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 gambrel roof is framed in very much the same way as is a pitch roof or a hip roof. The slope of the roof, however, is broken at a point between the plate and the ridge. The part of the roof above this break makes an angle with the horizontal plane of less than forty-five degrees usually, while the portion below the break makes an angle with the horizontal plane greater than forty-five degrees. This is shown in Fig. 185.
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. 185. 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, forming 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 7. 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.
Fig. 185. Framing for Gambrel Roof.
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. 18G. 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 E 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 E 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.