Q & A - Downspouts, How Many Do I Need for My Roof?

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February 3, 2014 at 12:00 a.m.

How many rain gutter downspouts should I have?

Tim H. from Birmingham, Alabama, asked, "How many downspouts do I need for my roof? My warehouse has 4 downspouts. Each side is 75 feet long by 33 feet wide gable sides. Is that enough downspouts?"

There are too many variables to this roofing question for just one answer. Typically a warehouse the gutter system may only need one downspout every 25 – 35 feet. The more roof the more rainwater.

As a gutter system catches the rainwater it moves it right or left to the closest downspout. On a good rainstorm, one downspout per 35 feet may not be enough to let all the rain exit the gutters properly and rainwater can back up into the roofing the eaves, soffits and even into the commercial buildings on some roof designs without overhangs.

It important to have your rain gutter drains properly. Most commercial buildings use an sq 3×4 inch downspout and these seems to work well having one every 35 feet for average size roof if kept clear of debris. For a roof that has more than about five hundred sq. feet you need to think about a second downspout. Not seeing the building, here is what I would do. I calculated the sq footage of your building to be about 28 sqs. @ 6:12 pitch. In my opinion, 4 downspouts are not going enough and I would install 6 total, 3 on each side. At 25 feet apart this will ensure a good working system.

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