33. Wooden fire doors may be covered with any kind of sheet metal which has a high temperature of fusion. Copper and zinc are not suitable for this work; galvanized iron and tin are therefore used, though tin seems to have the preference. The covering is generally put on as follows: The outer edges of the door are covered first with a strip of tin, which is returned over both faces to a distance of about 6 inches, the edges being nailed to the woodwork and turned over, as shown in Fig. 20. The flat part of the door is then covered with sheets 14 in. X 20 in., in a manner similar to that employed on flat roofing. This arrangement avoids seams around the edges of the door, and allows it to shut with a close contact.

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(a)

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(b)

Fig. 19.

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Fig. 20.

A heavy body metal should be employed for this work, because the efficiency of the covering in case of a fire depends upon the thickness of the metal, and not upon the thickness or quality of the protecting coat. The seams should all be closely nailed, and then locked and thoroughly pounded down. A layer of asbestos cloth should be placed between the tin and the wood.

Corrugated - iron doors and shutters are also extensively used, but they do not form as efficient fire stops as ordinary wooden doors or shutters which are properly covered with tin and asbestos. The corrugated iron must be thoroughly braced with an angle-iron or T-iron frame. The chief objection to iron fire doors and shutters is that they warp very easily when the flames touch them. Plain iron warps so much that it is practically useless as a fire stop. Fig. 21 furnishes a good illustration of this. The door shown is composed of one sheet of thick metal, and is provided with iron cross-pieces to which the hinges are riveted. The fire plays against the inside of the door and warps the sheet outwards, thus allowing an opening for the flames, as shown.

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Fig. 21.