Designing extensive and semi-extensive vegetated roofs for long-term performance

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Photo courtesy Xeroflor Canada/Jean-Jacques Laplace

By Kees Govers, B.Sc.(Agr), GRP, and Sasha Aguilera, B.Arch, GRP
Vegetated roofs are relatively new in Canada, with construction guidelines primarily brought over from Germany. Partially because of the climatic differences between Europe and large parts of Canada, there is a wide range of design options available to match the expectations of building design consultants, contractors, and owners alike.

The authors of this article, employed by two competing vegetated roof manufacturers, have research and development teams and a liberal sprinkling of opinion behind them in advocating for their preferred vegetated roofing assemblies. In assessing any given project’s requirements, they are likely to provide you with different ways of accomplishing the goals of the project. That is the very nature of living systems—there is more than one way to accomplish the same goal.

Nevertheless, there are many areas in which virtually all vegetated roofing specialists converge in their recommendations, regardless of the systems they represent. The first and most obvious is to only build vegetated assemblies on roofing systems selected, installed, and detailed for that purpose, whether a new building or a retrofit. This includes ensuring the building has the applicable structural loadbearing capacity and the roofing assembly has appropriate wind uplift and fire rating, drainage pitch, cover boards, and flashings detailing to perform with a vegetated roof overburden. Especially important to vegetated roof installers is leak-testing. Selecting and installing appropriate roofing systems for use with vegetated assemblies has been the subject of a significant body of research and many articles as vegetated roofs have come into regular use over the last 15 years. (For example, see the Construction Canada article, “Waterproofing Considerations for Green Roofs,” by Karen Liu, PhD).

The other key recommendation lies at the heart of ensuring long-term performance of vegetated roofs—regular maintenance. As with all building systems and components, nothing is truly maintenance-free; virtually every component requires periodic maintenance to maintain its ability to perform according to the requirements set out during design. Since vegetated roofs are a living building assembly with periodic needs for moisture and nutrients (as well as removal of unwanted plants), these systems especially need regular upkeep. Ensuring regular maintenance needs to start near the inception of project design and be carried through the entire project process from design to building handover and beyond.

This article’s purpose is to encourage consultants and contractors to think about their designs and implementations related to vegetated roofs in such a way that systems can perform as long as the building is in use.

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Vegetated roofs are essentially living building assemblies, with periodic needs for moisture and nutrients (as well as removal of unwanted plants). Ensuring regular maintenance needs to start with project design and be carried through the entire process—from design to building handover and beyond.
Photo courtesy Xeroflor Canada/Greg van Riel

Budgeting and design
Vegetated roofs are no longer a novelty for obtaining additional points under the Leadership in Energy and Environmental Design (LEED) program—rather, they are now a design option for all buildings with clearly defined purposes in mind. As part of the design process, clear budgets should be established based on the type of system chosen, according to the parameters important for the building.

Although maintenance requirements can vary slightly with different designs and plant materials, each assembly comes with a fairly clear maintenance budget—both during the establishment period and long-term. Once the system has been selected, it should only be deviated from if its purpose changes. This ensures the system capabilities match the expectations of the design team and the client. A lot of technical information is available to make decisions related to planting types and performance, stormwater management capability, and wind uplift performance. These must all be taken into account when specifying a product, as well as approving alternates—when an alternate product’s claims look too good to be true, they usually are.

Ensuring access
All roofs with vegetation must be safely accessible for installation and green roof maintenance crews. Freestanding ladders as a means of access to a roof are neither enjoyable nor safe, especially when a maintenance crew also needs to haul up their tools and materials. With that in mind, one should ensure all roof levels can be reached with permanent stairways or ladders and/or window or door access. In addition, if parapets are less than 1 m (3 ft) in height, it is important to include roof anchors and permanent lifelines to allow construction and maintenance personnel to safely access the roof levels and perform their duties while tied off.

Once the safe access points to a roof are established, inclusion of designated work areas and pathways should not be overlooked. While many vegetated roofs in the extensive and semi-intensive styles are very visible, they are often inaccessible to anyone other than authorized personnel. However, this does not mean maintenance crews are the only ones with a need to access a vegetated roof area. Activities that are infrequent and low intensity, such as the maintenance of the green roof, do not require the installation of dedicated maintenance pathways. (The original meaning of ‘extensive’ and ‘intensive’ vegetated roofs was related to their maintenance requirements. The former had the least amount and lowest frequency of maintenance, while the latter had the highest [‘semi-intensive’ was a blend of the two]. The current meaning has been expanded to include growing medium depth. An extensive system is now defined as having a depth of up to 150 mm [6 in.] and planted with low-maintenance plants such as sedums, alliums, and related succulent plants with limited height (up to 300 to 450 mm [12 to 18 in.]).

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An example of temporary protection with grass pavers—sensitive plants need to be protected from rooftop maintenance traffic.
Photos courtesy Liveroof

Servicing of rooftop equipment, including those for mechanical systems and window-washing applications, are examples of two other functions that have to be performed atop the building.

Good design work can make it easier for service personnel to do their work without damaging the vegetation on the roof. The installation of pavers or ballast areas at access points and roof hatches goes a long way in reducing foot traffic damage in those sensitive areas.

Additionally, directing pathways toward regularly serviced equipment aids in avoiding traffic tracks across the vegetation. Providing appliance service technicians with a space at least 600 mm (24 in.) wide all the way around the curb of an appliance, with a minimum 1.2 m (4 ft) wide area immediately in front of the service panel, provides a clean work area without having to work on the plants and potentially misplacing equipment and parts in the process.

The use of swing stages above a newly installed vegetated roof should be discouraged. Nevertheless, where swing stages need to be landed and launched on a green roof area, the plants must be protected, as the frequency of foot and materials traffic will easily damage or even destroy the planted area used.

During construction, installation of the vegetated roof plants in these areas should be avoided until the other work is done, if possible. However, if the activities have to occur simultaneously, the plantings can still be protected with the use of plastic ‘grass driveway pavers,’ which can be put down atop the vegetation either temporarily or permanently. (The plants in the vegetated roof will grow through the holes in the pavers and all but obscure them if installed permanently.) When a swing stage lands on these pavers, the traffic only damages the portion of the plants showing above the edges of the pavers. The crowns of the plants are protected; this allows them to regrow after the work has been completed.

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