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.
It is sometimes necessary to make an opening in a roof surface for the admission of light to the rooms under the roof. This is usually done by the formation of what is known as a dormer window, the method of framing for which has been already described, but often it is desired to admit light when the attic space is not of sufficient importance to justify the introduction of a dormer window in the roof. In this case recourse is had to a skylight. A skylight may be obtained by the use of glass in the roof surface in place of other roof covering such as roof boarding and shingles, but it is almost always necessary to provide some sort of frame for this glass, and the glass surface is usually raised about 6 or 8 inches above the shingled roof surface so as to keep the glass as free as possible from snow and to separate it from the general roof surface. The first thing to be done is to frame an opening, in the rough, between the rafters, by introducing trimmer pieces just above and below the place where the opening is to be. This completes the rough framing for the skylight. The finished opening may be formed in a number of ways, one of which is shown in Fig. 289. Here, AA are the pieces referred to above, which frame between the rafters BB and form the rough opening. D is the roof boarding which is brought up to the edge of the opening as framed, and sawed off flush with it as shown. CC are pieces of rough stuff which are nailed on top of the boarding to raise the skylight above the roof surface. They may be of any size desired, but in this case they are 4X6 inches. EE is sheathing about 7/8 inch thick which forms a finish for the inside of the skylight opening. It is continued to the top of the pieces CC and down to the plaster line inside under the roof so as to cover up all the joints. This sheathing may be V-jointed or beaded if desired. FF is furring on the under sides of the rafters, and G is the plastering under the roof. HH are finishing pieces covering the joint between the sheathing and the plaster. These pieces are called casing. MM is flashing of some kind of metal, which should be carried well under the shingles or other roof covering all around the skylight, and is carried inside to form a little gutter around the inside of the skylight just under the glass as shown, in order to catch drippings from the glass. A' is the sash which holds the glass. It should be hinged to open at the top at the point marked L and should project an inch or two beyond the frame all around and have a drip cut in it as shown at 0. Fig. 290 shows an elevation and a section through a sash suitable for use in a skylight. AA is the top rail which is ploughed to receive the glass as shown at B. The bottom rail CC is made thinner than the top rail so that the glass can pass over it and project beyond it as shown at D. EE are divisions called "muntins" running lengthwise of the skylight, their distance apart and the number of them required depending upon the width of the sheets of glass used. The muntins support the sheets of glass at the sides, as shown in Fig. 291, which is a section through a single muntin. In this figure, A is the wood muntin itself, BB is the glass on each side, and CC is the putty which is used to hold the glass in place and to make the joint tight. In a skylight sash there should be muntins running lengthwise of the sash only, and the glass should be supported only at the side. If the sash is so long that a single sheet of glass will not cover it, two or more pieces should be used and should be lapped on each other at the ends as shown at P in Fig. 289 and at F in Fig. 290. This lap should be from 1 1/2 to 2 inches. The side pieces of the skylight sash, GG in Fig. 290, should be cut similar to the muntins to receive the glass.
Fig. 2S9. Section Showing Skylight Construction.
COURTYARD AND STAIRCASE IN THE "BARGELLO," FLORENCE, ITALY A Strong and Picturesque Treatment of a Courtyard.
Fig. 290. Plan and Section of Skylight Window.
There are a number of other methods of constructing skylights besides the one shown in Fig. 289. The construction of the sash, however, is always about the same as there shown and as described, the difference being in the form of the frame. The pieces marked C in the figure are sometimes omitted entirely, and the sheathing E, which is only about 7/8 inch in thickness, is replaced by planking 1 1/2 to 2 1/2 inches thick. Such planking is stiff enough to be allowed to project 6 or 8 inches above the roof boarding and it thus takes the place of the pieces C. The sash then rests on the ends of this planking, as shown in Fig. 292. In this figure, AA is the rough framing similar to that in Fig. 289, BB are the rafters, CC is the planking mentioned above, which, it will be noticed, does not extend down to the plaster line on the inside, but is stopped about 3 inches above it and is pieced out with a strip of 7/8-inch stuff, DD the joint between the two pieces being covered and eased off by a molding EE. The architrave HH is made use of in this instance also. Other openings in the roof surface, which are parallel with that surface, and which are for other purposes than the admission of light, such as scuttles, trap doors, etc., may be framed and finished in a manner similar to that just described for skylight openings.
Fig. 291. Cross Section of Muntins.
Fig. 292. Another Form of Skylight Construction.