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

The design of a successful roof goes well beyond choosing plants and waterproofing. For example, direct exposure to air vents quickly dries out vegetation. Thoughtful placement of flora-free zones can help by making rooftop service work simple.

Water and irrigation
Closely related to the selection of the vegetated roof is the need for water and irrigation. Water requirements of the plants are closely related to climatic and weather conditions, as well as system design (e.g. plant selection, growing medium depths, and use of water retention layers). Not all vegetated assemblies require an irrigation system, but many do. The same roof that can be installed without irrigation in Vancouver or Toronto, almost certainly needs it in the climatic conditions of Kelowna, B.C., or Calgary.

As was evident in Vancouver during last summer’s drought, a climate traditionally thought of as ‘cool, temperate, and wet’ does not guarantee a green roof will never need irrigation. Many vegetated roofs that received no supplemental water during the summer suffered serious drought damage. Until they are repaired and re-established, both the functionality and the esthetics of the system and the plants will be severely diminished.

Designs should always include water access on every planted roof level, as all vegetated roofs require water immediately after installation. Many systems also require temporary irrigation during the establishment period, which can be up to three years under certain circumstances. The hose bibs installed should supply sufficient volume and pressure to run at least a simple hose and sprinkler irrigation set up. If the design includes an irrigation setup connected to either a stormwater cistern or potable water, its designer and the mechanical engineer must work closely together to ensure appropriate volume and pressure is available.

In the authors’ experience, irrigation of extensive roof systems is accomplished with the highest efficacy with a professionally designed rotary sprinkler irrigation assembly. If the selected plants and system exceed a certain combination of height and density combined with deeper growing media depths, then micro-spray and drip irrigation often become better choices. Water retention fleeces can increase water storage in the system (thus reducing irrigation needs), and assist in the distribution of irrigation water across the vegetated roof for more even distribution.

Wind uplift and high-rises
Most vegetated roofs have been installed on buildings less than 10 storeys high. In these situations, plant and system choices are based on a limited number of design parameters. Once a vegetated roof installation is contemplated at higher elevations, numerous factors come into play that determine success.

Splash pads and vegetation-free zones can be used to direct water from higher roof levels toward the drains. Space has also been left for gas pipes and rooftop units to allow for easier access and damage-free plants.

When designing vegetated roofs for tall and super-tall buildings, a number of additional considerations go into product selection, installation, and irrigation. (The Council for Tall Buildings and Urban Habitat (CTBUH) defines tall buildings as less than 300 m (984 ft) in height, while supertalls are 300 to 600 m (1970 ft). Megatall buildings are beyond that category). Since the recent introduction of Canadian Standards Association (CAN/CSA) A123.24-15, Standard Test Method for Wind Uplift Resistance of Modular Vegetated Roof Assembly, it is now possible to determine the suitability of a specific pre-grown vegetated roof system for application on a specific building. Comparing the wind uplift rating of the vegetated roof to the wind uplift study performed for the building answers whether it is suitable. (For more, see the Construction Canada article, “Designing Vegetated Roofing for Wind Forces,” by Jeff Truman, M.A.Sc., P.Eng., PMP).

What is not as evident are the other precautions for ensuring a vegetated roof thrives at high elevations, where wind is a much bigger factor than at lower levels. Not only does the wind have higher average speed and a higher gust speed because of the clear path around the building and the lack of shelter, but it also has a drying effect much more pronounced than at lower levels. In order to retain healthy plantings, even on extensive succulent green roofs, this drying effect often requires the inclusion and use of irrigation systems.

The management of the irrigation cycles is also important as the plants need to be well-hydrated entering the winter months to avoid the freeze-drying effect of high winds, low temperatures, and low humidity. Further, irrigation in the spring needs to start much earlier in the season than at lower levels. Unless tall screens are present, high elevation roofs do not have any significant snow accumulation—a major factor in winter moisture retention of the plants.

In addition to the constant drying effects of wind, there is always a concern for growing medium erosion at high elevations. The best way to avoid erosion is by using pre-grown vegetation, which allows the growing medium to be sheltered from wind. Plants are very effective at withstanding wind erosion as long as the roots are anchored into the growing medium.

The method of installation is also a factor in success. Due to the heights involved, installation with a tower crane or materials hoist are the only practical methods to transport the product to the rooftop. This means prior to the installation of the vegetated roof, all other major work at the roof level should be completed to prevent the destruction of the plants. Where construction activities are still required after the installation, the plants should be protected from foot and materials traffic as outlined elsewhere in this article.

Control the content you see on! Learn More.
Leave a Comment


Your email address will not be published. Required fields are marked *