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.
In Figs. 188 and 189 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. 188 resting entirely on the roof, while the one shown in Fig. 189 is merely a continuation of the wall of the building above the line of the eaves. The second type is often seen on buildings only one story in height, while the other kind is employed on larger structures.
Fig. 1S8. One Form of Dormer Window.
Fig. 189. Another Form of Dormer Window.
In order to construct a dormer window an opening must be made in the roof surface, and the window must be 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, are 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 carry the weight which comes from them besides supporting the walls of the dormer. Timbers 4X8 inches to 6 X10 inches, according to the size of the dormer, are usually large enough for the headers and often smaller timbers may be safely used.
The headers are shown in section at A and B in Fig. 191, and it will be noticed that they are not used in exactly the same way. The piece at the top A is so placed that its longer dimension is at right angles to the plane of the roof, while the piece at the bottom B has its longer dimension vertical. In the case shown in Fig. 189, where the front wall of the dormer is merely an extension of the main wall of the building, there is no need of the lower header B, the main wall plate taking its place and supporting the studding for the front wall of the dormer, as shown at the right-hand side of Fig. 191.
Fig. 191 shows sections taken through two dormers of the types mentioned above, parallel to the direction of the main rafters and at right angles to the main wall plate of the building. At the left is a section taken through the type of dormer shown in Fig. 188, while at the right a section of the other type is shown. The studs C C which form the side walls of the dormer, are notched over the trimmer rafters and roof boarding about 1 inch, and allowed to continue downward to the attic floor. This is shown at section D D. At E is a section of the trimmer rafter, C is the wall stud, G is the attic floor boarding, and H is a section of one of the attic floor joists. The studs C are in line with the studs forming the side walls of the attic room, so the studs I can not be carried down to the attic floor. They are stopped, at the bottom, against a 2X3 inch strip K which is nailed to the side of the trimmer rafter. At L is the ridge board, and M M M are the short rafters which form the pitch roof of the dormer. They may be very light, as they are short and carry little weight. They rest, at the foot, on a plate 0, and at the top bear against the ridge board L. In the dormer shown on the right of the figure the rafters P are in planes parallel to the main rafters, and a furring piece S may be nailed to each of them so as to give the dormer roof any desired curve.
Fig. 190. Roof Framing for Dormer Window.
Fig. 191. Framing Details for Both Types of Dormer Windows.
Besides the openings in the roof frame for dormer windows there must be other openings for chimneys and skylights. These are formed in the same way as explained for the dormer openings, with headers and trimmer rafters. A plan of such an opening is shown at E in the roof framing plan in Fig. 174.