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Appropriately Sizing Gutters for Your Project:

There are many sizes and styles of gutters that Atlantic Gutters offers.  To figure out what size gutter system will suit your project, you will need to calculate the square footage of the gutter’s drainage area.  On a simple gable-end roof, you will only need to make two calculations, one for each slope.  Hip roofs and intersecting roofs have multiple facets, and for those you will need to add up the area (length x width) of each surface within a drainage surface to get the total square footage.


Making Adjustments for Roof Pitch and Area Rainfall:

Once you have calculated the total square footage of drainage for each gutter, you will need to make adjustments for Roof Pitch and Area Rainfall.  


--Calculating Roof Pitch Factor:  You can determine roof pitch with a 2-foot level and a tape measure.  Begin by holding one end of the level against the roof.  Bring the level into balance against the roof and then measure the distance between the roof and the underside of the level at its midpoint.  This will give you a 12-inch run.  A 6-inch gap, for instance, is called a 6-on-12 pitch. Once you know pitch, you can find your roof-pitch factor in the table below:

Roof-pitch factors: 

-12 on 12 or higher = 1.30 Factor
-9 on 12 to 11 on 12 = 1.20 Factor
-6 on 12 to 8 on 12 = 1.10 Factor
-4 on 12 to 5 on 12 = 1.05 Factor
-Flat to 3 on 12 = 1.00 Factor


--Maximum Rainfall Intensity:  The U.S. Weather Bureau has developed a scientific method to calculate maximum rainfall that could possibly occur in a 5-minute period.  This factor is calculated in inches per hour, for various regions across the United States of America.  Higher unit number require larger gutter sizes in order to prevent gutter systems from being overwhelmed in heavy storm bursts.  The U.S. Weather Bureau table can be downloaded here:


Sizing of the Gutter System:  Multiply the drainage area by the roof-pitch factor and rainfall intensity to find out the adjusted square footage for your system.  Then use the chart below to select the correct size gutter system.

--K-Style System: 5-inch Gutter = 5,520 square feet 6-inch Gutter = 7,960 square feet


--Half-round System: 5-inch Gutter = 2,500 square feet 6-inch Gutter = 3,840 square feet



An Example:  Consider a home in Detroit that has a roof with a drainage area of 1000 square feet.  This home has a 6 on 12 pitch requiring a 1.10 factor while making calculations.  Based on the U.S Weather Bureau's rainfall intensity map, this area receives approximately 7.1 inches of rain per hour in a conventional storm.  We can use these bits of data in the following formula to calculate the appropriate size gutter system for this home:

1.1 Pitch Factor x 1000 Square Foot Roof Area = 1100 Square Foot Effective Area 

1100 Square Foot Effective Area x 7.1 Inches of Rain Per Hour = 7,810 Adjusted Yield

A 6" K-Style Gutter System, which is rated for 7960 square feet, is the proper system for this home considering the calculated Adjusted Yield of 7,810 square feet.