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

20160518_TTC-Steeles-W_0016-(fig-12)
An example of safe access to the vegetated roof.

Care during construction
The green roof contractor should ideally be the last trade on the roof to minimize potential damage to plants, but it may be impossible to organize a construction schedule allowing this. Consequently, it is ever more critical for the design/construction team to be aware that although different plants have different thresholds for damage, most common green roof plants can tolerate only light foot traffic.

If pedestrian traffic is unavoidable, protective measures must be undertaken to diminish the impact of construction damage. Materials like plywood sheets or insulation boards may be laid over the vegetation temporarily to help distribute the weight of a person walking or equipment stored on the green roof. The protective material must be removed at the end of each workday to allow plants to recover overnight. Temporarily installed ‘grass driveway pavers’ in traffic areas can also greatly assist in protecting the vegetation.

In cold climates, particularly when temperatures drop below 0 C (32 F), foot traffic over vegetation must be avoided at all costs as frost-covered plants are susceptible to immediate damage. When foot traffic occurs on frosted green roof plants it can prove lethal, leaving foot-shaped patches of dead plants. The damage will be obvious the following spring; recovery may take several months or, in cases of severe damage, may not happen at all.

If traffic on the roof is necessary during winter, workers must restrict their movement to designated pathways and work areas cleared of ice and snow with plant-friendly materials such as sand, cat litter, or beet juice (this last one can cause some ‘pseudo-mould’ that might be esthetically off-putting, however). De-icing chemicals that can harm vegetation should not be used. Alternate plant-friendly options include freeze-protection systems such as heat cables installed under the patio slabs that are to be kept ice and snow free.

Post-installation maintenance training
As maintenance is key to any roof’s long-term performance, it is important the person assigned to visit and maintain the system is properly trained, has horticultural knowledge, and fall-arrest certification. Working on the roof differs from working at ground level, and certified maintenance contractors are therefore generally recommended for this kind of maintenance.

If it is decided green roof maintenance is to be done ‘in-house,’ maintenance staff should receive both initial and periodic training by the green roof installer or manufacturer. This should be hands-on training with easy to follow, step-by-step, guidelines that include:

  • rooftop safety;
  • project-specific plant/system information;
  • troubleshooting recommendations; and
  • maintenance checklists.

A comprehensive maintenance manual is an essential companion for personnel performing routine maintenance.

If the roof membrane needs to be repaired or rooftop units need to be added, the owner should inform the green roof manufacturer or the installer to co-ordinate the work with the roofing contractor. This ensures the vegetated roof is properly removed and replaced to minimize disturbance to the plants and growing media. Systems vary in the ease of removal and re-installation, as well as the plants’ recovery time. It is therefore important to employ contractors familiar with what has been installed.

How-not-to-treat-a-vegetated-roof-(fig-10)
This is the result of a general lack of maintenance. For more on the topic of vegetated roof upkeep, see this article’s authors’ companion piece online at www.constructioncanada.net/vegetated-roof-maintenance.

Vegetation and overburden warranties
The vegetation warranty of any given vegetated roofing system will specify the expected performance, the warranty coverage, and the attached conditions. Under the Toronto Green Roof Construction Standard, for example, any vegetated roof has to achieve and maintain at least 80 per cent vegetation coverage within three years from installation. It should therefore be expected a vegetation performance warranty provides—at minimum—the same guarantee as the construction standard.

It is standard practice for the industry to provide clients with a vegetation warranty of at least two years from the date of installation. The limitations on the warranty stipulate the vegetated roof must be maintained according to the maintenance protocols provided by the manufacturer, with appropriate documentation of those maintenance activities. An experienced green roof inspector can generally determine the level of maintenance provided within minutes of seeing the project.

As discussed earlier in the article, damage done during the construction is not part of the coverage provided by these warranties. A vegetation warranty is specifically written for the post-installation period to ensure the selected plantings have an opportunity to mature and provide the required coverage while any bare spots in the vegetation will be patched as part of the maintenance required under the warranty.

Overburden removal and replacement warranties are typically warranty riders provided by roofing membrane manufacturers (or, increasingly, by green roof manufacturers) to clients to ensure that, in case of a leak in the membrane, the removal and replacement of the vegetated roof (overburden) is done at the cost of the warranty provider. Typical requirements, aside from a fee based on the length of the warranty, is the inclusion of a leak-detection system in the vegetated roofing assembly to provide quicker and easier location of the leak, thus reducing the area that needs to be removed and replaced for the repair. The overburden removal and replacement warranty will be of the same duration or shorter than the warranty for the roofing system on which the vegetated assembly is installed.

Conclusion
Across the country, vegetated roofs have become significant green technologies that solve a number of environmental challenges. To ensure excellence in performance, it is important the details in design and installation are considered as critically as the systems themselves. Attention to detail ensures long-lived, attractive, and well-functioning vegetated roofs for future generations.

Kees Govers, B.Sc.(Agr), GRP, is the technical sales manager for LiveRoof Ontario Inc. He has had a long career in production management of large-scale commercial horticultural operations for woody plants and perennials before joining the LiveRoof green roof network in 2006. Govers has served on the London Chapter, the Growers Commodity Group, and the Provincial Board of Directors of Landscape Ontario; he is currently the director for the London Chapter of CSC. Govers can be reached at kees@liveroofontario.ca.

Sasha Aguilera, B.Arch, GRP, is a client services and design consultant for Xeroflor Canada. A green roof consultant for nearly 10 years, she has considerable experience working with designers and contractors on vegetated roofs of various complexities, from retrofits to new construction. Based in Toronto, Aguilera regularly travels coast to coast, certifying installers and inspecting installations. She can be reached via e-mail at sasha@xeroflorcanada.ca.

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