One of the most frequently asked architectural questions SMACNA receives is on sizing gutters and downspouts. In response, SMACNA’s Technical Resources Department has created a free Downspout and Gutter Sizing Calculator.
The Downspout and Gutter Sizing Calculator is located on the Tools, CAD, and Apps page of the SMACNA website.
Now architects, engineers, designers, and contractors can easily and accurately size downspouts and gutters according to the specifications in SMACNA’s “Architectural Sheet Metal Manual,” 7th edition, 2012.
Includes Rain Intensity Wizard
The calculator includes a Rain Intensity Wizard enabling one to select the nearest city and the information taken from the tables in the “Architectural Sheet Metal Manual.” The rain intensity according to Table 1-2 (Rainfall Data and Drainage Factors) uses a 5-minute duration for a 10-year or 100-year storm, thereby providing a worst-case scenario for the downspout and gutter design.
Since the rain intensity data is constantly being updated by the National Oceanic and Atmospheric Administration (NOAA), this calculator also allows the user to manually enter the rainfall intensity. This feature allows one to use calculations according to the local authority with jurisdiction in that area. For the most up-to-date Rain Intensity Data, visit NOAA’s website and use “Precipitation Intensity” as the data type.
Design Area Wizard
SMACNA’s new Downspout and Gutter Sizing Calculator enables the user to manually enter the Design Area of the roof or use the Design Area Wizard for the calculation. This Wizard makes it easy to calculate the roof Design Area by providing various shape calculations and the calculation for Vertical Walls area per International Plumbing Code (IPC).
The gutter and downspout size requirement depends upon the number of sections and length of the gutter sections as well as the number of downspouts. Adding additional sections of gutter reduces the length of each section and will reduce the gutter size. Also, the gutter width in relation to height may also be adjusted. Plus, adding more downspouts will change the downspout volume, resulting in smaller gutters/downspouts.