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 (i.e. sometimes referred to as Storm Events). 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

Diameter of Gutter

1/16” Slope

1/8” Slope

1/4” Slope

1/2" Slope

3”

340

480

680

960

4”

720

1,020

1,440

2,040

5”

1,250

1,760

2,500

3,540

6”

1,920

2,720

3,840

5,840

7”

2,760

3,900

5,520

7,800

8”

3,980

5,600

7,960

11,200

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

Diameter of Gutter

1/16” Slope

1/8” Slope

1/4” Slope

1/2" Slope

3”

340

480

680

960

4”

720

1,020

1,440

2,040

5”

1,250

1,760

2,500

3,540

6”

1,920

2,720

3,840

5,840

7”

2,760

3,900

5,520

7,800

8”

3,980

5,600

7,960

11,200

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.

*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.