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  /  Opinions   /  Lessons Learned at RCI : Ponding Instability- Part 1

Lessons Learned at RCI : Ponding Instability- Part 1

A Safe, Streamlined Process for Analysis That Is Understood by All Is Required to Ensure the Risk of Catastrophic Failure Is Minimized

Many interesting and important building envelope issues were discussed in the educational seminars at the March 2018 RCI International Convention and Trade Show in Houston.  Dr. Stephen L. Patterson, RRC, PE with Roof Technical Services Inc., and Dr. Madan Mehta, PhD, PE with the University of Texas at Arlington inspired us with their presentation on roof drainage design, roof collapses, and the related codes. At Zero/Six Consulting, we regularly work on re-roofing projects, where the issues being discussed in this seminar prove especially relevant. We believe any and all life safety concerns should be studied and simplified as much as possible. A safe, streamlined process for analysis that is understood by all is required to ensure the risk of catastrophic failure is minimized.

Controlling a situation, rather than responding to it, is the key to this process. Ponding on low-slope roofs is a life-safety hazard that can be anticipated and mitigated with proper analysis. An issue that has been observed in the AEC community is the building code doesn’t clearly address the roof drainage situation for existing buildings. The different disciplines involved in dealing with roof drainage is another issue. Architects, engineers, plumbing consultants, and roofing contractors are all obligated to ensure the structure can handle the worst-case-scenario for rain loads. My opinion, as an engineer, is the engineer takes the lead and becomes responsible for the final roof drainage design. The engineer is understandably focused on structural design, but simply getting the rain load from the architect or plumbing consultant does not adequately address the potential issues. The structural engineer should likewise engage with the architect to ensure roof slopes will properly collect water at the primary drains. In the case of a primary drain backup, the overflow design handles water without ponding.

Finally, coordination with the plumbing contractor is necessary to ensure the roof drain size and height will be set properly, and the conductors are designed and installed to be fully independent from primary drains to superintend rain to the overflow as required.


The most important factor in preventing ponding on the roof is providing adequate slope to quickly and efficiently get water to the primary and overflow drainage. The newer building codes require one-quarter inch per foot of slope and define those areas without adequate slope as “susceptible bays”. Susceptible bays include areas where water is impounded – when the secondary drainage system is functional while the primary drainage system is blocked.

Progressive deflection or instability are worst-case scenarios preventable with adequate drainage. Two basic outcomes can occur with flexible roof supports and inadequate slope:

1. as ponding water gains depth and weight, it causes deflection in an area, which means an even greater water thickness will result. This progressive deflection will continue to expand until the ultimate bearing capacity is reached and the roof collapses;

2. if there is adequate stiffness, equilibrium will occur, and the deflection fails to increase and ponding maintains a constant depth while draining, which protects against progressive deflection and instability.

The orientation of joist members must be considered with different roof geometries and drainage options. The depth of water as it collects around the low areas on its way to primary and overflow drainage will increase the rain load in that area. Therefore, it is best to orient joists parallel to the drainage path so the highest rain loads are dispersed across multiple members at the lowest areas. Joist orientation isn’t typically considered when designing roof slope and drainage, but if the engineer takes responsibility for coordinating with architects, it is more likely to be incorporated into the design.


When designing for secondary (emergency) drainage, otherwise known as overflow drainage, typically the geometry and design of the primary drainage system drive the overflow design. If cost is a consideration, scuppers are preferred versus overflow drains due to an increase in plumbing costs to discharge the overflow to an endpoint separate from the primary system. Section 1108 of the International Plumbing Code (IPC) also requires all overflow to be discharged to a location above grade that would be observed by building occupants or maintenance personnel. This is to alert all parties the primary system has backed up or become clogged, and the overflow drain now has to manage the water. With larger areas and bigger drains, it is a common mistake to create a sump for both drains with a hydraulic head that ends up being higher than the overflow collar. This arrangement can cause the overflow to activate during regular rain events without the primary drains being backed up creating staining on the walls. This also defeats the purpose of the visible discharge for the secondary drainage system because it’s now unclear if it’s activated due to the sump design or a primary system backup.

When Designing for Secondary (Emergency) Drainage, Otherwise Known as Overflow Drainage, Typically the Geometry and Design of the Primary Drainage System Drives the Overflow Design


While new buildings provide roof drainage challenges of their own, factoring in proper analysis of rain loads during the design phase will result in roof members designed to carry the anticipated loads.

For re-roofing and evaluating existing buildings, this analysis is often overlooked and can quickly become a public safety hazard. Many older buildings were either built without overflow drainage or have inadequate overflow capacity due to code requirements and guidance at the time being unclear.

Rain loads may or may not have been properly accounted for with existing buildings. The guidance for calculating rain loads when sizing roof members has been in ASCE 7 since its beginning in 1988, but it may not have been considered or used properly. An often-overlooked issue with existing building roofs is a false sense of security can result based on how the roof has performed so far. The assumption is – since the roof hasn’t collapsed over the past several years of heavy rains, surely the roof structure is adequately designed to hold roof loads. The problem with making this assumption is the roof may not have had issues with any primary drain backups to this point. If the overflow drainage isn’t properly designed, all it takes is a “perfect storm” of primary drains collecting excessive debris, clogging or backing up, or having inadequate overflow drainage.

Some older versions of the building code adequately addressed the need for re-roofs to create adequate roof slope and analyze structural stability for rain loads. For example, in 1988, the Uniform Building Code (UBC) required re-roofing to conform to the roof drainage design for new roofs. This includes minimum one-quarter inch per foot slope and guidance for minimum overflow drains and scuppers that will adequately drain the roof in case of primary drain backup. This same 1988 UBC also includes a requirement for an initial inspection from a building official where the official inspects the roof prior to roofing to determine if any evidence of extensive ponding is apparent. The note for this requirement adds that if extensive ponding of water is apparent, an analysis is made of the roof structure and the appropriate corrective measures are undertaken. These may include relocation of roof drains and scuppers, re-sloping the roof, or structural changes…