Copper Gutter and Downspout sizing – Let’s do some math.

Let’s try to answer that age old question posed to Guttersmiths since the beginning of gutters: What size gutters and downspouts do we need?


For those who would like to save some time, check out the gutter and downspout sizing calculator below, provided to us by the awesome folks at Northclad, manufacturer of Rain Screen Cladding and Architectural Panel Systems. The calculator uses numbers from the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) Manual, whose standards and manuals are accepted worldwide by the construction community. Throughout my time in the industry, the SMACNA manual, including its charts and diagrams, have been referenced throughout the construction process and are definitely considered an authority in the construction world.


For those, who would like to manually calculate gutter and downspout sizing, you will need to complete the following steps:

  • Find out the rainfall intensity for the area you’re working in
  • Compute roof area and roof slope (steep slope roofs catch more wind driven rain and should be given a higher factor)
  • Decide downspout locations/spacing
  • Determine size of downspouts and gutters

Rainfall intensity 

The U.S. Weather Bureau records the maximum rainfall that could possibly happen in a 5-minute period, in inches per hour, for various regions. This number changes based on the likelihood of a storm occurring that delivers that amount of water every number of years. Typically the number of years charted is 10 years or 100 years, and the amount of rain varies by location. See this chart to determine rainfall intensity near you.

Calculate Roof Area

You’ll have to calculate roof area for each section of gutter. Simply put, multiply the width by length of the section of roof you’re measuring. Unfortunately, the larger, more complicated the roof, the more difficult this becomes. You have a few different options here. You can go up on the roof with a tape measure (dangerous) or grab approximate measurements from the ground. You can also pull a Google satellite view of your home, which offers a surprisingly accurate scale, but you will need to multiply the area by a roof pitch factor to account for slope. The easiest to use roof area calculator that I’ve been able to find online is located here.

The table also shows the measured roof area which can be drained per square inch of downspout. It is founded on the premise that during a rainfall having an intensity 1200 square feet of roof cans drain. If the intensity is doubled the downspout capacity is halved, or 600 sq. ft.; if it is tripled the capacity is one third, and so on.

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