To mark out the shape of sheet to form a right-angle elbow for a square valley gutter (Fig. 182) is an easy matter. The girth is first laid out (Fig. 183) by setting along the width of bottom, depth of sides, and breadth of flanges, the cut for the mitre being formed by making a b on the pattern equal in length to the line A B on the section, and c d equal to C D. The flanges or laps for riveting are added on as shown. After the joint is riveted, it should be carefully soldered along to prevent leakage.