Design Area of Pitched Roofs for Copper Gutters: The roof area to be drained is a key factor in designing copper gutters and copper downspouts. The area of roof contributing runoff to each copper gutter and copper downspout should be determined. The maximum accumulation of rainfall occurs when it falls perpendicular to the roof plane. With flat roofs, it is a simple matter of calculating area, since the true roof area is equal to plan area.
When a roof is pitched, its plan area is less than its true area. However, using the true area in the calculations has typically resulted in oversized copper gutters, copper downspouts and drains. Table 4.4.1 shows the factors that should be used to determine the design area for pitched roofs. The plan roof area should be multiplied by this factor. The result is the design roof area that is used to calculate the required sizes of copper downspouts.