The design process — green roof-part 1 — Home tips

The design process - green roof-part 1 - Home tips

The design process

The green roof design process, like a green roof itself, is a hybrid. Most landscape projects do not take place on a building or as part of the construction of the building itself, and most building projects do not involve living elements. Thoughtful and appropriate design choices are always important, but on a green roof project the impact of these choices can show up in unexpected places. The cost of careful design is almost always lower than the cost of failure. Interdisciplinary cooperation is especially important to ensure that all of the elements of the project, from the structural to the horticultural, complement and reinforce each other.

Establishing objectives: educate yourself up front

Owners or developers considering a green roof should learn what living with one entails. Try to find projects similar to your own with green roofs and see how they are working after installation. Do they look good? Have there been leaks or other problems? Are the plants well established? Would those owners choose a green roof again? If you are sure that a green roof is right for your project, the more clearly you articulate your objectives for that roof to your design team at the earliest stages of the process, the more likely it is that the design will satisfy those objectives.

Similarly, it is important for the designer to clarify the client’s objectives before the design process begins in earnest. “Don’t put pencil to paper unless you know why you want a green roof,” says green roof engineer and designer Charlie Miller. Some building owners might be focused on performance, others might want to incorporate as many green features into the building as possible on altruistic, marketing, or other grounds. If a client proposes a green roof, it’s incumbent on the designer to understand what, exactly, the client wants to achieve and to make sure the owner understands what a green roof is (not necessarily a roof garden), what it will look like (especially early in its life), what level of effort is needed to sustain the roof (this will depend on the design), and how much it is going to cost, both up front and over its lifetime.

This kind of inquiry, of course, is an important part of the design process for any landscape or construction project and any significant aspect of a large building project. But the green roof is a new design concept to most people. This might make it a more desirable option in some cases and more intimidating in others. Some people will be familiar with green roofs only through articles and images that highlight spectacular but unique projects that have little relevance to a standard commercial or residential job. In other cases, a client might have objectives that a green roof can effectively satisfy, but the client might be unaware of this potential.

Determine whether green roof is a good way to help achieve those objectives

Even if a green roof appears to be a good potential design solution, it might not be the best choice in the context of the project. It might be difficult to keep plants alive, for example, on a roof that is shaded by other buildings for most of the day. A low-rise building surrounded by plants, such as maple trees, that produce and disperse a lot of viable seed, might make maintenance too onerous for a green roof to be practical. On a suburban site with a lot of available land, measures such as rain gardens and porous paving might be easier to implement and a more cost-effective way to manage stormwater.

Even when a green roof is theoretically feasible on a project, if you leave aesthetics aside and compare it to other sustainable building or ecosystem service tools by discrete function, a green roof might seem like an unacceptably expensive solution. A white roof can save energy and usually costs less to install. Rain gardens and bioretention swales in paved areas can capture storm-water runoff and can be designed and installed without directly implicating the building, making the process much easier and less expensive.

But here, too, the context and details of the project matter. In a densely built city, with less land available for at-grade measures such as rain gardens, a green roof might be one of the few viable green options for a new project or retrofit. An owner focused on long-term costs and performance might choose a more expensive but durable green roof over a shorter-lived white roof.

In addition, green roofs provide an aggregation of benefits that can make their higher initial costs worthwhile. A green roof designed primarily to satisfy stormwater code requirements might also serve as an accessible or visual amenity space or help to lower energy costs on a low-slung industrial building. Public benefits, such as a single building’s modest contribution to mitigating the urban heat island effect and reducing stormwater runoff into local rivers and streams or providing a patch of urban green and a pleasant view for workers and residents in neighboring buildings might be impossible to quantify but important to the client. The visible evidence of the owner’s commitment to sustainability might enhance the building’s value or the owner’s reputation in the community.

Deploy a full spectrum of knowledge and skills

While green roof design and construction require a range of disciplines, there is no template or checklist of necessary professionals for a green roof design team. No one person will hold all of the necessary expertise, and each member of the team should be fully aware of his or her own specific responsibilities and those held by others. Most important is that everyone, especially the lead designer, should be cognizant of gaps in knowledge and how to fill them in a manner that strengthens the design.

Good roofing details are an important part of green roof design and installation. The flashings on this vent were done well, the drains are well clear of planted areas, and rainwater and any condensation will drain quickly away from the area through the large-graded stone.

The team should include people who are experienced and well versed in green roof materials, components, standards and testing methods, and design paradigms as well as sound roof construction and horticulture. An architect, for example, might be able to design a structurally sound roof, but might not appreciate materials compatibility and drainage issues introduced by the overburden of a green roof. That’s not a problem as long as the architect recognizes the need to consult others who do understand those issues.

Similarly, the green roof team must ensure that the non-green aspects of the roof, such as waterproofing and flashings, are integrated successfully with the green aspects and are not undermined during installation of the overburden. Roof leaks most often occur where horizontal and vertical sections meet, such as parapet walls and penetrations, so flashings and other details must be carefully and appropriately designed (NRCA 2009).

The leader of the design team need not engage separate individuals or firms for each subset of required knowledge, but the team should, across its membership, have access to all of the information necessary to produce a good result. On a design/build project, the team should also have a reliable network of suppliers to ensure that materials are consistent and of high quality. If the project is being put out for bid, the designers should know enough about the components of a green roof assembly that they can write specifications.

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