For the homeowner, figuring out what to install in their gutter system can be confusing at best. A downspout is obviously important, since you don’t want your gutters to overflow, but where should you install it? Where should the water go? What size should you use? Fortunately, once you know the basic facts, it’s easy to figure out. All you need is some patience and a calculator.
Start with the total square footage of your roof. Remember, it’s not your floor plan, because your roof likely has a peak to slough off water when it rains. Just multiply the length (in feet) by the width (in feet) of each side of your roof, and you have your square footage. If your roof has triangular sections, just use the Pythagorean Theorem; don’t worry, unlike high school, you can use a calculator for this math problem. Just sketch out the triangle to scale, divide it into two right triangles, and the rest is easy.
Once you have the square footage, it’s just simple math. A 2”x3” downspout will drain about 600 square feet of roof, while a 3”x4” will drain about 1200 square feet of roof.
For the security and stability of your gutter system, you should err on the side of larger. So, if your roof is 750 square feet, it makes sense to install just one larger downspout instead of two smaller downspouts. This limits the number of seams you’ll have in your gutters, which means fewer leaks and a gutter system that’s easier to repair. It also keeps you from having to find two spots to place a downspout, which can be a tricky business.
To double-check, measure how many feet of gutter you have and compare to the number of downspouts you’re installing. A good rule of thumb is that for every 30 to 40 feet of gutter, there should be one downspout. Unless you have a larger home, one downspout will probably suffice.
Once you know that, installing the downspout is a snap. Use a level to determine the slope of your gutter. Don’t eyeball it: the slope of the gutter can be very subtle, especially in short lengths. Find a location for your downspout at the downward part of the slope that’s not near any important features, such as driveways, electrical junction boxes, water meters, or other features that should be protected from rainwater. Cut a hole, preferably at the corner, and solder in your downspout.
And, pretty as a picture, it’ll drain your roof quickly and easily. No more worries about leaks!