This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
A mansard roof is framed in very much the same way as is a gambrel roof, as may be seen in Fig. 187. Resting on the main wall plate A, we have a piece B which is inclined slightly inward, and which supports at its upper end a secondary 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 studding, 2X4 inches to 4X6 inches in size, depending upon the size of the roof. It supports 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 A. The piece of studding I is merely a furring stud to form the wall of the attic room. It may be omitted entirely if desired, or if the attics are to be unfinished. The ceiling 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 ceiling joist K fastened to the upright B rather than to the furring stud I.
Fig. 186. Method of Finding Contour for Gambrel Roof.
Fig. 187. Framing for Mansard Roof.