be seen in Fig. 157. Resting on the main wall plate A, we have a piece B which is inclined slightly inward, and which §upports at its upper end a second ary plate C. On the plate C rests the outer end of the deck rafter D which is nearly horizontal. The piece B is a piece of stud ding, 2X4 inches to 4X inches in size, depending upon the size of the roof. It sup
ports the whole weight from the rafters, carrying this weight to the main wall plate and thence into the walls of the building. This member should always be straight, and the curved shape which is usual on mansard roofs is obtained by the use of the furring piece E. The piece E is nailed to the upright member B at the top, and at the bottom it is secured to the lookout F, which also forms a support for the projecting cornice. The floor joist G is supported on a ledger board H, or it may rest directly on the plate ii. The piece of stud ding I is merely a furring stud to form the wall of the attic room. It may be omitted en tirely if desired, or if the attics are to be unfinished. The ceil ing joist K may be supported on a ledger board as shown, or may be simply spiked to the studding I or to the upright B. The studding I may rest directly on the floor joist G with a sole piece L at the bottom as shown. The plate C should be of a good size, at least 4X6 inches, and should not be placed more than 2 or 3 feet above the ceiling joists K. The ceiling joists act as ties across the building and prevent the plates C from spreading apart, as they receive the thrust from the rafters D. For this reason it is better to have the ceil ing joist K fastened to the upright B rather than to the furring stud I.
Dormer Windows. In Figs. 1SS and 1S9 are shown what are known as dormer windows, this name being applied to all windows in the roofs of buildings, whatever may be their size or shape. The figures show two different kinds of dormer windows which are in general use, the one shown in Fig. 1SS resting entirely on the roof, while the one shown in Fig. 1S9 is merely a continuation of the wall of the building above the line of the caves. The second type is often seen on buildings only one story in height, while the other kind is employed on larger structures.
In order to construct a dormer window an opening must be made in the roof surface, and the window must he built up over the opening. Headers are framed in between two of the rafters as shown at A and B in Fig. 190, and thus a rectangular opening is formed in the roof frame. The rafters C and D, which form the sides of the opening, arc called trimmers and should be much stronger than the common rafters. Usually the trimmers are made by doubling the ordinary rafters. The headers receive the ends of the rafters which are cut by the opening, and must be large enough to earn, the weight which comes from them besides supporting the walls of the dormer. Timbers 4X S inches to 6X 10 inches, according to the size of the dormer, are usually large enough for the headers and often smaller timbers may be safely used.