Another use of metal for exterior work is found in the framework of skylights. These, if large, are made up of light steel angles or T-irons, and covered with sheet metal, sometimes with, a lining in parts of terra-cotta blocks. If of ordinary size they will be formed up of heavy sheet metal, usually galvanized iron or copper, and in a variety of designs suited to various uses. The two principal forms of skylight are the hipped skylight, Fig. 165, and the gabled skylight, Fig. 166. All skylights should be provided with a ventilator of some sort and often a greater amount of circulation of air is obtained by building a vertical wall containing sashes or louvre blinds.

Fig. 165. Hip Skylight.

Fig. 165. Hip Skylight.

Fig. 166. Gable Skylight.

Fig. 166. Gable Skylight.

Of construction similar to large skylights but without the glazing or louvres in the walls, are roof houses; which must often be constructed to give access to roofs or to contain machinery. These, if above forty feet from the sidewalk, will be required by many cities to be of fireproof construction and are usually made of T-irons with a terra-cotta filling, and covered with sheet metal. (Fig. 167.) A precaution which must be taken in regard to the construction is to be sure that the T-irons are set with their flange inside or on the opposite side from the metal covering, or the pounding down of the seams of the metal will be likely to start the terra cotta out of place. On account of this pounding down of the seams, it is often better that the blocks should be of solid terra cotta and not hollow tiles. The connection between these houses and the main roof must be taken care of as described for roof and wall flashings, due allowance always being made for expansion of the metals. In this connection, it may be well to note that zinc for flashings should never be used where it will come in contact with iron, lead, or copper, as this contact will produce voltaic action which will finally destroy the zinc.

Fig. 167. Hollow tile Wall.

Fig. 167. Hollow-tile Wall.