Copper Gutter Sizing: The minimum required size of a copper gutter is related to the intensity of rainfall and the area of roof that drains into the copper rain gutter. The two depends on the length of the copper gutter, which is related to locations of copper downspouts, copper expansion joints, and copper gutter ends.

Other factors considered in the design of copper gutters, include the size and spacing of outlets, the shape of the copper gutter, and the pitch of the roof that the gutters will be associated with. The copper gutter size must be capable of handling even fast moving water from a step roof.

Copper Downspout Sizing: Copper Downspouts should have a cross-sectional area of at least 7 square inches, except for small areas such as porches and canopies. Their size should be constant throughout their length.

The design roof area is divided by the area of roof shown in Table 4.4.2, column A2 or B2 (see discussion above), to give the minimum required area for each downspout.

Table 4.4.4 is used to determine the required width and depth of a copper gutter. To do this, a ratio, M that equals the depth divided by the width, is initially assumed. Starting with the length of the copper gutter, L, follow a vertical line until the ratio, M, is reached. At this point follow a horizontal line to the left until the vertical line of rainfall intensity x design area, IA, is crossed. The required copper gutter width can be read from the diagonal lines. If the intersection lies between two lines, use the higher value. Finally, the width is multiplied by the ratio, M, to determine the depth.

The size of copper gutters with an irregular shape can be determined by calculating the required size of a rectangular copper gutter, which closely matches in profile and cross-sectional area, the irregularly shaped copper gutter.

Table 4.4.5 shows an example of the complete process.