Things You'll Need
House blueprints or measuring tape
Pen and paper
While it may not always be looked at as such, a roof is one of the most essential aspects of a building because it protects the people and things inside of it from inclement weather and extreme heat. Critical to the effectiveness of a roof is its drainage system, including gutters and downspouts. When installing a new set of downspouts, you must make sure they are large enough to handle the capacity of rain that may flow off the whole roof area. Knowing your roof area and the average amount of rainfall you receive will help you get the right size and number of downspouts for your building.
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Determine the area of your roof by examining the building plans or blueprints for your house which indicate the length and width of your roof; if you cannot locate these documents, use a ladder to carefully climb up to your roof and measure its length and width with a measuring tape. Write down the figures for your roof and multiply them (length x width) to get the area, in feet, of your roof. Realize that roofs are rarely purely rectangular so you may have to add various width and length measurements together before multiplying them to get your total area.
Determine the average rainfall your city receives (use Resource 1 if necessary by clicking your state, county and city and referring to the rainfall rates listed on screen). Use that figure to calculate what size downspouts are needed for your roof.
Understand that it takes 96.15 square feet of surface with 1 inch per hour of rainfall to correspond with 1 gpm of flow rate. Downspouts are sized in relation to rainfall on this basis. Use table 1-2 (in Reference 1) to find the amount in square inches of downspout that will drain a specific amount of roof area (given in square feet). For example, if you are measuring downspouts for a roof in Boston, you would use the table to find that 1 square inch of downspout drains 170 square feet of roof area.
Divide your total roof area by 4 to determine the amount of roof area that each downspout will drain. For example, if your roof is 8500 square feet total, divide that number by 4 to determine that each downspout will drain 2338 square feet of roof space. If you live in Boston, you would use your calculation from Step 3 to determine the minimum area that each downspout must have to properly drain rainfall by dividing the square footage served by each downspout by the square footage that each downspout can drain (2338 divided by 170 = 13.56 square inches). In this example, you would calculate that each downspout must have a minimum area of 13.56 square inches to adequately drain the roof.
Use table 1-3 (in Reference 1) to determine the diameter of downspout you need to install. In the preceding example, you could use a five-inch diameter plain round, corrugated round, rectangular corrugated or plain rectangular downspout to drain your roof.
Measure the distance from the end of your gutters to the ground and then an additional three to four feet out from that spot to determine the length of downspout that you need to install. Multiply this figure by the total number of downspouts needed (probably four) to determine the total length of downspout you need to install.
Don’t use downspouts measuring less than 7 square inches in diameter except for with small areas like a porch or canopy. Make sure the downspout diameter is consistent throughout its length.
Make sure the downspout size suits the bottom width of the gutter so that a tight fit is achieved.