=44. Material for Roof Covering.=—The roof covering of this class of building is either of slag on 2-inch spruce plank, spiked to nailing strips bolted on to steel purlins from beneath, with lagscrews, or of slate laid on 1-inch or 2-inch sheathing boards. Even galvanized iron is used for the roofing of some of the cheapest class of buildings, especially those which, owing to the process of manufacture, are subjected to a high temperature. =45. Construction of Sides of Building.=—The sides of these buildings may be covered with either expanded-metal lath on metallic furring strips, plastered inside and out with cement mortar so as to form a fireproof and rigid screen wall about 2 inches in thickness; or, the walls may be 9-inch or 13-inch brick walls built part way up the height of the columns and leaving the columns exposed on the face; or, corrugated galvanized iron lapped 6 inches and secured either by riveting to metallic supports or nailed to wooden studding secured to the steel frames. Of these constructions, probably the first is the most expensive and also the most satisfactory. [Illustration: FIG. 21] =46. Partially Supported Steel-Frame Building.=— In Fig. 21, there is designated a type of construction that may be built for about $1 per square foot of the area covered. This consists of steel =I= beams, or angle-and-plate columns, used for column supports carrying the usual angle iron steel-roof truss. The roof is sheathed with 2-inch spruce tongued-and-grooved planking, covered with a good quality of roofing felt and slag, with a stop-gutter _a_ at the edge. Owing to the fact that the steel columns are supported in a direction of their minimum radius of gyration by means of the brick walls, they can be made very light. The building illustrated has what is known as a _saw-tooth roof_. By this means, light is obtained on the side next to an adjacent and higher building by means of a sash _b_. This sash is usually made hinged or pivoted, to provide the necessary ventilation. =47.= In Fig. 22, there is illustrated, diagrammatically, the framework of a one-story skeleton-construction building. In the design of all such buildings, where there are no end gable walls, the several columns and trusses must be braced diagonally, as indicated at _a_, _a_, and frequently it is necessary to introduce a secondary system of horizontal bracing from one panel point on the lower chord to another, as indicated at _b_, _b_. [Illustration: FIG. 22]