Are there any general guidelines to follow to determine the number of downspouts that should be provided from a pitched, shingled roof? I’m assuming that roof area would factor into the calculation, but I’m sure that there are other factors like length of eavestroughs serviced.

I’m inspecting a house next week (new construction) that is about 3600 square feet in 2 storeys. I’m guessing that the total length of eavestroughs is 160 feet. The builder has provided one downspout. Seems to me that at least one more should have been installed.

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.

Local gutter guys use the total roof surface and slope to determine gutter size and number of downspots. General rule around here is slope and area determines gutter size and approx. 600sqft of roof surface per downspout or every 25’ of gutter…ymmv