The finished product — a LEED-platinum certified building — with a successfully strengthened structure supporting a new green roof.

In the race to build green, many building owners and contractors have turned to an unlikely place: the roof. A green roof is a green space created by adding a growing medium and plants on top of an existing or new roofing system. A distinction should be made between a green roof and traditional roof garden. A garden roof is typically done with containers and planters located on a roof terrace or deck while a green roof system is made of several layers that are installed directly on the roof. From the bottom up, these layers include: the roof structure, a waterproofing membrane, a drainage layer, a growing medium, and plants. Green roofs have been shown to reduce heat loss and energy consumption in winter conditions and help keep temperatures down, particularly in urban areas.

Green roofs have many environmental benefits. Chief among the benefits is the ability to better regulate stormwater runoff, help keep temperatures under control — both inside and outside the building, filter dust and smog particles, provide a habitat for wildlife, insulate noise, as well as increase the lifespan of the roof. For property owners seeking Leadership in Energy and Environmental Design (LEED) certification, a green roof can contribute to several credits toward green certification of new and existing buildings. Because of these factors, green roofs are currently experiencing a boom in popularity in urban environments. Many green roofs are designed like gardens, with pathways and manicured landscapes, and are popular amenities for office buildings, hotels, and condominiums looking to create green spaces for tenants and guests. What was a fad a few years ago is now developing into a quantifiable building improvement for natural aesthetics, lower utility costs, and reduced local watersheds.

Basic green roof terminology
There are two types of green roofs: intensive roofs, which are thicker and can support a wider variety of plants, and extensive roofs, which consist of a light layer of a growing medium and vegetation. Intensive green roofs are generally heavier, include a deeper layer of growing medium, and have greater needs for irrigation and maintenance. These roofs are designed as amenity space that can be used by building tenants or the general public. Soil depth for intensive green roofs varies from 6 to 24 inches (or more) and can weigh from 80 to 200 pounds per square foot (psf). Extensive green roofs, on the other hand, are generally designed to be lightweight to maximize the performance and environmental benefits for the least increase in design loads. Extensive green roofs feature a layer of growing medium that is 6 inches deep or less and weighs 15 to 50 psf. They have low maintenance requirements, but are generally not maintenance free. Extensive green roofs are not designed for public access and require less initial investment, making them ideal for owners who are simply looking for the energy benefits a green roof can provide.

Green roofs can also be classified as integrated or modular. Integrated green roof components are installed as a series of layers. Modular green roofs are partially assembled off-site and are installed in units. Some modular systems feature plastic or metal trays that are filled with growing medium and placed on the rooftop.

Before any installation can take place, it is important for a licensed structural engineer to determine if the existing structure can support a green roof. The engineer will need to know the following information to complete the structural analysis:

• green roof type, layout, and loads;
• type and condition of the existing roof membrane;
• existing and required electrical and water supply;
• roof accessibility for installation and maintenance; and
• current structural system capacity.

Green roof loading is one of the main factors in determining both the viability and the cost of a green roof installation. For a green roof installed on an existing building, the design can be limited to the carrying capacity of the existing roof, or the existing roof system can be upgraded to support the new green roof loads. Typical green roofs weigh between 30 and 100 pounds per cubic foot. This is a heavier load, considering that most existing roofs are typically designed for a live load of 30 or 40 psf. In general, more complex green roof systems will require more robust structural upgrades. However, strengthening solutions have been successfully used to increase the load carrying capacity of structural systems and are typical considerations for green roof installation projects.

Strengthening solutions
There are different methods available to strengthen existing buildings, including externally bonded fiber reinforced polymer (FRP) composites, span shortening, externally bonded steel, external or internal post-tensioning systems, and section enlargement. A thorough analysis of the existing capacity and the effects of the new loads on the roof structure including slabs, beam, and columns is a critical first step. The strengthening techniques will depend on the specific requirements of the project in regard to the type of deficiency (flexure, shear, and/or torsion), magnitude of strength increase, constructability issues, aesthetics, and economics; see www.gostructural.com for more details.

Green roofs in action
Today’s savvy owners are looking to add green roofs to their properties to enhance value. One example is an eight-story office building located at 1225 Connecticut Avenue in downtown Washington, D.C., owned by Brookfield Properties. In September 2008, the building was undergoing a complete renovation. The owner originally registered the project as LEED EB (Gold) certification, but decided to add a green roof to complement the new penthouse and to earn enough credits for a Platinum rating — the highest allowed under the LEED system. A strengthening program was needed to address the increase in required capacity from 30 psf to 75 psf — part of which came from the 4 to 6 inches of saturated growing medium. It was determined that a carbon fiber reinforced polymer (CFRP) system could supply up to 100 psf (10 psf superimposed dead load and 90 psf live load).

In certain places on the roof structure, the team investigated using rebar equivalents, but the roof needed underside reinforcement because of the penthouse wall. This wall required the use of CFRP as approved by the engineer-of-record, SK&A Structural Engineers, PLLC.

The strengthening contractor, Structural Preservation Systems LLC (SPS), worked to develop a space-saving solution involving the following:

• CFRP sheets on the underside of the slab;
• near-surface mounted (NSM) carbon fiber rods on the top side of the slab;
• concrete drop panel enlargements designed for optimum structural enhancement and minimal space disruption; and
• column enlargement modified for load requirements, minimization of labor, and usable tenant space.

Projects involving strengthening of an existing structure offer different challenges than new construction. A significant one with this project was access to the roof. A crane was not used during the project; therefore, all equipment and repair material (including thousands of bags of concrete) were taken up to the roof via a small service elevator. Some of the equipment had to be dismantled to fit in the elevator and then be re-assembled on the roof. Three mobilizations were required to accommodate the overall project schedule.

These challenges were addressed and the solutions implemented with great success. The building now has a green roof that not only enhances the value of the property, but also helped the owner attain LEED-platinum certification.

“The green roof addition to the project presented logistical, scheduling, and engineering challenges to the project,” said Chris Voros, project manager at James G. Davis Construction Corp. “Without the capability of the CFRP, we would have been limited in our options to include this part of the project, and we would have not been able to achieve a LEED-platinum rating for Brookfield Properties.”

Structural strengthening leveraged many strategies including carbon fiber reinforced polymer (CFRP) sheets installed on the underside of the concrete slab. A final strategy included adding a drop cap at the column and enlarging the existing columns to support the increased loads of the green roof.

Key to a successful green roof
Depending on the type of green roof system selected, installation can be a major undertaking. Upgrading a roof for additional capacity requires careful consideration to ensure the structure is prepared to handle the additional load. Because every element of the existing structure carries a share of the load, the effects of strengthening or removing part or all of a structural element must be analyzed carefully to determine its influence on the global behavior of the structure. Failure to do so may overstress the structural element and cause serious problems. In addition, performing work on an existing structure requires attention to critical issues such as access to the work area, noise and dust control, and compatibility of construction materials.

Tarek Alkhrdaji, Ph.D., is an engineering manager with Baltimore-based Structural Group. He may be contacted at 410-796-5000 or at talkhrdaji@structural.net. Dan Schneckenburger, LEED AP, works for Structural Preservation Systems in its Baltimore office and can be reached at dschneckenburger@structural.net.