The one and only purpose of an aluminum downspout is to carry rainwater from your gutters to the ground and away from your foundation. Downspouts are identified by their size and available in the same color as your gutter. The two most common sizes are 2x3 and 3x4.
Downspouts are made in a roll forming machine, similar to the machine that forms the gutter. Downspouts come in 8 ft. and 10 ft. lengths and directs water away from the foundation.
The size of the outlet which connects the gutter to the downspout plays a very important part in keeping gutters flowing. Clogs always occurs at the outlet. If your outlets clog, water will slowly seep through the clogged outlets and your gutters will overflow in a heavy rain.
By using large outlets, your gutters will not become clogged with small debris, but allow it to be flushed down the downspout. Another advantage for large outlets is that it greatly increases draining capacity.
Two straps of the same color as the gutter downspouts should be used to secure an 8 to 10 foot downspout. The straps should be installed with 1 1/2 wood screws into wood siding or a hardened nail insert into brick, then wrapped around the downspout and secured to itself with painted rivets or screws.
Getting the water out of the gutter is the most important thing. Remember that gutters are only there to catch the water, not hold it. Heavy downpours can overwhelm a gutter system that just meets the minimum requirements.
As a general rule about aluminum downspout capacity, a 2x3 downspout will drain the average rainfall for every 600 square feet of roof area and a 3x4 downspout for every 1200 square feet of roof area.
Downspouts can handle about 100 square feet of watershed area per square inch. Round the watershed area up to the next even 100 then divide by 100 to determine the square inches of downspouts required.
For example, a 1,550 square foot watershed area needs at least 16 square inches of downspout. A 2 inch by 3 inch downspout has a 6 square inch cross section (2 x 3 = 6) while a 3 inch by 4 inch downspout has a 12 square inch cross section. In this example, you need three 2 by 3 inch downspouts (a total 18 square inches cross section) or two 3 by 4 inch downspouts (24 square inches total cross section) to equal the minimum 16 square inches required.
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