in overflow drainage design theory. Historically, most primary and secondary roof drainage systems were designed based upon the 1-hour duration, 100-year rainfall rate. This 1-hour duration, 100- year rainfall rate is the amount of rain that is likely to fall in one hour once every 100 years. However, in 1991, the Standard Building and Plumbing Code changed the requirements for overflow design. The 1991 Standard Plumbing Code adopted the 15- minute duration rainfall rate for design overflow drainage systems, which is the amount of rainfall that can be expected to fall in 15 minutes once every 100 years. To put it in perspective, the 15- minute, 100-year rainfall rate is approximately twice the 1-hour, 100-year rainfall rate. 1507.3 Maximum Rainfall Rate for Secondary Drains. Secondary (emergency) roof drain systems or scuppers shall be sized based on the flow rate caused by the 100- year, 15-minute precipitation as indicated on Figure 1507.3. The flow through the primary system shall not be considered when sizing the secondary roof drain system. The National Standard Plumbing Code has also adopted this criterion. The rationale for using the 15-minute duration, 100-year rainfall is based on the concept that roof drains may become blocked during extreme storms like hailstorms and hurricanes. The Standard Plumbing Code was formerly the Southern Building Code, which was adopted primarily throughout the Southeast part of the United States, including the hurricane-prone Gulf and Atlantic Coast states. Blowing debris during hurricanes commonly block roof drains, and hurricanes often produce extraordinary rainfall. The combination of blowing debris and extraordinary rainfall can be catastrophic for roof drainage systems. The first International Plumbing Code (1997) also adopted this 15-minute, 100-year rainfall rate, but reverted to using the 1-hour, 100-year rainfall rate for overflow systems in 2000. Also gone is the old Uniform Building Code standard of using Proceedings of the RCI 22nd International Convention Patterson – 129 overflow scuppers three times the area of the roof drains. The Uniform Building Code first provided the overflow scupper option in the 1967 Edition, which follows: (c) Overflow Drains and Scuppers. Where roof drains are required, overflow drains having the same size as the roof drains shall be installed with the inlet flow line located two inches (2″) above the low point of the roof, or overflow scuppers having three times the size of the roof drains may be installed in adjacent parapet walls with the inlet flow line located two inches (2″) above the low point of the adjacent roof and having a minimum opening height of four inches (4″). Overflow drains shall be connected to drain lines independent from the roof drains. There is a major flaw in this drainage criterion. This criterion does not take into account the depth of water that develops at the scupper based upon the geometry of the scupper. The fundamental function of the scupper is to control the depth of water, so as to limit the load on the roof structure and prevent a collapse. The geometry of a scupper is critical. For example, a 6-inch drain has an opening of approximately 28 inches. Using the old Uniform Building Code’s standard of providing an overflow scupper three times the area of the roof drain, the overflow scupper size would be 84 square inches. The problem with this standard was that a designer could select an 8- inch-wide by 10.5-inch-high scupper and meet the code requirements. However, the head (depth) of water at the scupper would have to be 8.4 inches to achieve the flow rate required. The net result would be a depth of water of 10.4 inches at the scupper, as the overflow scupper is located 2 inches (10.4 + 2) above the roof. This depth of water could easily cause a collapse in many circumstances. Based upon the old UBC, it was possible to collapse a roof using overflow scuppers three times the area of the roof drains. Roof drains are roughly three times more efficient than scuppers in terms of flow. The 8-inch-wide by 10.5-inch-high scupper has the flow capacity of almost 790 gallons per minute, which is slightly more than a 6-inch drain. Unfortunately, the head of water required to achieve that flow through the scupper is 8.4 inches, while the head of water required to achieve approximately the same flow is only about 3.6 inches. In 1991, the Standard Plumbing Code introduced a requirement to determine the depth of water that can accumulate on the roof and to notify the structural engineer to design the structure based upon this depth of water, assuming the primary drains are blocked, which is today’s standard. The excerpt from the 1991 Standard Plumbing Code (above) shows the chart used to determine the depth of water that can accumulate at a scupper. The 1991 Standard Plumbing Code began requiring designers to assume that the primary drains are blocked and to calculate the depth of water that could accumulate over the secondary or overflow drainage system. This is the basis of the design requirements provided in the initial ASCE 7 in 1988 and the initial International Building Code in 2000. This is the most logical approach and simply reinforces the concept that the structural engineer should design the structure to support the loads that could be expected to occur on a building. ASCE/SEI 7-05 provides a basis for calculating the head of Patterson – 130 Proceedings of the RCI 22nd International Convention water over drains and scuppers. There are also methods for calculating the head of water over scuppers and drains provided in Roof Drainage. There are methods for determining the head of water (depth) in scuppers and over drains. At the right is the chart from data included in ASCE/SEI 7-05. The overflow drainage system controls the depth of water that can accumulate on a roof and is the most critical part of the drainage system. The roof structure should be designed to support all the loads anticipated on the roof, and water build-up is one of the most important sources of loads on a roof. The requirement for overflow systems goes back to the beginning of the modern national codes, and these requirements have been refined over the years. Today, the codes are pretty simple and straightforward. Calculate the depth of water that can accumulate on the roof assuming the primary drainage system is blocked, and design the structure to support those loads. The structure must be evaluated for ponding instability any time the slope is less than 1/4:12. These are basic concepts that have been articulated in codes and standards for years. Location of Overflow Drains and Scuppers Overflow drains and scuppers should be located above the low point of the roof to help prevent the overflow drains and scuppers from becoming blocked. Debris is carried by the flow of water on the roof, which is to the low point. The primary drains are located at the low point, which is what makes them susceptible to becoming blocked by debris. Placing the overflow drain or scupper approximately 2 inches above the low point of the roof reduces the likelihood that debris will flow into the overflow system and become blocked. Additionally, overflow drain outlets should be located in a prominent location so that maintenance personnel can readily observe water flowing out of the overflow. This is an indication that the primary drains may be blocked and maintenance needs to be performed. Sometimes it is difficult to place overflow scuppers 2 inches above the low point of the roof, due to the location of the drains. Today, there is no specific requirement to locate the overflow drain or scupper 2 inches above the low point of the roof. The requirement is to find out how much water can accumulate above the overflow system and design the structure accordingly. However, the overflow drain or scupper should be located approximately 2 inches above the low point to prevent debris from blocking the overflow, but the scupper can be located at a higher elevation, provided the structure is adequate to support the load. PRIMARY DRAINAGE DESIGN General Requirements The primary drainage system is an important element in drainage designs, but it is the least important of the three elements of drainage design. These elements include the slope of the roof and ponding instability, the overflow drainage system, and the primary drainage system. However, it is important that roofs drain freely, and the primary drainage system is designed to remove water efficiently. This author has investigated roof drainage systems that were under-designed, resulting in water depths deep enough to cause leaks at low curbs and expansion joints. The primary drainage system generally consists of either roof drains or scupper drains. Most roof drains today are manufactured by companies like Josam, Zurn, and J.R. Smith and have standard flow rates, which are reflected in the various plumbing codes. These drains are designed with sumps and strainers that conform to the codes. Strainers are important, as they block the debris from getting into the drains’ lines. Also, strainers can actually improve the flow into drains by breaking up the vortex of the water flowing into the drain. Scupper drains, on the other hand, are generally shop- or fieldfabricated, and generally, flow rates must be calculated. There are no standard strainers designed to promote water flow and function with debris. Scuppers also generally require a greater depth of water to achieve the designed flow, so the depth of water at scuppers can be significant. Scupper drains and overflow drains should be separate and should not be connected. Proceedings of the RCI 22nd International Convention Patterson – 131 Drainage Rates for Roof Drains The various plumbing codes generally agree on the design criteria in principle. However, there are variations in the flow rates between the rates provided in the International Plumbing Code and the other two national plumbing codes – the National Standard Plumbing Code and the Uniform Plumbing Code. It should be noted, however, that the flow rates in the International Plumbing Code are the same as the old BOCA Plumbing Code, old Standard Plumbing Code, and old Uniform Plumbing Code (prior to 1997). Adjacent is a comparison of the flow rates. The National Standard and Uniform Plumbing Codes (after 1994) are more conservative. All these flow rates are based upon Manning’s equations, but there are slightly different assumptions regarding the amount of open area in the pipe, which results in the differences in the charts. Roof drain manufacturers also publish drainage design literature, and most of these manufacturers use the same drainage design assumptions as the International Plumbing Code’s standards. Drainage Rates for Scuppers Scupper drains must be designed, and there are also variations in the formulas used for calculating the flow through a scupper. It is important to understand that water has to build up to a relatively high elevation in order to achieve the design flow rate through the scupper. The depth of water that develops is primarily an issue of the width of the scupper. The wider the scupper, the lower the head of water that will develop at the scupper, which is desirable even if the structure is designed to support the loads from a large head of water. The flow rate of water through scuppers is generally determined by the derivation of an equation known as the Francis Formula, which is Q = 3.33LH1.5 where Q is the flow rate, L is the length of the weir (scupper), and H is the head of water. Because experiments have shown there is a contraction in the water flowing through the weir, the equation has been modified to adjust for this reduction. The modified form is Q = 3.33(L- 0.2H)H1.5. Below is a chart derived from this equation. The design of the primary drainage system is relatively straightforward. There are variations in flow rates of drains and scuppers, and further research into these variables would be helpful in establishing consistent design guidelines. DRAINAGE DESIGNS FOR ROOF REPLACEMENTS Requirements to Modify Drainage for Reroofing The 1967 Uniform Building Code added Chapter 32 to the appendix of the code, which was titled Re-Roofing, and the first section (3209) in that chapter stated that all re-roofing had to comply with Section 32 in the Building Code. This was a significant change in the code. However, the most significant change came in 1988 with the addition of the statement that roof systems shall be sloped a minimum 1/4- Patterson – 132 Proceedings of the RCI 22nd International Convention inch in 12 inches for drainage. This requirement to provide a minimum slope in Chapter 32 of the 1988 UBC in combination with the changes made to Chapter 32 in the appendix for Reroofing had major implications for reroofing design. Chapter 32 in the appendix required a re-roof to conform to Chapter 32 of the code, which required the roof be sloped a minimum 1/4 in 12 inches. There was no reference to draining within 48 hours or allowing 1/8-inch per 12 inches for coal tar pitch. From a fundamental design perspective, this was the most appropriate code dealing with roof drainage and roof slope. A minimum slope of 1/4-inch in 12 inches has long been recognized as the most appropriate minimum slope for low-sloped roofs. A minimum 1/4-inch per 12 inches is also important from a structural design perspective, as any roof with less than this slope has to be designed for ponding instability. Deflections in structural elements with less than 1/4- inch per 12-inch slope can result in a progressive collapse due to deflection. In other words, the roof deflects, allowing more water to accumulate until the roof collapses. Clearly the authors of the 1988 UBC were addressing the issues that cause roof collapses. Below is Section 3210 from Chapter 32 in the appendix to the 1988 UBC. Inspections Sec. 3210. New roof coverings shall not be applied without first obtaining an inspection by the building official and written approval from the building official. A final inspection and approval shall be obtained from the building official when the re-roofing is complete. The pre-roofing inspection shall pay particular attention to evidence of accumulation of water. Where extensive ponding of water is apparent, an analysis of the roof structure for compliance with Section 3207 shall be made and corrective measures, such as relocation of roof drains or scuppers, resloping of the roof, or structural changes shall be made. An inspection covering the above-listed topics prepared by a special inspector may be accepted in lieu of the preinspection by the building official. These changes in 1988 were met with less than an enthusiastic response from elements of the roofing community. In fact, this change was a mind-altering event for many in the roofing industry. A great number of existing buildings did not have a minimum 1/4- inch per 12-inch slope. In some cases, it was not only impractical but it was virtually impossible to provide the minimum slope. Then there were the issues from the coal tar pitch industry, where 1/4-inch per 12-inch slope could be too much slope for the system. The result of these issues and others was a watering down of the requirements. Current Design Standards Today’s International Building Code is relatively ambiguous regarding positive drainage. As defined, “positive drainage” is based upon ensuring drainage within 48 hours of precipitation. As previously stated, water standing for 48 hours does not constitute good drainage. What does “48 hours from precipitation” mean? Does it mean 48 hours in summertime conditions or wintertime conditions? A properly sloped roof should drain freely. Other than anomalies in the roof created around penetrations or crickets and valleys, there should be no water ponding after a rain. Clearly, the code requires reroofs to have positive drainage, but since the definition is ambiguous, enforcement is difficult. Some building officials have ruled that the design professional is responsible for making the determination of what constitutes positive drainage, but often there is no design professional in the case of reroofing. Certainly, positive drainage is a benefit in terms of roofing longevity and performance, and providing 1/4-inch per 12-inch eliminates many structural concerns. In those cases where achieving 1/4-inch per 12-inch is not practical, care should be taken to help limit the amount of water that can accumulate on the roof and involving a structural engineer should be considered. It is important to understand that simply re-sloping a roof with tapered insulation may not be adequate. It is imperative that the drainage system function properly after the tapered insulation is installed. Often, increases in insulation thickness can restrict drains and overflow systems. One of the most important requirements for reroofing is to make sure there is a proper overflow system. This is a code requirement that is often overlooked by many in the roofing industry, but the overflow system is critical in terms of limiting the amount of water that can accumulate on a roof and in preventing roof collapses. As a rule of thumb, some try to limit the depth of water to a maximum of 4 inches, which prevents the load from water buildup from exceeding the minimum 20-psf live load used throughout much of the southern regions of the country. It is also important to understand that even though there is a minimum 20-psf live load requirements in the code, there are live-load reductions that are allowed by code, which can reduce live loads on certain structural elements to 16 psf and even 12 psf in some cases. Proceedings of the RCI 22nd International Convention Patterson – 133 CASE HISTORIES Drainage design problems come in all types, from poor drainage deteriorating a roof to causing major roof collapses. Most of the roof collapses investigated by this author have involved defects in drainage design, usually in combination with other factors. The build-up of rainwater is nature’s way of load-testing structure, and sometime structural design or construction deficiencies are identified. Realize that most drainage systems are designed based upon the 100-year rainfall occurrences, so it may take a long time before the structure really gets tested by one of these rainstorms. Below are some examples of some of the collapses investigated by this author. • The first roof collapse investigated was a simple case of the contractor roofing over the scuppers during construction. Unfortunately, that roof received its first load test before the contractor had time to cut in and flash the scuppers. • One of the first really large losses involved a computer assembly facility. A looselaid, ballasted, single-ply had been installed over an existing built-up roof without evaluating the structure. The extra dead load for the roof dramatically reduced the live-load capacity of the structure, resulting in deflection between the drains. This structure failed as a result of a progressive deflection of the joists as the water depth kept increasing between the drains. • One of the most expensive collapses occurred in a building that had no overflow scuppers. Instead of a conventional strainer, screens were installed in front of the drainage scuppers to catch debris. The screens worked efficiently and soon became blocked with debris. One eyewitness reported water lapping over the top of the 12- inch parapet wall just before the roof collapsed. • Another large collapse in volved the irregular spacing of the roof drains and a defective joist girder. There were enough drains if the drains had been spaced evenly, but the end bays were 50% larger than the typical bays, resulting in 50% more water. Additionally, there were no provisions for overflow. The head of water over the drain was high enough to cause the defective joist girder to fail. • One of the most dramatic collapses involved a concrete structure. The roof was designed to drain to an outside wall through drainage scuppers, but the roof had deflected as a result of long-term plastic deformation. The roof sagged in the middle, resulting in ponding. The roofer decided to install drains in the center and add slope from the outside walls to the center of the roof. The drains were too small, and the drain lines were not sloped, restricting drainage. There were no overflow scuppers or drains, and the roof collapsed during a heavy rain. • One collapse was actually predicted by this author after a routine roof inspection. The core sample indicated that there were multiple roofs, one on top of the other, weighing more – than 20 psf. The recommendation was to remove the roofs immediately, as the roof could collapse during a heavy rain. The roof was flat, and the addition of all the roofs over the years reduced the live load capacity to zero. Unfortunately, the owner waited, and the roof collapsed several months after the inspection. This sampling of collapses illustrates some of the issues related to collapses involving roof drainage design. Lack of overflow is a common problem, along with inadequate drainage and too much weight from the roof(s) installed. Sometimes the weight of the water simply finds the weak link in the structure. FINAL COMMENTS The basic concepts of proper roof drainage design have been around for many years, and there are extensive data and design guides available. Roof Drainage, published by the RCI Foundation, provides a much more complete discussion of roof drainage, and every roof consultant should have a copy in his or her library. There are still issues that need clarification and additional research is needed, particularly in the area of water accumulation on roofs and the appropriate flow rates of drains and scuppers. The roof consultant can play an important role in preventing roof collapses. It is essential that there is a properly functioning overflow system on a building. Checking roofs for an appropriate overflow system and recommending corrective action to add or enlarge overflow drains and/or scuppers should be a standard part of the roof investigation process. Further, improving the drainage and the design and installation of overflow systems should be part of a reroofing project where these systems are inadequate. Patterson – 134 Proceedings of the RCI 22nd International Convention
