Large skylights, and those having a gable or hipped roof, can be made much better of galvanized iron or copper than of wood, but small skylights or glazed scuttles, when necessary for lighting an attic room, may be constructed of the latter material when not within the fire district.

Such skylights usually consist of a glazed sash through which light is admitted, and the frame on which the sash rests and to which it is usually hinged.

Fig. 165.

When on a pitched roof, the skylight or sash is usually placed parallel with and about 8 inches above the roof. The proper method of constructing such a skylight is shown in section in Fig. 167. An opening is first framed in the roof by means of header and trimmer rafters and the frame spiked to the inside of the opening, This frame should be made of 2-inch or 2 -inch plank 11 inches wide. Quite often the frame is made of 6-inch or 8-inch rough plank nailed on top of the roof, the inside flush with the rough opening, and the opening and frame cased with finished boards or ceiling. This method, however, is not as good as the one shown, as the wide planks add to the stiffness of the frame and opening and prevent the two from separating.

The sash is framed together in the same way as window sash, but should have no cross bars or muntins, and the lower rail should be made so that the glass will pass over it. The rails and stiles should be 2 inches wider than the thickness of the frame, and a 7/8-inch strip should be nailed to the under side of the stiles, outside of the frame, to protect the joint.

Fig. 166.

For economy in the glass, and also to stiffen the sash, the latter is usually divided into lights, about 12 inches wide, by longitudinal muntins or sash bars, as shown in the isometric view. The glass is usually set in putty at the top and sides, but at the bottom the top of the glass is left free to shed water. If the length of the sash is not more than 36 inches each light should be of one piece of glass. When it is greater than this the lights may be glazed with two or more pieces lapped over each other {about 1 inches), as shown in the section.

Fig. ,67.

Greenhouse roofs are glazed in this way, the divisions often being 8 or jo feet long and glazed with small lights of glass. The thickness of the sash should not be less than 1 inches, and if the frame opening is greater than 3x4 feet the thickness should be increased.

The most important items in connection with a skylight of the kind shown are the flashing and provision for taking care of the condensation that always forms on the under side of the glass, if the room below is warmed or occupied.

Behind the top of the frame a gutter should be formed as shown, the board B being cut so as to be highest at the middle and falling to each side. The lining of this gutter should extend well up on the roof, and should be turned over the edge of the frame into a groove which should be graded to drain off the water at the sides. If the sash is to open it should be hinged at the top and a strip of lead nailed to the top rail, as shown at H, to form a counter flashing, If the sash is stationary a simple fillet may be nailed to the under side of the sash above the frame. The sides of the frame should be flashed with tin (or zinc) shingles the same as around a chimney, the flashing being carried to the top of the frame.

At the bottom of the frame it is better to use a wide piece of galvanized iron for the flashing, as this will stay in place better than tin or zinc.

For taking care of the water of condensation a small gutter should be formed in the flashing, as shown at D. As the water forms on the glass it runs down until it strikes the lower rail and then drops into the gutter.

For a small skylight the water in the gutter will evaporate so that it will not overflow, but on larger skylights provision should be made for draining off the water by means of a small pipe carried through the frame.

On large skylights, also, if made of wood, the sash bars should have a cross section like that shown by the enlarged section, gutters being formed at G to receive water that may run down on the sides of the bars. These gutters should empty into the gutter under the lower rail. Unless some such provision is made for receiving the condensation, much trouble will be experienced by water dripping on the floor.

The sash is usually fastened by a flat iron bar, provided with holes to slip over a pin, so as to both secure the window and to hold it open at certain distances. The frame and sash should be made of clear, well-seasoned cypress, white pine or redwood.

When a skylight of the style described above is placed on a flat roof it may be made in the same way, only making the frame higher at one end than at the other, so that the sash will have an inclination of about 2 inches to the foot. On flat roofs the frame or curb may be set on top of the roof.

Fig. 167a shows another detail, which is in some respects superior for large skylights.

Scuttles in a roof should have a frame and be flashed in the same way as described for skylights, and the corner should set down over the frame and be covered with tin or copper.