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.
When it is desired to obtain the admission of light to the space under the roof surface in a way more elaborate and satisfactory than is possible with a simple skylight such as has just been described, recourse is had to dormer windows. They are so called probably because in early times nearly all of the sleeping-rooms of the houses were under the roofs and were lighted by means of such windows. They are formed by framing an opening in the roof surface in the same way as described for skylights, but on the rafters and on this framing are built up vertical walls of a height sufficient to take a small window set in vertically. The method of building the rough framework has been already described. The walls of dormer windows are treated in the same way as the walls of the main building, being covered with shingles or clapboards or, in some cases, with slates. Fig. 293 shows the most simple form of dormer-window roof. This is a simple roof with a pitch in one direction only, less steep than the pitch of the main roof so as to allow of enough vertical wall in the front part of the dormer to accommodate the window, and uniting with the main roof at a point farther up on this roof. The sides of the dormer are covered solid with shingles or whatever other covering is used, the front only containing an opening. This arrangement is the same for all dormer windows, there being no advantage in putting openings in the sides. The roof, shown in Fig. 293, sheds water onto the main roof in front of the dormer window and, therefore, there should be a gutter along this front and here the eaves should project somewhat over the wall, as shown. At the sides there need be no projecting eaves. A section through the gutter would be similar to that shown in Fig. 283, though the gutter itself may be a little smaller and the rafters are of course smaller. The piece marked A in Fig. 293 serves as a fascia to cover the ends of the rafters and this fascia should be continued up on the sides of the dormer as shown at B in Figs. 293 and 294. (Fig. 294 is a section to a larger scale through the side wall of the dormer window near the roof.) This fascia is cut out as shown, to receive the shingles which cover the side walls. The gutter marked C runs along the front of the dormer and should be made to miter at the end with a molded board marked D, which runs up on the side. A section through this molded board is shown in Fig. 294. The distance E in this figure must be the same as the distance E in Fig. 293, but as the distance G is not the same as the distance F (both in Fig. 293), the profile of the molded board D will not be the same as that of the gutter C. This is a principle which will often be met with in gable finish. In Fig. 294, H is the roof surface of the dormer, J is the end rafter, K is the boarding on the wall, and L is the boarding on the roof. If it is desired that the eaves shall project beyond the walls on the sides as well as on the front for the sake of effect or to better protect these side walls, this result may be accomplished, as shown in Fig. 295, by blocking out the fascia and the molded board as far as necessary. In this figure A is the fascia, B is the molded board, C is the blocking, and D is the roof surface of the dormer, E is the wall of the dormer. The blocking C should consist of pieces of 2-inch stuff cut to the required shape and spaced 1 1/2 feet to 2 feet apart along the line of the eaves to receive the finished pieces A and B. F is a soffit piece added to box in the eaves and G is a secondary fascia added to receive the shingles or other covering.
Fig. 293. Front and Side View of Simple Dormer Window.
Fig. 294. Section through Side Wall Dormer Window.
Fig. 295. Section of Dormer Side Wall Showing Boxed Construction.
The type of dormer roof just described throws the water forward onto the main roof of the building, but this arrangement may be varied by allowing the roof to drain sideways like an ordinary double-pitched gable roof. In this case there will be gutters on the eaves at the sides of the dormer window and a gable on the front. Such a window is shown in Fig. 296, in which A represents the line of the finished roof surface, while B is the roof of the dormer window shown in side elevation, C being the side wall of the dormer itself. D is the gutter at the eaves. A section through the eaves at D would be very much like that shown in Fig. 283. E is a fascia covering the ends of the rafters and the eaves may be either open or boxed in. The fascia E is usually continued around the front of the dormer window as shown at F, projecting as far as may be desired beyond the front wall G. A molded board, cut in such a way as to miter with the gutter, is carried up the side of the gable end, and this piece is generally known as a "raking molding," or a "raking mold." It is similar in shape to the molded board shown at D in Fig. 294. A section through the fascia where it runs across the face of the dormer is shown in Fig. 297. Here A is the fascia board which is nailed to the ends of pieces called "lookouts," about 2 inches thick and spaced from 1 to 2 feet apart, as shown at B. The lookouts may be nailed to the studs as shown in the figure or they may be merely nailed to the outside boarding, but the method shown is the better one, as it gives the lookouts a very much firmer support. The under side of the lookouts should be sheathed with 7/8-inch stuff as shown at C, with a piece D to receive the shingles. The upper surfaces should be covered also with sheathing and on top of this a covering of galvanized iron, copper, or tin to shed water. Besides this the tops of the lookouts should be cut with a pitch outward, as shown at E, to facilitate the shedding of water. In this figure, F is the studding and G is the outside boarding which comes in the triangular space marked H in Fig. 296. A section through the raking molding K, in Fig. 296, is similar to that shown in Fig. 295.
Fig. 296. Dormer Window Construction with Gutters on Side of Dormer Roof.
Fig. 297. Section Through Fascia Board.
Fig. 298. Gambrel Roof Finish.
Although the two types of dormer windows described are the basis from which all other types have been developed, still there are many kinds of dormers which have quite a different appearance. They are all, however, similar in construction to the two types shown, the difference being in the way in which the wall covering is applied and in variations in the proportion and in the shape of the windows. The ones shown are the very simplest of their respective kinds, but they serve to illustrate the manner in which all should be constructed.