What size and how many downspouts?


I have been puzzled with two questions recently:
How to calculate the number of downspouts on a house and how to size them, but by somehow using the flow rate of the gutters?

I found that there are a number of sources which offer the sizing of downspouts by the usage of tables.
I also found that some USA manufacturers recommend defining the number of downspouts by putting one downspout on ever 20 to 40 feet length of gutter.

But is it possible not to use the tables approach nor 20-40 feet rules of thumb?

Is it possible to to somehow calculate the number of downspouts by knowing the flow rate of the gutters?

Let's say that my roof produces 0.2 cfs (cubic feet per second) of runoff.
I can use Manning's formula to size my gutter. If I pick 6 inch rectangular gutter, it can take 0.21 cfs.
Can I now size the downsprouts and their number, based on this 0.21 cfs gutters's flow rate ?

I apologies for not asking the question directly related to SWMM.
Thank you in advance.

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  • I've also found out that one of the way is to use the Wyly-Eaton equation for vertical downpipes, to determine the minimal diameter. The equation is used in sanitary systems, but looks like it can be used to size the circular cross-section downpipes aswell:

    D (inches) = { Q/[27.8*r^(5/3)] } ^ (3/8)
    Q is the flow rate from the gutter in gallons per minute.

    I imagine that the area of the circular cross-section downpipe might suit the cross-section area of the rectangular one too, or am I wrong?

  • Thank you Lori.  Is this a visual way to count the number of downspouts or some other way?

  • Thank you, for sharing the information and link. There can even be a way to get it done in a professional way. Recently, when one of my friends encountered the similar analysis problem about evaluating downspouts and other more confusions, he got in a way to make it up with a professional's actual norms and concern by getting it done in an inexpensive way. There are many ways and reference to get it in a way to have an eye on and get it resolved.

  • In my neeighborhood there are normally a downspouts on each corner of the house - it does not matter the shape or the length of the gutter,  what matters is that each individual gutter has one downspout.  The number of gutters per house depend on the shape of the roof and the number of odd roof angles.   It varies from house to house but is normally 4 to 6 (if you include the center of the front and back of the house).

    However, there are guidelines - here is one site with guidelines for gutters


    with this comment

    "I don't go by code when determining the adequacy of gutters and downspouts on a roof edge. I do like to see one (4" or 5" gutter) with a 2" X 3" downspout at each end of a gutter. If I see a gutter that is over 30 or 40 feet long, with only one downspout, I always want to see the bigger 4 or 5 inch gutters installed with the 3" X 4" downspout, in order for it to handle the amount of water it will be discharging."

    • Dear Mr. Dickinson,

      Thank you for the useful reply and link!

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