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Weekly UpdatesENERGY EFFICIENCY AND ENVIRONMENTALLY SOUND DESIGNS ARE BECOMING INDUSTRY STANDARDS. AT THE SAME TIME, MANY CORPORATIONS AND MUNICIPALITIES ARE REQUIRING GREEN AND SUSTAINABLE PRACTICES THROUGHOUT THEIR BUILDINGS. In this environment, you must always seek ways to improve the performance of new been used for and existing buildings.
Green roofs have been used for decades in residential and commercial construction in Europe, but their use is a recent development in the United States. Green roofs can provide many benefits that a typical roof system cannot, such as decreased heating and cooling costs, better property aesthetics and increased roof life.
So how can you determine if greening your roof is a good option for your properties? It's important to first understand the basic idea behind green roofing systems. There are two kinds of green roofs in the United States marketplace: intensive and extensive green roofs. They represent the two ends of the spectrum of green roofing options, but any particular system may fall somewhere between the two extremes.
WHICH ROOF FOR YOU?
Intensive green roofs are typically called roof gardens. They have soil depths greater than 8 inches, contain a multitude of vegetation and can act as an outdoor space for building occupants. These roofs typically also provide aesthetic views for building occupants and abutters, as well as varied vegetation--including the potential for vegetable and flower gardens. On the flip side, they require additional cost for installation, additional landscaping maintenance and need a higher structural roof capacity for additional weight.
Extensive green roofs are more like meadows than gardens. They are typically not meant for occupant use on a regular basis. They vary in soil depth from 3 to 8 inches, and the vegetation is generally limited to grasses and other easily maintainable plant life. While they do provide an aesthetic view, they don't offer outdoor recreation space due to the fragility of system. Since these roofs require drought-resistant plants, the plant choice is also more limited. But unlike intensive green roofs, extensive green roofs are cheaper to install and require less landscaping maintenance and a lower structural roof capacity for additional weight.
KEY COMPONENTS
Structure is the most critical of all green roof components. In new design, green roofs can add cost to the structural framing. For existing buildings, the structure governs what additional weight the roof can hold. While it is possible to reinforce existing roof structures to support the increased loads of a green roof, it is often not financially feasible.
Roof structure and the capacities of the supporting structural elements should be the first items reviewed before a green roof is applied. As part of the structural capacity review, have an engineer consider the added weight of fully saturated planting media. Often overlooked, the weight of saturated soil may easily double or triple the dry weights of the planting media.
Another key element is waterproofing. Appropriate systems will depend on the type of roof deck, details required (penetrations, terminations, etc.) and the green roof system used. Oftentimes, the entire system is supplied by one manufacturer, from the waterproofing to the growing medium.
Other components of the system--such as protection board and drainage composites--will be dependent on the type of waterproofing system and the manufacturer. The selection of these components can significantly impact the cost, as some manufacturer warranty requirements may not allow for mixing and matching of the components. Carefully consider the type of waterproofing system selected. Waterproofing assemblies with reliable long-term performance service records will be the optimal choices for the garden roof system.
Contractor installation is also a very important factor in the success of a green roof installation. Like a conventional roof, the waterproofing membrane cannot have any installation flaws or it will leak, regardless of how reliable the waterproofing membrane may be. The risk for leakage can be minimized if the following precautions are taken during installation:
* Increase site observations to identify problems and resolve conditions not anticipated in the design drawings.
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* Inspect all installed membranes.
* Complete flood testing of the waterproofing membrane before installation of any green roofing components--including insulation, drainage board, protection layers, etc. Flood testing should be to the full soil depth to account for the possibility of blocked drainage.
The design must not allow base flashings at rising walls to fall below the level of the overburden or overtopping of the flashing and subsequent leakage may occur. In cold weather regions, base flashings should also extend above the level of normally anticipated snow drifts.
Successful green roofs go beyond the installation of a leak-free waterproofing system. The vegetation on the green roof must also be aesthetically pleasing and flourish in different conditions--from saturation to drought. Therefore, hiring a good landscape architect is a key component. A landscape architect can realize an owner's vision for what the green roof should look like and select appropriate plants that will thrive in a variable environment.
BREAKING DOWN THE PROS & CONS
Owners may realize some of the advantages of green roofs immediately, while other advantages may occur over years of operation. In addition to the owner's benefits, the occupants and public will likely realize positive results from a green roof, including:
* Potential for decreased heating and cooling costs because green roofs provide additional insularion and decrease urban island heat effects.
* Reduced storm water runoff, there by reducing the buildings load on the city drainage system.
* Better aesthetics for the surrounding buildings that view the roof.
* Potential for quicker permitting and possibility of tax credits, depending on local, state and federal government requirements.
* Increased roof life between replacements from the protective nature of the plantings over the membrane.
* Improved air quality due to the filtering effects of vegetation, reducing toxins and creating oxygen.
* Increased sound insulation for the top floor.
* Potential for good public relations and marketing.
In spite of the positive results green roofs can deliver, they do have some drawbacks, which must be closely evaluated in order to make the optimal roofing decision, including:
* Additional initial cost and installation time due to the special layers of material.
* Maintenance costs for the installed vegetation.
* Difficulty identifying water leakage locations and repairing any leaks should they occur.
* Additional roof overburden weight requiring a more substantial structural system in new construction or structural system upgrades in existing construction.
If you are considering a rooftop garden, it might be a good idea to conduct a cost/benefit analysis first with these ideas in mind. If the positives outweigh the negatives, it might be worth building into your budget.
Emily W. O'Keefe (ewokeefe@sgh.com), engineer; Peter M. Babaian (pmbabaian@sgh.com), senior staff engineer; and Michael J. Louis (mjlouis@sgh.com), principal, work in the Waltham, Mass. office of Simpson Gumpertz & Heger Inc.
Simpson Gumpertz & Heger (SGH) designs, investigates and rehabilitates structures and building enclosures. For more information visit www.sgh.com.
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