The Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) is now providing a simple and effective Downspout & Gutter Sizing Calculator. This tool allows engineers, architects, designers, and contractors to easily and accurately size downspout and gutters per SMACNA’s Architectural Sheet Metal Manual.
The calculator’s Rain Intensity Wizard allows you to select the city nearest you and utilize the information taken from the tables in Architectural Sheet Metal Manual 7th Edition – 2012 for your calculations. The Rain Intensity per ASMM Table 1-2 uses the 5-minute Duration for a 10yr or 100yr storm, thereby providing a worst-case scenario for your 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 manual entry of the Rainfall Intensity, thereby allowing you to calculate per the local authority having jurisdiction in your area. For the most up to date rain intensity data, NOAA’s website is http://hdsc.nws.noaa.gov/hdsc/pfds/index.html. (Use Precipitation Intensity as your data type.)
The gutter and downspout size required is highly dependent on the number of sections and length of those gutter sections and the number of the downspouts.
- Adding additional sections of gutter thereby reducing the length of each section will reduce the size of the gutter required. Also, the gutter width relation to height may also be adjusted.
- Adding additional downspouts will change the downspout volume resulting in smaller gutters/downspouts.
You may manually enter the Design Area of the roof for the calculation or use the Design Area Wizard. 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.)