The commonest form of a gutter angle is perhaps of a square elbow for a half-round gutter (Fig. 176). It may be made out of thin galvanised sheet, say 24 to 20 gauge, having a bead or flange along the edge and a soldered joint, or, as in the sketch, made out of 16 or 14 gauge black iron, riveted at the joint and galvanised or painted afterwards. The setting out of the pattern, which is explained by Fig. 177, is a simple matter. A semicircle is described, as shown, and divided into six equal parts, and the girth line of the pattern made the same length as the semicircle either by calculation or marking along six lengths, each equal to one of the parts on the semicircle. From each of the division points on the girth line a perpendicular is run up, and from the points on the semicircle lines are drawn parallel to the girth line. The intersection of corresponding lines will give points on the pattern curve. Thus, where the line drawn up from 2 on the girth line intersects the line drawn through 2 on the semicircle will give point 2 on the pattern curve. In the same manner all the other points can be determined. A free curve being drawn through the points, the net pattern is complete. The lap for flanging is added, as shown by the dotted line. The arm of the gutter angle which fits inside will, of course, not require any lap. It will also be an advantage to have the girth of this arm a little less than the other, the side lines of the pattern being cut slightly tapered, as shown, by the two dotted lines running along the sides of the pattern.
A good deal of care is necessary in the flanging, this being best carried out in the case of thick gauges by throwing over when hot. The holes for rivets should be punched in the plate after flanging, the holes for the inside arm being marked from these and punched by the use of a burr.