Q:

The article says you should have 1 downspout for every 600-800 sf of roof. What determines the proper size of the gutters & downspouts? Won't a four inch carry at least 25% more water away than the three inch? Should I use a 5"?

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Dja, that number is an estimate based on a 4" "K" styled gutter/downspout system, which is the most common type of gutter system used.  If you have a situation where you'd exceed that estimate (such as when an upper roof downspout discharges to a lower roof gutter), then you might want to upgrade the size of the lower gutter to handle the entire load. 

This is a good rule to follow as in heavy rains, undersized downspouts can become overloaded and the excess water overflows and creates flooded basement and potential foundation damage.

It's also important to make sure downspouts extend away from the foundation, at least 4 to 6 feet is recomended.

Thanks,

Tom

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