Gutters and downspouts
Half-round gutter size selector
Compare listed half-round gutter diameters. Preliminary size comparison for one section and outlet.
Compare half-round sizes
Choose the smallest listed diameter from the July 2007 half-round capacity table. This dated comparison covers the half-round profile only.
Adjustable example. Change these illustrative inputs to match your task, then recalculate. Reset restores the example and its result.
Preliminary table comparison only. Outlet, downspout, overflow, supports and local-code requirements are not checked. K-style, rectangular profiles and unlisted sizes remain outside this selector.
Capacity values used
Maximum design area in sq ft at 1 in/hr. Divide by the entered rainfall intensity to find the design area served.
| Diameter (in) | Level | 1/8 in/ft | 1/4 in/ft |
|---|---|---|---|
| 3 | 680 | 960 | 1,360 |
| 4 | 1,440 | 2,040 | 2,880 |
| 5 | 2,500 | 3,520 | 5,000 |
| 6 | 3,840 | 5,440 | 7,680 |
| 7 | 5,520 | 7,800 | 11,040 |
Only the listed 3–7 in rows are compared. Larger diameters are outside this selector. No extrapolation is made.
Understand the result
How this calculation works
The size selector compares a listed half-round capacity table with a rainfall condition.
Steps
- Enter plan area, rainfall intensity and section length.
- Select the listed roof factor and gutter slope, then compare the returned candidates.
The form starts with the worked example below. Change its values and selections to match your task; Reset restores the example.
Formula
factored area = plan area × roof factor; demand = factored area × rainfall intensity; select the smallest listed capacity at the selected gutter slope that is at least demand.
Variables and units
Plan area: square feet. Rainfall intensity: inches per hour. Section length: feet, up to 50. Roof factor and gutter slope: only the listed table options.
What the result means
Only the listed 3–7 inch half-round rows and table conditions are compared. No interpolation or code approval is made.
Worked example
Inputs: Area = 600; Intensity = 4; Length = 40; Roof pitch band = 6:12 to 8:12 (factor 1.10); Gutter slope = 1/4 in/ft.
Calculation: For 600 sq ft at 4 in/hr, a 6:12–8:12 roof band (factor 1.10), a 40 ft section and 1/4 in/ft gutter slope: 600 × 1.10 = 660 sq ft; demand is 2,640. The 3 in row holds 1,360 and fails; the 4 in row holds 2,880 and fits.
Result: 4 in half-round.
No listed half-round size fits
Inputs: Area = 100000; Intensity = 6; Length = 40; Roof pitch band = 12:12 (factor 1.30); Gutter slope = 1/4 in/ft.
Calculation: For 100,000 sq ft, factor 1.30 and 6 in/hr, demand is 780,000 sq ft·in/hr. The largest listed quarter-slope capacity is 11,040, so no listed size fits.
Result: No listed size fits.
Frequently asked questions
Can it size K-style gutters?
No, the current table is half-round only.
Does the roof factor multiply sloped area?
No, it is the source table band.
What if no size fits?
That is a valid outcome requiring a separate design review.