Getting the Green Roof Right
For living green roofs and landscapes over structure, this can become a particularly difficult complication if conduits must be larger than anticipated, raising invert elevations and designed finish floor or grade elevations.
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Inaccurate or missing survey data can also have structural impacts — for example, making it difficult to achieve designed top-of-slab elevations and resulting in sections that are too shallow to support the designed site load.
Unforeseen geotechnical conditions can impact the designed horizontal or vertical layout, but more significantly, they can have limiting structural implications. An example of this is an unforeseen high localized water table, which can have a "bathtub" effect. A different type of slab construction may then be required, affecting the horizontal elevations.
Subsurface Water and Drainage
Similarly, if the construction itself causes an interruption of subsurface flow, drainage throughout the landscape over structure can become a problem, particularly when the project is at street or grade level: too much water with no outfall can saturate soil. This may not become apparent for some time, until plants decline and die.
Another major issue that can negatively affect site and roof deck drainage is poor coordination of trades and protection of materials. Early in the construction phase, during excavation or concrete work, overcompaction with heavy machinery can produce impermeable layers of soil, which in turn will impede drainage and airflow.
Later in the construction sequence, it is important to protect not only the waterproofing membrane from damage, but also the drainage system. Drainage boards, drainage pipes, and drainage aggregate can easily become crushed or clogged with construction dust.
Location of Utilities
Site utilities may be discovered to be located differently than documented, both horizontally and vertically. Sometimes this is because of unforeseen subsurface conditions, and sometimes it occurs because the contractor or civil engineer is not aware of the tight tolerances that are essential in green roof systems. At a minimum this can cause numerous field changes, but when a project has very tight tolerances, particularly in top-of-slab elevations or where core drilling for later connection to utilities is not feasible or will disturb or damage completed work, the negative consequences can be significant.
For living green roof systems where concrete must be installed, perhaps the most important consideration is to ensure the correct elevations, pitches, and finishes of horizontal concrete surfaces that will support green roof systems. In paved areas it is important to ensure that the expansion joints are in the correct locations. Likewise, it is important that vertical and horizontal surfaces for planter walls, site walls, stairs, fountains, or footings for lights or other site elements are in the correct locations and at the correct heights.
Drainage structures should be properly located and any coring for utilities should be completed in the correct locations before any subsequent work is completed, particularly the installation of the waterproofing membrane. It is very difficult to achieve proper installation and performance of any of the remaining systems — drainage, planting, paving, other site elements — if horizontal and vertical concrete surfaces are not installed in the proper locations and to the correct elevations.
Installation and Protection of the Waterproofing Membrane
After the concrete is correctly installed and cured, and openings for utilities are correctly located, the waterproofing membrane will be installed. The waterproofing consultant and manufacturer's representative should be present during its installation. Flood testing should be completed and the protection material installed immediately. If a leak detection system is installed, it should also be tested and immediately protected.
Insulation and Drainage Systems
The installation of the insulation is usually quite conventional, particularly if board insulation is being used. If tapered or grooved, it needs to be installed in the correct orientation to achieve appropriate slope and facilitate drainage. If the insulation was chosen for its high density and compressive strength to support paving, site elements, or some types of vehicular traffic, it is essential that it is the correct material and that it is being installed properly. Often it is placed in interlocking layers to limit movement.
Getting the drainage system in correctly is perhaps one of the most important aspects of successful green roof system projects. This starts with proper pitch of the slab, with the water directed to drains of appropriate size and in the correct locations. Everything that makes up the drainage system — drainage mat, drainage/ aeration mat, drainage panel, drainage aggregate with drainage pipes, filter fabrics, moisture retention mats — must be installed correctly.
Sometimes because of contractors' lack of familiarity with materials, components may be installed in the wrong order or even upside down. Because these components and systems will soon be covered with soil mixes, plants, paving, or other site elements, if they are not installed correctly, the problem will likely go unnoticed until the entire system starts to fail. Replacement is usually complicated and costly.
If underdrainage pipes are being used, ensure that there are adequate locations for cleanout and that cleanout access is installed in the correct locations and visible.
Specify that testing of the drainage system is required, and ensure that testing is performed and approved prior to the placement of any paving or soil mixes.
Lightweight fill can consist of a number of materials, ranging from extruded polystyrene (XPS) board insulation to large blocks of expanded polystyrene (EPS) and lightweight concretes. Each of these components should installed to the manufacturer's instructions and specifications. Large blocks of EPS can easily be wire-cut on site. If the placement of them is quite specific in terms of the profile the finished grades, it may be necessary to specify that shop drawings indicate their configuration, stacking pattern, and any connection details such as cleats or taping.
When paving over a structural deck, many of the same construction issues encountered will be similar to those in conventional paving projects. However, the two most challenging are ensuring positive drainage and establishing the proper depth of the setting bed. Positive drainage should be both across the paved surface to surface drains and below across the top of the deck to the deck drains.
Often if the elevation of the deck has been poured too high or too low, the temptation is to make up the difference in the thickness of the setting bed. If a setting bed is too thick, however, the pavers will rock and tend to chip at the corners, leading to deterioration of the paving system.
If a setting bed is too thin, it will be difficult to achieve a uniform finish grade, because there is generally an allowable tolerance in pavers, both in width and in depth. A variance in depth can cause a "lip" that can alter drainage flow and cause the same rocking and chipping as noted above. A setting bed that is too thin can also be detrimental to the interlocking of pavers and most importantly to the protection of components below, such as the drainage system and the waterproofing membrane.
If an open-joint paving system is used, where pavers are set dead flat on pedestals, allowing the surface drainage water to flow through the joints and across the surface of the sloped deck, the placement of supporting pedestals must be extremely accurate, as should be the joint size and spacing. Ensure that the concrete deck has been poured to the proper elevation and pitch and that drains have been located in the proper locations.
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Susan K. Weiler, FASLA, is a partner at Olin, a landscape architecture and urban design firm in Philadelphia, Pennsylvania. She lectures widely and teaches landscape architecture at Temple University, where she developed its landscape engineering and construction curriculum. She is a recipient of a 2003 Green Roof Award of Excellence, given by the Green Roofs for Healthy Cities industry association.
Katrin Scholz-Barth is a consultant from Washington, D.C., currently working to help establish green roofs in the Middle East. A national expert in green roof technology, she leads efforts in ecological planning and design in both the United States and the Middle East. She taught at Harvard University and the University of Pennsylvania and is former director of sustainable design for the HOK Planning Group, a business unit of HOK. She is also the recipient of a 2003 Green Roof Award of Excellence.
This article is excerpted from Green Roof Systems by Susan K. Weiler and Katrin Scholz-Barth, copyright © 2009, with permission of the publisher, John Wiley & Sons.
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