This depends on your roof and spacing of your downspouts but we normally install a 3”x4” rectangular downspout. Placement of the downspouts is almost more important than size but we do not recommend anything smaller than the 3”x4”. The 3″x4″ downspout carries twice the water capacity of a 2″x3″ that we commonly replace and is normally the best solution for most residential homes.
We also offer a 4” round downspout. This is becoming more & more popular because it looks more decorative & traditional.
For some commercial applications a 4”x5” downspout may be necessary.