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Green Roofs

Green Roof

Diagram of a green roof application (taken from the Low Impact Development Center website).

Green Roof Background

Green roofs, also known as “living roofs”, consist of vegetation or soil planted over a waterproofing membrane and offer many aesthetic, economic and environmental benefits. Environmentally, they have the ability to provide temperature and acoustic insulation which increases energy efficiency and reduces the urban heat island effect, provide biodiversity preservation, improve air quality, and retain and cleanse storm water.

Typical green roof systems consist of the following layers: vegetation, a growing medium, the landscape (to contain the growing medium while allowing water penetration), a specialized drainage layer, waterproofing/roofing membrane, and the roof structure with the traditional insulation above or below.

Roof Types

The two main types of green roofs are extensive roofs, which have a thinner and lighter layer, and intensive roofs, which are thicker and heavier.

Extensive Green Roof

Extensive green roofs are suitable for roofs with relatively low bearing capacity because of their low weight, lower cost, and minimal maintenance requirements. They typically vary in depth between 2″ to 6″, corresponding to a saturated weight of approximately 15-35 psf, and have a typical initial cost of approximately $8 to $20 per square foot.

Intensive Green Roof

Intensive green roofs offer a wider plant diversity and individuality, creating a higher potential to develop a more complex ecosystem. However, they are heavier, more costly, and require more maintenance. They typically range from 8″ to 24″ in depth, with a saturated weight of 60-200 psf, and have a typical initial cost of approximately $15 to $25 per square foot.

Semi-Intensive Green Roof

Semi-intensive green roofs fall between the extensive and intensive green roof. They have a slightly higher plant community, have a mid-range cost, and require periodic maintenance. They typically range from 5″ to 10″ in depth, corresponding to a saturated weight of 25-40 psf.

Engineering Considerations

When designing a green roof system it is important to consider the specific project’s requirements such as its function, location, the structure itself, roof accessibility, installation, and maintenance. Therefore, it is a process that involves coordination between the structural engineer, owner, architect, landscape architect, and the M&E. Some structural considerations when designing a green roof include:

Additional loads of green roof

Wind pressure

In reference to ASCE7-05, Chapter 6: Wind Loads, wind pressure is related to geographic location and terrain, and the building’s shape, size, and features. The green roof system must prove to be stable under the specific wind pressure of the building.

Uplift

A pressure differential can be created when wind is redirected and accelerates, which can then in turn cause an uplift force on the rooftop. There is minimal information regarding uplift of specific green roofs, so green roofs are often treated as if they are pavers of similar mass, and special attention is given to negative wind pressure.

When taking a large uplift force into account, it may be necessary to include additional applications such as: limiting the green roof to the center of the rooftop while using heavier ballast along the perimeter, overlaying the green roof with netting, or fully adhering the green roof.

Parapet Design

This relates to the positive and negative wind forces and depends on the building’s size and height. Depending on the parapet shape and design, it may help to moderate and reduce uplift pressures, especially for taller buildings.

Roof Slope

This depends on the friction present and the force exerted against the parapet. Buttresses may be necessary for long roofs and roofs with great slopes to distribute the load from the parapet. The North America’s Green Roofs for Healthy Cities determined that the maximum slope for a green roof should be 40 degrees based on containment challenges and soil moisture.

The following website provides more detailed engineering considerations:
http://liveroof.com/architecture-and-engineering/

The Role Of The Structural Engineer

Whether determining basic loading capacity and other safety concerns or undertaking more detailed measures to avoid costly building code errors down the road, the structural engineer is a critical team member of any green roof project.

As the public continues to recognize their benefits, green roofs are becoming more commonplace in the design of both new buildings and retrofits to existing buildings. The participation of a structural engineer in the design process can vary widely depending on the specific challenges of the project—ranging from a minimal, almost passive involvement to being actively engaged in planning, shaping the design, and contributing to the final detailing of the project.

Preliminary Code Analysis

At this earliest phase of the project, the design team should include a code analysis of the building relative to the governing codes and insurance standards to establish the impact of the green roof. Depending on the use and access to a green roof, live load can vary from 12 to 100 pounds-per-square-foot (psf). With existing structures, a change-of-use often necessitates a related code-upgrade that may result in requirements for other improvements unless special consideration is given by the building official.

Structural Feasibility Investigation

For existing buildings, a feasibility analysis should occur during a predesign phase, prior to a formal design process. In this phase, information about the existing building is gathered and load capacities of the existing structure are established. Various potential green roof systems can then be evaluated according to their weights and characteristics.

The determination of structural capacities can range from simply identifying the design loadings shown on the original drawings, to performing independent calculations of the structural members shown on the engineering drawings. If no drawings are available, filed measurements and possible material testing may be necessary. Calculations will aid in identifying any available reserve capacity. Note that it is not uncommon to find reserve capacity in some, but not all of the structural elements. It is important to document the scope of this initial investigation and to identify what additional testing is required, if any, prior to proceeding with the green roof design and construction.

In the event that the capacities of any or all of the structural elements are deficient, the engineer can identify whether it is possible to utilize the existing capacity of the roof creatively. By planning the layout of various elements, such as the locations of deeper plantings and publicly accessible areas (i.e. developing a structural strategy), the need for structural reinforcement can be minimized or eliminated. The engineer can also evaluate whether it is structurally feasible to reinforce the limiting structural elements from above or below (depending on access to areas below the roof) and can comment on the relative costs and disruption of reinforcement schemes. For example, structural reinforcement options might include local reinforcement of existing members, or retrofit of the framing with supplemental full-length members. While the first may be more costly, the latter may exhibit lesser impact on other building systems and less disruption to tenants in the space below the roof.

During this phase, the design team should evaluate any existing roofing, insulation, and drainage systems, and consider upgrades if those systems are near the end of their useful lives. Other structural considerations may include an increase in snow build-up as a result of changes to the insulation profile of the roof, or an increase in accumulated rainwater due to changes in drainage patterns (with respect to the existing roof drainage systems and considering redundancy).

It is at this stage that the scope and quality of the structural engineer’s input may be most meaningful: a conservative approach might render a project unfeasible while too cursory a review of the technical issues may cause cost or code issues to be overlooked, only to surface in later phases. Conversely, identifying constraints (or costs) related to occupant access to areas of the roof are important to establishing realistic expectations for the project.

Concept Design

For existing buildings, once feasibility of a green roof system is established, the design begins by implementation of the structural strategy. The strategic locations of various green roof elements – including extensive/semi-intensive/intensive planting areas and their associated soil and plant weights, maintenance pathways, occupancy areas and ancillary structures—all come into play in a chess game of structural analysis governed by the limits of occupant access and cost reinforcement. For example, heavily loaded areas on a green roof such as areas of public assembly or tree plantings may be located near columns to minimize stress in the structural framing, or conversely may be located in the center of a bay (a bay refers to the space between framing structural supports, either columns or walls) to reduce the load transmitted through any single framing member (that is, a structural element that spans between supports) to a single column.

Roof slope is an important consideration. If changes to the existing drainage slope are to be implemented, the weight of added material must be considered and the elevations of the final roof surface must be carefully planned. Fill material (the base layer used to achieve an elevation height and required slope for drainage) and substrates must be coordinated with the roofing system to assure the testing and warranty requirements necessary successful installation of green roof components.

Specialized Training for a Fuller Knowledge

Additional knowledge of how a green roof system operates can supplement a structural engineer’s performance on these projects. Accredited Green Roof Professionals (GRPs) are ideally positioned to be a critical member of any green roof team, possessing knowledge of the special requirements and challenges of green roofs from design through to maintenance. A project’s success can be greatly aided by using a GRP. A GRP has passed a written exam showing a body of knowledge about green roof design, installation, and management and can assist project teams in the successful installation of a green roof. The GRP exam is delivered by Prometric, whose other clients include Microsoft, IBM, the European Personnel Selection Office, the Medical College Admission Test, the Uniform Certified Public Accountant Examination and the Architect Registration Examination.

Because of the inherently interdisciplinary nature of green roofs, the Green Roof Professional (GRP) accreditation is a unique program, drawing on a variety of skills and providing a common language between different professions. As the demand for this technology increases, so will the demand for green roof professionals, such as suppliers and manufacturers of green roof components, engineers, architects, installers, landscapers—and structural engineers.

In conclusion, green roof systems offer great opportunities for owners of new and existing buildings to reap many benefits of living architecture for people, communities and the planet. Knowing the appropriate structural engineering services and having appropriate structural expectations for each phase of the design process is critical to helping these owners realize these benefits.

Source: Kirk I. Mettam and Steven W. Peck, Green Building Pro

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