Gutter size calculation article

[FONT=Verdana, Arial, Helvetica, sans-serif][size=6][size=3]To all:
I recall someone on the message board asking about sizing a gutter. I got permission by the author to reprint the following article. Hope this helps. Thanks Doug Pushard from

Residential Gutter Sizing[/size]
by Doug Pushard[/size][/FONT]

Gutter sizing is an aspect of rainwater collection that has been studied extensively and can be calculated based on published guidelines in the plumbing codes. For example, the Uniform Plumbing Code (UPC) recommends that a gutter system be able to carry the runoff of the heaviest 60 minute downpour recorded in the last 100 years. The International Plumbing Code (IPC) has a similar, but not exact, published sizing recommendation. Both plumbing code manuals include the necessary sizing calculations to properly size a gutter system.

                           With water  weighing over 8 pounds a gallon, it poses a significant threat to a  house and it’s inhabitants. The weight of rainwater standing on a roof  has caused roofs to collapse. The gutter system must be capable of  draining the roof fast enough so that the structural limits of the roof  are not exceeded. Both plumbing codes use the highest hourly rainfall  event recorded in the last 100 years as a way to ensure the gutter  system planned for and ultimately installed can handle the largest known  rain event to occur in recent times. 

                           In properly  sizing the gutter system (i.e. sometimes also referred to as the  drainage system), any of the following can be varied: roof size, number  of downspouts, slope of the gutters and the  size of the gutters and  downspouts.   In most situations, the roof size will be a given and the  examples below assume the roof size is fixed and has been constructed to  meet  building codes.

                           To determine  system sizing alternatives, know the roof size to be drained.  In a  typical home with two slopes, each side will have  gutters and be sized  separately.

           Next, find the amount of rainfall that  needs to be  carried off the roof.  This number can  be located by calling the local  building department or by looking the number up in either Appendix  D,  Table D1, of the UPC or Appendix B of the IPC manual. Copies of these  manuals are   generally available at the local library. 

                            For example, for [Phoenix*****](  it is 2.2 inches of rain per hour or .023 Gallons Per Minute Per Square  Foot of Roof Area according to  Table D1 in the UPC manual. 

                           After locating the rainfall number,  determine the total rainfall that needs to be drained by multiplying  the Gallons Per Minute Per Square Foot of Roof Area  by the Square  Footage of Roof area. On a 2,500 square foot roof, this works out to be:

                           .023 Gallons Per Minute Per Square  Foot of Roof Area x 2,500 square feet = 57.5 gallons per minute. 

           Consequently, the guttering system needs to be  designed to handle upto 57.5 gallons per minute or 3,450 gallons per  hour (i.e. multiplying 57.5 gallons by 60 minutes). 

           **That is over 10 tons of water!**  

           So what size of gutters need to be installed to  handle this incredible volume of rainfall?  The width of the gutters,  the slope of the gutters and the number of downspouts all come into play  to determine the correct system sizing.  For example, the greater the  width of the gutter (i.e. going from a 3” gutter to a 5” gutter), the  lower the slope required (i.e. going from ½” to ¼”) to handle the same  amount of rain.

           With the above information in hand, refer to the  tables in either the  UPC or the IPC manuals to determine the gutter  system size alternates.   (In the IPC 2006 manual it is provided in  Table 1106.6. [Note  there are significant differences in how to use the tables in each  manual.]( The Uniform Plumbing Code table below illustrates the  various design options.  Locate the amount of water to drained in the  table (i.e. 3,450 gallons per hour), then look to the first column to  determine the required gutter diameter and to the column heading to  determine the slope of the gutter. 

UPC Table 11-3-1 would dictate that a roof needing to handle 3,450 gallons per hour would require a system designed to the highlighted specifications below.

TABLE 11-3-1 Uniform Plumbing Code
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Any of the above highlighted combinations could be installed, per the UPC.
The example above assumes only one downspout. Alternatively multiple downspouts could be installed and would change the gutter size and slope requirements. To determine these changes, divide the hourly rainfall number by the number of downspouts.

           For 2 downspouts, divide 3,450 by 2 and it  reduces  the requirement to 1,725 gallons per hour; therefore, the gutters could  be reduced to  5” and the slope of the gutters to 1/8”(**noted by yellow below**). Adding  another downspout (i.e. 3 downspouts) would further reduce the  requirements to 1,150 (i.e. 3,450/3) gallons which results in either 4”  and 1/4” slope or 5” with 1/16” slope (**noted  by green below**) gutter system. Adding another downspout  (i.e. for a total of 4 downspouts) equals 862.5 gallons (i.e.  3,450/4=862.5) would result in a system as small as 3” with a 1/2“ slope  (**noted by blue below**). As  shown in this example, adding downspouts can greatly reduce either the  diameter or the slope of the required system or both. 

TABLE 11-3-1 Uniform Plumbing Code
Picture 2.png

With the size determined, choosing a material is the next logical step. Many different materials are allowed by both the IPC and UPC. However, Schedule 40 PVC, factory-painted aluminum or galvanized steel sheet metal, all of which can be found in most large hardware stores, are generally recommended for rainwater harvesting systems. There are numerous materials that meet the IPC and/or UPC standard, but should not be used with rainwater systems, including: lead, copper, bronze, and cast iron pipe.

                           Lastly, it is important to note  that the rainfall rate  used in the IPC and the UPC manuals may differ  and [neither  may be the correct number*]( The International Plumbing Code  uses  the “a storm of one hour duration and a 100 year period”, while the  Uniform Plumbing Code uses recorded rain based on data from the U.S.  Weather Bureau. Which you use depends on your local permitting entity.  If you are sizing a system, spend a few extra minutes and call your  lcocal building department. Ask which plumbing code is used locally and  the rainfall rate that should be planned for in your location.               

Please remember water is extremely heavy and large rainfall events can put a ton of added weight on the roof surface. Plan for it. Gutter sizing can be and should be calculated and installed accorded to generally accepted building code requirements. Using either the IPC or UPC manuals provides an excellent way to correctly size a gutter system.

           **Related Links**
           [Maximum  Rainfall Rates for Various Cities]( 
            [Rainfall  Rates Vary Depending on the Plumbing Code You Use]( 
            [IPC  Plumbing Code Manual](
            [UPC  Plumbing Code Manual]( 
            [Making  Raingutters Work!](
             [Getting  the Numbers - Let me do the work, send me your location and Roof Size](http://javascript%3Cb%3E%3C/b%3E:void%280%29)                 **[Getting the Numbers - Let me do the work, send me your location and Roof Size](**   ($10.00 fee)

           *Phoenix is a good example as  the rainfall rate required to be handled needs to be verified by the  local building officials.  The UPC table states 2.2 inches, while the  IPC table states 2.5 inches; but the local building code says it must be  designed to handle 6 inches per hour.

           It is important  to note that the IPC and the UPC tables differ significantly. The IPC  table contains the square footage of the roof area and not the amount of  rain that can fall on the roof. Do not confuse the numbers to be the  same in each manual's table.

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