Things You'll Need
A properly designed gutter and downspout system will take water from the roof of your house and direct it away from the house. Without a gutter system, water will simply fall off the roof and likely form pools near the base of your house. The water will then flow through any fault in the base of your home until it reaches the inside, where it sits and forms mold. Designing a gutter system is a little complicated.
Draw a map of your roof on a piece of paper. The map needs to be as close to scale as possible. You can use your tape measure on the ground to approximate the size of the roof edges.
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Cross off any edge that isn't at the bottom of a slope. A little water will fall off these edges, but not enough to make it worth installing a gutter. Every edge at the bottom of a slope will need a gutter installed.
Find the points where the gutter will have to stop. Mark these on the design with an "X." These are almost always corners where the gutter can't continue since the roof isn't sloping down once you round the corner.
Measure the distance, in feet, between two "X" marks where a gutter will be installed. Add one foot to the measurement for every turn in the gutter. Divide this number by 20, and round the result up to the nearest whole number. This result will tell you how many downspouts you need for that stretch of gutter.
Find a place to put the downspouts, and mark their location with an "O" on the design. The most common location for the downspouts is the corners of the house, since it is easier to move the water away from the house there. You will need to have a downspout for every 20 feet of gutter, and turns in the gutter add 1 foot to the total. You can position a downspout in the middle of a side of your house, but make sure it doesn't cross any windows or doors on its way down.
Walk around the outside of the house with your design in hand. Check the elevation where each of the downspouts is coming down. If the land isn't sloping away from your house there, you will either have to adjust your downspout location a few feet to somewhere where the land is sloping away, or you will have to dig a hole and lay a pipe to funnel the water away from your house. Failure to do this defeats the purpose of the gutter, as water will simply pool next to your house. Make any necessary changes to your design.
Measure the distance between each downspout and draw a thick line through the exact middle point between them. This will mark a high point for your gutter, and the gutter leading from this point will gently slope downward, leading in a different direction on different sides of this point. The proper slope is 1 inch down for every 20 feet of gutter.
Remember that downspouts in the middle of a run of gutter will take water from both sides, so you can’t have more than 20 feet of gutter total leading to a single downspout.