Secondary Drainage and Ponding Requirements in the IBC and IEBC

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November 28, 2017
By Wanda Edwards, PE, RCI Senior Director of Technical Services

Editor’s Note: Code and standards issues of relevance to the building envelope industry are discussed in this ongoing series of articles by RCI’s staff experts.


Requirements for reroofing projects are found in both the International Building Code (IBC) and the International Existing Building Code (IEBC). Because the IEBC is not adopted by all jurisdictions, the requirements for reroofing are also found in the IBC. Projects must comply in their entirety to one code or the other; you cannot take some requirements from the IBC and use some requirements of the IEBC. You must use all of the requirements of the IBC or all of the requirements of the IEBC. As will be apparent upon reading this article, the requirements in the IBC do not completely match the requirements of the IEBC. We will also highlight some often-overlooked requirements.1


Section 1511 of the 2015 IBC is entitled “Reroof.” The IBC contains a new provision in Section 1511.1 of which you may not be aware. The section is as follows:

Section 1511 Reroof
1511.1 General.

Materials and methods of application used for recovering or replacing an existing roof covering shall comply with the requirements of Chapter 15.


  1. Roof replacement or roof recover of existing low-slope roof coverings shall not be required to meet the minimum design slope requirement of one-quarter unit vertical in 12 units horizontal (2-percent slope) in Section 1507 for roofs that provide positive roof drainage.
  2. Recovering or replacing an existing roof covering shall not be required to meet the requirement for secondary (emergency overflow) drains or scuppers in Section 1503.4 for roofs that provide for positive roof drainage. For the purposes of this exception, existing secondary drainage or scupper systems required in accordance with this code shall not be removed unless they are replaced by secondary drains or scuppers designed and installed in accordance with Section 1503.4.

Exception 2 was added to the 2015 edition of the IBC. Before 2015, the code required that reroofs must meet all the requirements of Chapter 15, which would include the provisions of Section 1503.4, which requires a secondary drainage system or scuppers.

[P] 1503.4 Roof drainage.

Design and installation of roof drainage systems shall comply with Section 1503 of this code and Sections 1106 and 1108, as applicable, of the International Plumbing Code.

[P] 1503.4.1 Secondary (emergency overflow) drains or scuppers.

Where roof drains are required, secondary (emergency overflow) roof drains or scuppers shall be provided where the roof perimeter construction extends above the roof in such a manner that water will be entrapped if the primary drains allow buildup for any reason. The installation and sizing of secondary emergency overflow drains, leaders and conductors shall comply with Sections 1106 and 1108, as applicable, of the International Plumbing Code.

1503.4.2 Scuppers.

When scuppers are used for secondary (emergency overflow) roof drainage, the quantity, size, location, and inlet elevation of the scuppers shall be sized to prevent the depth of ponding water from exceeding that for which the roof was designed as determined by Section 1611.1. Scuppers shall not have an opening dimension of less than 4 inches.

The backup created by too much rain creates water pressure and shoots water out of a roof drain in New Orleans. Photo courtesy of Steve Patterson.

Under the provisions of the 2015 IBC, Section 1511.1 has no requirement to analyze the existing system or to provide a secondary drainage system. If adequate drainage is not provided, structural damage—and, in the worst cases, roof collapse—can result. The best opportunity to require the drainage on the roof to be analyzed is when the roof is being re-covered or replaced. This change in the code reversed 50 years of codes requirements. All roofs should have primary drainage designed to the requirements of the IBC and the plumbing code. All roofs should be provided with a secondary drainage system to prevent water accumulation on the roof. It is the opinion of RCI that the lack of such provisions in the 2015 IBC will increase the probability of roof collapses.

The code proposal to add exception #2 was initiated by the National Roofing Contractors Association (NRCA). In their reason statement, they declared that many existing buildings were built without secondary drainage systems. The rationale for not requiring secondary drainage is that if positive drainage is provided, that is adequate to drain the roof. Exception #1 of Section 1511.1 states existing roofs do not have to meet the requirements of the code if positive drainage is provided. The minimum slope the code allows is ¼ in./12 in. This means that even less than the minimum of ¼ in. per foot would be acceptable. This logic does not consider that lower-sloped roofs will not drain water at the same rate as roofs with greater slopes. This is going to cause water to collect on the roof, and if enough collects, it can cause the roof to collapse. The standing water will also cause deflection in the roof members, and exceeding that design requirement can also cause collapse. Additionally, the orientation of structural members can influence the performance of the roof. Joists that run parallel to the drains will not distribute the load as well as joists running perpendicular to the drains. These are life safety issues and can result in loss of property and life.

Ponding water on a roof. Courtesy Edmonton Roofing.

In a report in 2014, entitled “National Climate Change Assessment,” prepared by the U.S. Global Change Research Program, it states, “Heavy rainfalls are increasing nationally, with the largest increases in the Midwest and the Northeast.”2 A record 64 inches of rain fell during Hurricane Harvey—the heaviest rainfall ever in the U.S. These increases in intensity are attributed to the air being warmer and therefore capable of holding more moisture than cooler air. Analyses have confirmed that the air is holding more moisture over land and oceans. “Projections of future climate over the U.S. suggest that the recent trend towards increased heavy precipitation events will continue. This is projected to occur even in regions where total precipitation is projected to decrease, such as the Southwest.”3 Witnesses at many roof collapses reported they occurred during heavy downpours.

RCI submitted an emergency code proposal to the International Code Council (ICC) to remove exception #2 of Section 1511.1. The board denied RCI’s request. However, RCI will submit a proposal to remove exception #2 in the next round of code hearings.


As sometimes happens with code changes, provisions that appear in multiple codes often get overlooked. The reroofing requirements of the IEBC and IBC are a good example of this. The code proposal to add Exception 2 in the IBC did not include a like change to the IEBC. Therefore, the IEBC Section 706 Reroofing does not match the IBC Section 1511 Reroofing. Section 706.1 reads as follows:

Section 706 Reroof
706.1 General.

Materials and methods of application used for re-covering or replacing an existing roof covering shall comply with the requirements of Chapter 15 of the International Building Code.
Exception: Reroofing shall not be required to meet the minimum design slope requirement of one-quarter unit vertical in 12 units horizontal (2 percent slope) in Section 1507 of the International Building Code for roofs that provide positive roof drainage.

Section 706.1 begins the same as Section 1511.1. As you can see, there is only one exception to Section 706.1. Exception #2 in the IBC section 1511.1 is not included in the IEBC. Looking at this section, we were uncertain whether Exception 2 in the IBC applies to reroofs when using the IEBC. The IEBC commentary does not include any verbiage about Exception 2; however, it discusses the first exception. The IBC commentary discusses both exceptions. The ICC provides informal telephone interpretations.

ICC’s interpretation is that Exception #2 is not in the IEBC and does not apply. The reference to Chapter 15 is there for reference to the material requirements for the reroof. This would mean that projects constructed using the IEBC would require secondary drainage be provided on reroof projects constructed under the IEBC.

While most of the roof design requirements are located in Chapter 15 of IBC, an often-overlooked section is Section 1611, Rain Loads. This section states:

Each portion of a roof shall be designed to sustain the load of rainwater that will accumulate on it if the primary drainage system for that portion is blocked plus the uniform load caused by water that rises above the inlet of the secondary drainage system at its design flow.
2015 IEBC
Cover of the 2015 International Existing Building Codes. Courtesy of ICC.


A code provision sometimes overlooked is in the definitions in Chapter 2 of the 2015 IBC, which defines a susceptible bay as:
     Susceptible Bay. A roof or portion thereof with:

  1. A slope less than ¼ inch per foot (0.0208 rad); or
  2. On which water is impounded, in whole or in part, and the secondary drainage system is functional but the primary drainage system is blocked.
    A roof surface with a slope of ¼ inch per foot (0.0208 rad) or greater towards points of free drainage is not a susceptible bay.

Section 1611.2 Ponding Instability, states:

For roofs with a slope less than ¼ inch per foot [1.19 degrees (0.0208 rad)], the design calculations shall include verification of adequate stiffness to preclude progressive deflection in accordance with Section 8.4 of ASCE 7.

How does ponding instability relate to Section 1511.1, Exception #2? Exception #2 states no secondary drainage is required if positive drainage is provided. Chapter 2 of the IBC provides the following definition for positive drainage:

POSITIVE ROOF DRAINAGE. The drainage condition in which consideration has been made for all loading deflections of the roof deck, and additional slope has been provided to ensure drainage of the roof within 48 hours of precipitation.

Forty-eight hours seems like a long time to drain a roof, and considerable water could be ponded on the roof during this time. It is unclear whether the requirements of Chapter 16 would apply only to new construction. If Chapter 16 doesn’t apply, how does one confirm that positive drainage is provided when reroofing? The International Building Code commentary often provides clarification and the intent of code provisions.

The commentary for Section 1511.1 states:

For low-sloped roofs, the exception indicates that reroofing (i.e., re-covering or replacement) is not required to meet the ¼”:12” minimum slope requirement of Section 1507, provided that the roof has positive drainage. The term “positive drainage” is defined as the drainage condition in which consideration has been made for all loading deflections of the roof deck, and additional roof slope has been provided to ensure drainage of the roof area within 48 hours of rainfall.

What does “consideration” mean? Would that be an analysis done in accordance with Section 8.4 of ASCE 7? I am unsure of the answer to that question.

The commentary for Section 1611.2 states:

In roofs lacking sufficient framing stiffness, a condition known as “ponding instability” can occur where increasingly larger deflections caused by the continued accumulation of rainwater are large enough to overload the structure and result in a roof collapse. This must be countered by providing adequate stiffness in order to prevent increasingly larger deflections due to the buildup of rainwater. Another means to minimize the accumulation of rainwater is to camber the roof framing. This section
requires a check for ponding instability at susceptible bays (see definition in Section 202). A ponding instability check is to be made assuming the primary roof drains are blocked. The determination of ponding instability is typically done by an iterative structural analysis where the incremental deflection is determined and the resulting increased rain load from the deflection is added to the original rain load.

It is suspected that most reroofs are done without any “consideration” of all loading deflections of the roof deck. But be aware that on new construction, this requires an analysis in accordance with Section 1611.2.

Our next article will highlight the changes to the 2018 codes that are now available from the ICC.


1. Article, in part, based on a presentation prepared by Stephen L. Patterson, RRC, PE, of Roof Technical Services and Dr. Madan Mehta, PhD, PE, of the University of Texas at Arlington.
2. National Climate Change Assessment by the U.S. Global Change Research Program, http:/
3. Ibid.