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WSRCA INFORMATIONAL BULLETIN 2017-S2 - Regarding Gutters, Downspouts & Drainage

Posted By WSRCA, Wednesday, November 8, 2017
Updated: Wednesday, November 8, 2017


Greetings to Members of Western States Roofing Contractors Association:


As the severity of surface weather storms appear to be occurring with greater frequency and in some areas with increased amounts of rainfall, WSRCA’s Steep-slope Committee wants to remind roofing designers, roofing contractors, and all of our members of the importance for appropriate roof system storm water run-off management. This bulletin in particular relates to steep-slope roof drainage and specifically gutters and downspouts.



Within the two primary model building codes that address storm water run-off management for the most of the Western region of the United States (The International Code Council’s [ICC] International Plumbing Code (IPC), and the International Association of Plumbing and Mechanical Officials’ [IAPMO] Uniform Plumbing Code [UPC]), the code requirements vary depending on the adoption of the model code and any amendments that local jurisdictions may have implemented.


In the International Plumbing Code (IPC), Chapter 11 Storm Drainage, published by the ICC, the installation of gutters on steep-slope roofs is not directly dictated or required. As with other model building codes, tables are included to assist with the sizing of gutters and vertical leaders (i.e., downspouts), often based on 100-year storm reoccurrence and 60-minute rainfall data. The most recent and current editions of the IPC do not provide language dictating the installation of gutters. However, the 2015 Uniform Plumbing Code and other previous editions, published by IAPMO, does state that gutters are required within Chapter 11, Storm Drainage. Therefore, it is critical to be aware of the Codes that are in effect in the jurisdiction of your roofing or reroofing project and understand the requirements for storm water run-off management. Please note that not all Jurisdictions adopt the most recent edition of the Codes, so it is important to also confirm and verify the year of the code that is in effect.


This WSRCA Technical Bulletin is intended to provide an overview of the varying code criteria and preliminary guidance for understanding model building code requirements that may be in effect for your project. In light of the magnitude of code changes over the last decade or so, WSRCA believes it is prudent that you verify which code is in place and understand the requirements that relate to your roofing and reroofing projects.


Background Information: 

Whether your steep-slope roofing project is commercial or residential, accommodations for roof drainage and proper run-off management are among many of the most critical and primary items to consider. Model Plumbing Codes have traditionally provided sizing tables for gutters and downspouts, although gutters may not always be implicitly required. Similar language for the International Residential and Plumbing Codes as well as the Uniform Plumbing Code generally state within the first few paragraphs of the Storm Drainage chapter, as a minimum baseline, that if there is not a requirement for connection to an approved storm water system, that rainwater from roofs shall be discharged so that storm water drains away from the building.



Also, as storm water run-off management requirements are becoming increasingly stringent, some code jurisdictions are providing amendments to the Codes that state or require that roof rainwater run-off shall be collected and managed onsite in rain water cisterns, bio-swales, or rain gardens, for example, as a means to keep the below-grade infrastructure of the city’s sewer and/or storm water systems from being overloaded during heavy rain events.



Individual states within the Western region of the United States, have adopted varying editions and combinations of the Codes, with many states and local jurisdictions providing their own Amendments to base line codes. In Washington State for example, the primary model code adopted for the State includes the International Code Council series of Codes (i.e., IBC, IRC, etc.), but continues to use the Uniform Plumbing Code in lieu of the International Plumbing Code. California’s Plumbing Codes are also based on the Uniform Plumbing Code, and therefore, include the requirements, further discussed below, that states that “Roof areas of a building shall be drained by roof drains or gutters.” The following sections provide a general overview of select code language from the primary model codes that may be pertinent to your roofing project.



International Code Council (ICC) Roof Drainage-Related Information: 

The International Code publishes the series of Codes, which include the International Building Code (IBC), the International Residential Code (IRC), and International Plumbing Code (IPC), etc. along with a number of other related codes sections. The most recent edition of the ICC Codes is published as the 2015 series of codes. Many jurisdictions have adopted the recent 2015 Codes; however, as mentioned above, not all city building departments or Authorities having jurisdiction adopt the most recent edition of the Codes and the effective Code for your specific project(s) need to be verified.


Within IBC’s Chapter 15: Roof Assemblies and Rooftop Structures, the Code states that roof drainage systems shall comply with sections from the International Plumbing Code (IPC). The IPC provides language in Chapter 11: Storm Drainage, Section 1101.2 Disposal, stating that rainwater from roofs shall drain to an approved place of disposal as well as language stating that “For one- and two-family dwellings, and where approved, storm water is permitted to discharge onto flat areas, such as lawns or alleys, etc. provided that the storm water flows away from the building. Section 1106.6 references Table 1106.6 for sizing requirements of roof gutters. In the 2015 International Residential Code, Section R903.4 Roof drainage, states that “unless roofs are sloped to drain over roof edges, roof drains shall be installed at each low point of the roof.”


Uniform Plumbing Code (UPC) Roof Drainage-Related Information: 

The Uniform Plumbing Code is published by the International Association of Plumbing and Mechanical Officials (IAPMO). The most recent edition of the UPC was published in 2015. The following language reference is provided in Section 1101.12.1 Primary Roof Drainage stating:


“Roof areas of a building shall be drained by roof drains or gutters. The location and sizing of drains and gutters shall be coordinated with the structural design and pitch of the roof. Unless otherwise required by the Authority Having Jurisdiction, roof drains, gutters, vertical conductors or leaders, and horizontal storm drains for primary drainage shall be sized on a storm of 60 minutes duration and 100 year return period. Refer to Table S 101.1 (in Appendix D) for 100 years, 60 minute storms at various locations.”


Please Note: For those States and Jurisdictions that are still utilizing the Uniform Plumbing Code, gutters and downspouts are required as part of your primary roof drainage system for steep-slope roof systems.



As with all building codes, the language and requirements can change from edition to edition, it is important to remain up to date on the current set of Codes and verify the current codes that are in effect for your specific project. It is also important to be aware and understand what the base line size of a storm means when Codes utilize the 60-minute rainfall duration, and 100-year return period or reoccurrence interval as the basis for design of roof run-off storm water management. Recent weather data indicates that severe rain events appear to be occurring with greater regularity, sometimes far exceeding the base line threshold. With the more severe rain events, it is likely that storms may have shorter duration bursts of heavy rain with large volumes of rainfall that can over burden the storm water management systems (e.g., some gutters and drain spouts, or tight lines), which may lead to potential rain water intrusion into buildings.


Studies have also been conducted in recent years that have shown many of the industries assumptions and design criteria for dealing with rainwater run-off have been historically undersized and not adequate to properly and efficiently drain rainwater from roof systems during some storms and heavy sustained rains. New drainage design methodologies and code requirements have been introduced in select Codes, and as the information is better understood and more testing is completed, it is likely that substantial roof drainage updates will be introduced and perhaps become more widespread. As an example, the 2015 International Plumbing Code has altered the methodology for sizing of roof drains on low-slope roofs, based on recent research, and other Code groups, such as the Uniform Plumbing Code, are likely to follow in a similar manner. WSRCA’s Low-Slope Committee is working on a separate Technical Bulletin regarding the updated roof drain design requirements for low-slope roofs, and we recommend that WSRCA Members become aware of that Bulletin and the related recent updated standards.


In Closing: 

We trust this information aids you to promote quality steep-slope roof and drainage systems. Thank you for your participation in the western roofing industry and for looking towards WSRCA to provide its members with industry-leading technical assistance for use on your steep roofing projects.


Thank you for your support of Western States Roofing Contractors Association, and our active efforts to strengthen and advance technology and science in our industry, as well as to promote the art of good roofing and waterproofing practices.


Copies of the IBC, IRC, and IPC may be purchased by contacting ICC, International Code Council at 800-786-4452 or visiting their website at 


Copies of the UPC may be purchased by contacting 909-472-4208 or visiting their website at 


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