Commercial Downspout Sizes In these cases, 4x5 inches or even 6x6 inches downspouts are commonly used. They are designed to handle the increased water flow from larger roof surfaces, ensuring water is efficiently directed away from the building.

Plain Square Aluminum Downspout Typically 2"x3" is selected for 5" K Style and 3"x4" is used with 6" K Style.

They both carry approximately the same amount of water and the choice usually comes down to looks. Aluminum round downspouts are only nominally more expensive than the standard 3×4″ downspouts and really make your home pop! In some scenarios, you may not want a downspout but instead something more decorative.

Downspouts must also be sized to handle this level of rainfall. Rectangular 2 x 3-inch downspouts have a capacity of 600 square feet, while 3 x 4-inch downspouts can handle 1,200 square feet.