Pathways to Professionalism Proceedings of the RCI 20th International Convention & Trade Show Miami Beach, Florida March 31 – April 5, 2005 © Roof Consultants Institute 1500 Sunday Drive, Suite 204 • Raleigh, NC 27607 Phone: 919-859-0742 • Fax: 919-859-1328 • http://www.rci-online.org A New Look at Ballasted Roof Performance David L. Roodvoets DLR Consultants Chantilly, VA ABSTRACT Ballasted roofs gained popularity in the 1960s. Since that time, they have become one of the most installed sheet membrane systems. After these many years, we have learned much about their installation and performance, including where they work best and some of the limitations and concerns. The wind design of the systems must meet the building code requirements using RP- 4. These requirements are reviewed, as well as new data on the energy performance of the systems. Energy saving opportunities of ballasted systems, including photovoltaics and reflective pavers, will be discussed. The consultant will be able to discern where the systems work well and where they should be avoided. New opportunities will also be presented. SPEAKER DAVID L. ROODVOETS is technical director of SPRI and has been involved in ballasted roofing systems since the 1970s. He has participated in the development of ANSI/SPRI RP-4 Wind Design of Ballasted Roofing Systems. He has worked with the wind design of photovoltaic systems and the use of pavers. Mr. Roodvoets has been involved in several energy related studies, which he most recently reported on at the 19th Annual RCI Convention, as well as the wind performance of asphalt shingle roofs; also reported , also reported at the 19th Annual RCI Convention. Roodvoets – 123 Roodvoets – 125 HISTORY The concept of the ballasted roof is one of the oldest in the history of roofing. Sod roofed huts were around long before any of the common roofing systems existed. One big problem with sod roofing was a lack of a roofing membrane beneath the sod. Some early sod roofs had animal hides that were covered with earth or flax; however, this was an expensive and limited solution to the problem of water getting into the building. The Egyptians and Babylonians discovered the use of tar as waterproofing and this may have been introduced to other cultures. Tar, or naturally occurring accessible asphalt, became the means to waterproof a roof. This helped make flat roofs possible in these areas of the world over 5,000 years ago. The roofs in Egypt were used for sleeping, bathing, and as an evening escape from heat. The sticky tar needed to be covered, and hence the ballasted roof. Although first-hand evidence is lacking, it is likely that natural paving stones were used, as well as existing clay. We don’t know if they worked well; however, we find temples buried in sand that are perfectly dry inside. (Doesn’t rain much there, either.) Some of the ballasted roofs in the 1930s and 1940s were garden roofs. These roofs were constructed on buildings with concrete decks that were waterproofed with asphalt or coal tar products. These roofs demonstrated long, useful lives and some are still performing. The next phase in ballasted roof development was the use of stone ballast for sheet membranes and the protected membrane roofs (PMR). These systems used the ballast stones or pavers to hold the membrane and/or insulation in place. This is contrasted to the pea gravel-covered, built-up roof systems where the gravel was used as UV protection and additional fire protection as well as a clean walking surface. In the ballasted roof, the concept of just the weight of ballast keeping the system on the roof was proven, both for individual insulation boards in the protected membrane system and sheet membranes. Skepticism still runs high, with those in doubt about the wind performance of ballasted roofs still populating the ranks of code officials and structural engineers. The skepticism was attacked head on in the 1970s and 1980s with wind tunnel studies at the National Research Council of Canada and field studies sponsored by major membrane suppliers. Wind tunnel research conclusively proved that ballasted systems can withstand high winds. Gravel ballast systems’ wind resistance is proportional to the mean size of the ballast, and, of course, the larger the ballast, the greater the wind resistance. Field evaluations confirmed the wind tunnel results and 40 years of practice have supported the research. The result of the research was a monograph by Kind & Wardlaw which, combined with field research, resulted in the development of ANSI/SPRI RP-4. The American National Standard was first developed in a combined effort from The Rubber Manufacturers Association and SPRI. SPRI assumed sole responsibility for the standard in the 1990s. A New Look at Ballasted Roof Performance Photo 1: Gravel-ballasted roof. Roodvoets – 126 Types of Ballasted Roofs Today, ballasted roofs come in many forms. The most common is the gravel-ballasted roof. It weighs 10 pounds per square foot or more and utilizes ASTM D-448 #4 stone or larger. Manufacturers require that river-bottom rounded stone be used. If this type of stone is unavailable, a separator must be placed between the membrane and the stone. EPDM is the most commonly used membrane. TPO, KEE, and some modified PVC membranes are also used. The advantages of the system are its ease of installation and overall excellent performance. The stone provides an ASTM E-108 Class A fire resistance; it protects the membrane from UV and reduces the temperature swings due to the diurnal cycle. Hail resistance is an outstanding feature of all ballasted roofs. Because the membrane is unattached except at the perimeters and penetrations, it easily accommodates building movement. This system is mostly used on large low-rise buildings with few penetrations. Paver-ballasted roofs come in a variety of forms. There are lightweight (10 to 12 pounds per square foot) interlocking pavers, intermediate weight (14-20 pounds per square foot), and heavy weight pavers (22 pounds per square foot and heavier). The heavy weight pavers are most frequently installed on pedestals and are used to create plazas and walking decks. The lightweight and intermediate weight pavers are most often placed directly on the membrane. Photo 2: Heavyweight concrete pavers. Photo 3: Lightweight interlocking pavers. Roodvoets – 127 Garden roofs are also a form of ballasted roof. These systems provide the benefits of the other ballasted systems, with the added benefit of having a vegetative surface that will provide positive benefits to the environment. Membranes used in these systems can be adhered or loose-laid. Protected membrane roofing is a variation of ballasted systems that place extruded polystyrene insulation above the membrane. PMR systems can be used with any topping. They protect the membrane from temperature extremes, and they have been shown to provide increased system life with lowered maintenance. One unique PMR system tops the extruded polystyrene with a cementitious topping. This product, which is interlocking and lightweight (4.5 lbs per square foot), can be used where the structural system does not provide adequate support for traditional ballasted surfaces. It also provides a superior walking surface for ease of equipment access. Why Ballasted Systems are Used There are many reasons for choosing a ballasted roof. Frequently, the owner does not want screw penetration of a metal deck; ballasted systems then are one choice. When roofing over a concrete deck, it is easier and less costly to install a loose-laid membrane and insulation ballast over the deck; then ballast over the system. Ballasted systems are installed quickly, allowing a large area of roof to be covered. Ballasted roofs resist hail; they are almost always ASTM D-108 Class A fire resistant, including over plywood decks, and can be used effectively in hourly rated fire resistant designs. When installed according to the requirements of ANSI/SPRI RP-4, they are wind resistant, and systems with superior high wind resistance can be designed. Installation of a ballasted roof can easily be taught and monitored. Installation crews learn quickly how to do it right. Monitors can observe and critique the roof both during installation and after completion. Precautions and Limitations Ballasted roofs, like all other roofing systems, must be installed according to the building code, manufacturers’ requirements, and good roofing practices. One of the most important assessments to be made before choosing a ballasted roof is the determination of the structural load capacity of the roofing system. Most systems weigh 10 pounds per square foot or more. The roofing system must have the dead load structural capacity for the weight of the system, plus a safety factor. The dead load safety factor often chosen is 1.5 times the expected dead load of the system. The International Building Code (IBC) requires that low-slope roofs have a secondary drainage system. This becomes the law when the authority having jurisdiction adopts the code. Secondary drainage is important in a ballasted system, because live load that exceeds the design requirements can result in building failure. Recent tropical storms, hailstorms, and hurricane experience have shown that the isopluvial maps used to design drainage can provide inadequate information in some of these extreme storm events. Ballasted systems typically have been installed on well-designed buildings with structural redundancy, so have not been the source of failure in this type of event. This practice should continue. When loose-laid membranes are used, it is important that all membranes be secured at all penetrations and perimeters. It is recommended that this securement always occur on the horizontal plane so as to use the greatest strength of the system and to prevent any shrinkage from loading the vertical surfaces. Membrane seams should always be in the plane of the roof so that shear loading is the only loading on the seam. Rossiter’s work in the 1980s and ’90s demonstrated Photo 4: Concrete topped extruded polystyrene. Roodvoets – 128 that the shear strength of EPDM systems is greater than peel strength. He also was able to quantify the improvement made by switching to taped systems. Later, Hoff provided data that clearly demonstrated the longterm performance of taped seams. Complying with ANSI/SPRI RP-4 is critical for wind design requirements. RP-4 provides the information needed to size the stones used and defines the buildings where the system can be safely used. When systems are installed according to RP-4, there is no expected stone scour or ballast movement in a design wind event. In the recent code cycles, the International Building Code has been modified to further limit the use of stone ballast in hurricane- prone areas. Code changes have been proposed that will again make the requirements of RP-4 the standard. The option remains to use pavers in areas where stone is excluded. Manufacturers of single-ply membranes also allow the use of nonrounded stone if there is an approved separator layer between the ballast and the membrane. The ballast required by RP-4 is divided into two classes – type 4 and type 2. The type 4 ballast is based on the performance of ASTM D-448 #4 stone. Systems that perform similar to #4 stone are classified as ballast type #4. The several field and wind tunnel studies have been based on replicating the stone sizes of ASTM D- 448. This means that the stone used in the wind tunnel tests included the various sized stones that are typically found in ASTM D-448 gradations. The stones on the roofs that were used in the studies that formed the basis of RP-4 were also representative of the gradations found in ASTM D- 448. These roofs included both the large stones and the very fine stones that are allowed. Type 2 ballast is the largest gradation required. This larger gradation significantly increases the wind speed required to dislodge stone. RP-4 also eliminates all stone ballast from the perimeters of roofs exposed to very high winds. Factory Mutual does not officially “approve” ballasted systems, but they have published design requirements and allow these systems on many buildings insured by FM Global insurance. Factory Mutual requires that ASTM D-448 # 3 stone be used in place of the #4 stone required by RP-4. Factory Mutual Research and Engineering believes this larger stone provides the increased margin of safety they require for insured buildings. Since #3 stone is not available in some areas, #2 stone is used. This larger stone does perform better than #3 or #4 stone, so Factory Mutual’s requirements results in their desired effect of having a more wind-stable roofing system. Ballasted roofs can utilize the largest membrane sheets available from the manufacturer. This reduces the number of field seams and the potential for workmanship leaks. The seams need to be made right in the first place. There are two options to ensure better seaming of ballasted roofs. The first is double seaming. This system is easiest to test (by means of inflation) and provides redundancy. The second is to use wider seams. This is easily accomplished with seam tapes and adds little to the cost of the system. For heat-seamable roofs, the double seam is likely to be most practical and efficient. When the seams are well made, the penetrations and perimeters become the most vulnerable leak locations. This is the same for almost all roofs, and leaks at these locations are usually easier to find and repair. Specifying and installing ballasted roofs compares with other roofing systems. Each has its set of precautions that must be understood and applied for a successful project. What’s new? One of the most promising fields is the use of photovoltaic panels (PV) as ballast. This system consists of an extruded polystyrene foam base with a cementitious topping and a photovoltaic panel mounted on stanchions on top of the system. In this system, the extruded polystyrene foam is tongue and grooved on all four sides, making a very strong interlock and the white coating protects the foam base from UV degradation. The PV weighs nearly 4 lbs per square foot. This system can be placed on about 90% of a roof, taking advantage of the space and providing electric power to the building. Because the PV is placed above the insulation with an air space between, the heat generated by the PV when active is largely dissipated above the surface of the insulation. Keeping the PV cooler adds to the life of the PV and provides increased output. The PV units have been installed on all types of membranes with success. As in all other cases, the membrane and seams need to be well made to function in this system. The membrane requirements are basically the same as all other ballasted roofing systems. Early results from an ongoing study sponsored by SPRI and being conducted at Oak Ridge National Laboratory show that stone ballasted roofs can provide energy savings similar to reflective roofs. The stone heats up more slowly than uncovered membranes, reaches lower peak temperatures, and loses the heat energy more slowly than the uncovered membranes. The lowered temperatures and the shift of the peak temperature reduce the Roodvoets – 129 peak demand on cooling systems, providing both total energy savings and lowered utility costs. Reducing peak demand is key to eliminating or postponing power plant construction. White reflective pavers, whitecoated pavers, and white-coated, cementitious, extruded polystyrene are now available. These products can be Energy Star® rated and can provide additional cooling energy savings to ballasted roofs. Rubber playing surfaces have been used as ballast. This provides additional functionality to the roof and safety for those using the roof. The rooftop garden can also be considered a ballasted roof. Rooftop gardens are a way to get more from a building site. They have been proven to slow or reduce water runoff and they provide valuable energy savings in cooling climates. They are also expected to reduce the heat island effect in cities. CONCLUSIONS Ballasted roofs have provided a viable and useful roofing option for the last 40 years. Their record of performance is outstanding; and when properly designed and installed, they will provide long, useful protection for the building. A wide variety of ballasted roof choices exist. They all add to energy savings and can provide outstanding aesthetics. Ballasted roofs continue to be one of the most specified roofing systems in the marketplace. Photo 5: Photovoltaic array.
