Gutters and Downspouts:
Sample Gutter Sizing Calculation

Table 4.4.5 - Example Calculation

Select round downspouts and size rectangular gutters for a building in Chicago, Illinois. The building is 120' x 80' with a gable roof having a pitch of 5 in. per foot. The slope is toward the long side. Maximum rainfall conditions will be used to determine downspout size.

Downspout spacing is restricted by two factors: each downspout should drain no more than 50 feet of gutter; and gutter expansion joints should be spaced no more than 48 feet (see Plate 4.4.3). Three downspouts will be used on each side, with expansion joints in the gutters 40 feet from the ends. Each downspout therefore, will drain 40 feet of gutter.

Downspout Selection:

The roof plan area that is drained by each downspout is,
PLAN AREA = 40' x 40' = 1600 SF

Given the Area Factor, B, in Table 4.4.1, the design area is,
DESIGN AREA = PLAN AREA x B = 1600 x 1.05 = 1680 SF

From Table 4.4.2, column B2, the area drained per square inch of downspout is 130 SF. The minimum downspout size is,
MIN. DOWNSPOUT AREA = 1680 / 130 = 12.9 SQ. IN.

From Table, plain round 5" downspouts, with an area of 19.63 square inches, will be used.

Gutter Sizing:

The roof area that is drained by each gutter is,
AREA = 40' x 40' = 1600 SF

From Table 4.4.2, column B1, the rainfall intensity is,
I = 9.3 in/hr.

IA = 9.3 X 1600 = 14880

On Figure 4.4.4, draw a vertical line representing IA = 14880.

Initially assume the gutter width ratio, M, is 0.75. On Figure 4.4.4, find the vertical line representing L = 40'.
Follow the vertical line to its intersection with the oblique line representing M = 0.75.
Follow a horizontal line to the left to the intersection with the vertical line drawn previously representing IA = 14880.
This intersection occurs on the oblique line representing a gutter width of 7".

The gutter depth should be at least,
MIN. GUTTER DEPTH = WIDTH x M = 7 x .75 = 5.25"