Example 3: PMRA garden roofs and water retention and detention features (blue roof)
The final example is a blue roof design in an urban area where water retention is essential (see Figure 7). Designed by Perkins + Will, the Northwestern University blue roof project involves phased construction: The 14-story base building was constructed first, and a 20-story tower addition is planned for construction five to 10 years into the future. Site conditions required a more creative approach to stormwater management than conventional, below-grade options.
The City of Chicago Department of Stormwater Management required the project account for the total building mass and footprint and a substantial detention requirement of 283 m3 10,000 cf) of stormwater. Retention volumes are different than detention volumes. Retention occurs within garden roof growing media where the water is retained until used by the plants or evaporated. Detention occurs in a blue roof where water is detained for a specific period until fully released to enter the storm sewers. This prevents flooding the sewer system. The City of Chicago Stormwater code required this stormwater volume be released through the flow control drains within 48 hours.
The primary stormwater detention requirement was met by creating two blue roof assemblies on the entire Level 13 and 14 roof structures. These blue roof areas were created using structural voiding units which were subsequently overlaid with garden roofs and paver assemblies for retention capacity and to provide proper ballasting. A conventional garden roof assembly was used on Level 2. The combination of the garden roofs on all levels reduced the overall detention requirement required by the City of Chicago.
To detain the blue roof water volume, roof drains were modified with a special insert that passively regulates stormwater drainage. The blue roof volume spaces were created using a structural plastic voiding component that is 95 percent open (see Figure 8). To optimize the blue roof storage, the roofs on this building were constructed with flat, zero-slope roof decks. These decks were waterproofed with a hot, fluid-applied, rubberized asphalt membrane and insulated with 178 mm (7 in.) of type VI extruded polystyrene insulation. The ability of XPS insulation to resist the wet conditions in a blue roof and green roof make it the perfect choice for thermal insulation.
Versatile solutions
As can be seen from the examples above, depending on the needs of the building environ, PMRAs can be engineered to drain water quickly, as in the case of the Dickies Arena; or retain water and gradually release it, as in the case of the Northwestern University blue roof. A PMRA can also be designed with a conventional drainage system at the roof covering level, as in the case of the aggregate topped federal building.
Another seldomly recognized benefit of PMRAs is reduced landfill. The average lifespan of a PMRA is 25 to 50 years, and the XPS insulation from such roofs can be and often is reused when the building is recovered at the end of the roof life cycle. Roof replacements send billions of cubic yards of to landfills annually. Over the lifespan of the building, PMRA roofs are better for the building owner and better for the environment.
PMRAs provide a platform for a variety of commercial and urban residential roof assemblies, including reflective and photovoltaic roofs for energy efficiency, extended roof life cycles, and vegetative roof assemblies for healthy cities. PMRAs support solutions to stormwater detention, mitigating flooding in urban areas, and contributing to water conservation. Finally, they also allow valuable outdoor areas to be repurposed as occupiable spaces.
John Woestman is director of codes and standards to the Extruded Polystyrene Foam Association (XPSA). He has more than 30 years experience in the construction and building products industry with various responsibilities in construction, manufacturing, human resources, marketing, and codes, standards, and regulations. He has a diploma in building trades, a degree in mechanical engineering, and an MBA.
