How much traffic can a roof system bear? The fact is, live loads on roofs are getting much bigger as building developers and owners seek to allow more indoor-outdoor uses and rooftop amenities, such as seating areas, gardens and even fire pits and pools, which draw people to the roof. Plus, the dead load may be increasing thanks to those living material installations, such as planters and vegetative roof gardens. These assemblies usually require or hold water—adding to the dead load—as well as frequent maintenance and inspections, which mean a few more people (and more live load).
“We’ve known the benefits of a green roof from a water-management point of view for some time,” says Joshua Zinder, AIA, principal of JZA+D, Princeton, N.J., noting that more than 70 percent of the water that hits the roof is absorbed. “Increasingly, we see the roof as an opportunity for generating revenue or enhancing the value of the building. One of the ways we’re now helping developers reposition older office and industrial properties is by determining if we can create roof farms or indoor-outdoor spaces not only on the ground floor, but also on the roof planes.”
The case of the rooftop garden with public access is a growing trend, too, and “one must ensure that the roof structure has the necessary structural capacity to support rooftop activity,” notes Kelly Luckett, author of Green Roof Construction and Maintenance. Local codes vary for live loads and dead loads, he explains, and the project team calculates the green roof assembly as part of the total dead load. “Water in excess of that which saturates the growth media, snow and people visiting the green roof are all considered part of the live load of the structure,” Luckett adds.
Just as important, the roofing system has to resist the wear and tear of the live loading. The three main concerns for exposed structural elements, such as roofs, balconies and terraces, are protection from weathering, water ingress and environmental damage. Pedestrian walkways must also ensure long-term durability.
A look at the latest trends in “activating rooftops” reveals even more reasons for roofing contractors, architects and facility owners to look more carefully at specification documents and installation methods for these live-load roof zones.
Skylife and Community
For residential projects with rooftop terraces, careful specifying and installation of green roof assemblies is critical. “We like using liquid membrane roof and extensive green-roof systems, such as sedum carpet,” says Andrew Franz, AIA, LEED AP, principal of Andrew Franz Architect PLLC, New York, adding that the systems work well because the drainage mat is modular, lightweight, and easy to install and adjust—something that is important on uniquely shaped urban rooftop terraces.
Recent projects by Franz include a 2,800-square-foot garden terrace for a family of four in Manhattan. A bluestone floor extends from the dining area to the terrace’s softscape herb garden, further blurring the boundary between in-doors and out. “The green roofing system also includes a protection mat, which protects the roof membrane, a filter sheet of very lightweight soil to protect the drainage mat and the sedum carpet,” Franz says.
Other recent projects with active green roofs demonstrate the benefits of strong PVC membranes, such as at the modern 93 Bright Street townhouse in Jersey City, N.J., designed and developed by Jorge Mastropietro, AIA, whose firm JMA is based in New York City’s Soho neighborhood. Another example, called Trouthouse, designed and built by the Brooklyn-based thread collective, is a showcase of “passive design” principles that reduce energy use, recapture water and even allow for a roof-mounted shade structure that doubles as photovoltaic panels.
The new LEED Gold-certified facility for Gateway Community College in New Haven, Conn., was designed with a vegetative roof to create a new community area on the top floor. According to construction manager Providence, R.I.-based Dimeo Construction, which worked with Providence-based Gilbane Building Co. and the New York office of architect Perkins+Will on the project, the “multi-level student gathering area steps up from the ground floor to a rooftop garden. The green roof also supports photovoltaic panels on a special framing system.”