Another use to which tin is frequently put is the making of metal-covered doors for the fire protection of exposed windows and other openings. Many city laws and insurance rules require that all windows within thirty feet of a source of danger from fire shall be protected by metal-covered shutters of some kind, and a wooden shutter completely encased in tin has been found to be an effective protection. For protecting openings in party or division Avails, the same kind of doors are used, one on each side of the wall, leaving an air space between of about the thickness of the wall, as in Fig. 164, and these doors are usually required to be set in rebated frames, and hung by various automatic devices of weights and fusible members, so that they will close at once if attacked by fire.

Fig. 163. Framing of Metal Bay Window.

Fig. 1(54. Fire Doors.

The best construction of these doors consists of a frame made up of pine 1 3/4 inches in thickness with flush panels, covered with tin tacked on as described for roofing, but for shutters and small doors, two thicknesses of 7/8-inch pine, nailed together and crossing each other, are used. As much care is necessary to preserve a tight interior for outdoor shutters as for a roof, as a small leak will soon rot out the pine core. The joints are all made by locking and tacking the tin, and no solder can be used, as the heat of a fire would melt it at once.