I am thinking that we need more downspouts because I could hear the water overflowing in the gutters last night. Is there some recommended ratio between length of gutters and number of downspouts needed for the length? Or do I need a more complicated formula that takes into account the pitch of the roof and/or other factors? Thanks!
Dirt and debris are the most likely reasons for overflowing gutters. The primary variable for calculating the number of downspouts needed is related to local climatic conditions - amount and type of rain. For example: in an area like Seattle, we get most of our rain in small amounts and don't need an unusual number of downspouts. Areas with heavier rain storms benefit from more downspouts. A basic rule of thumb is one downspout for every 25' of gutter.
Make sure that all downspout drainage is diverted away from the house. George