Size of Rectangular Roof Gutters and Downspouts According to the SMACNA Design Guide



This sheet determines the required size of rectangular roof gutters and downspouts according to the SMACNA design guide. Nomographs were programmed using tables of discrete values which might cause rounding errors. Check values returned by hand in the nomograph.

Calculation Reference
Fluid Mechanics

The Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) provides guidelines for designing and sizing roof gutters and downspouts to ensure proper rainwater drainage. Here's a summary of the steps to size rectangular roof gutters and downspouts according to the SMACNA design guide:

  1. Determine the rainfall intensity (in inches per hour) for the location. This information can typically be found in local rainfall intensity-duration-frequency (IDF) curves or tables.

  2. Calculate the roof area (in square feet) draining to each gutter and downspout. The roof area includes all sections of the roof that contribute to the flow into the gutter or downspout.

  3. Refer to the SMACNA gutter size chart, which provides the maximum roof area that can be drained for various gutter sizes and rainfall intensities. Select the smallest gutter size that can accommodate the calculated roof area and the local rainfall intensity.

  4. Calculate the flow rate (in gallons per minute) for each downspout using the Rational Method formula:

    Q = CiA

    Where: Q = Flow rate (gallons per minute) C = Runoff coefficient (dimensionless, between 0 and 1, depending on the roof material) i = Rainfall intensity (inches per hour) A = Roof area (square feet)

  5. Refer to the SMACNA downspout size chart, which provides the maximum flow rate that can be accommodated for various downspout sizes. Select the smallest downspout size that can handle the calculated flow rate.

  6. Ensure that the selected gutter and downspout sizes meet local building codes and regulations.

Keep in mind that this is a simplified overview of the sizing process, and additional factors, such as gutter slope, downspout spacing, and roof configuration, may also need to be considered. Always consult the complete SMACNA design guide and local regulations when designing roof gutters and downspouts for a specific project.

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29 Oct 2007
Last Modified
27 Apr 2023
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Comments: 1
roofguy 10 years ago
Can you please send me the runoff.xls? I am having trouble downloading it. Thank you very much!-Shawn