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42. The Conical Roof Flange.—Whenever a smoke pipe is to pass through a roof, it is necessary that a hole much larger than the pipe be cut in the roof in order to lessen the fire risk. In order to render this construction water-tight, a conical roof flange, as shown in Fig. 126, must be used.

The Elevation.—First, the roof line is drawn at the angle demanded by the job specifications (in this drawing 30°). The roof fine immediately becomes the miter line. Next, a "vertical" center line, line 4 in Fig. 127, is drawn in. Upon each side of this center line a distance equal to one-half the diameter of the smoke pipe is set off. A short horizontal line is put in to represent the joint between the pipe and the flange. One-half the diameter of the hole in the roof is set off on each side of the vertical center line. This will locate the low point (point 7) of the miter line. From this point the base of the cone should be drawn at right angles to the center line. The slant height lines may now be drawn by connecting the ends of the base and the ends of the short horizontal line of the junction between the flange and pipe. These lines must be prolonged until they meet at the apex.

The Profile.—A half-profile. Fig. 127, is drawn and divided into equal spaces. The divisions are numbered and an extension carried upwards from each division as far as the base of the cone. From each intersection thus obtained, extension lines are drawn to the apex of the elevation. Where these extension fines cross the miter line, numbers that correspond to the numbering of the profile are placed. Horizontal extension lines from each intersection of the miter line are drawn over to the right-hand slant height line.

The Pattern.—With the apex of the elevation as a center, and a radius equal to the slant height of the full cone, the arc of stretchout is drawn. The spacing of the profile is transferred to this line, and the number of spaces doubled to provide for a whole pattern. The divisions are numbered to correspond. The measuring lines of the stretchout are drawn from each division of the arc of stretchout to the center point (apex).

Starting from point 1 of the profile, the extension lines are