Designing a low-maintenance vegetated roof

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A vegetated rooftop requires maintenance to keep it at optimal performance. Photos courtesy Xeroflor

By Sasha Aguilera
Throughout Canada, there are many beautiful, living vegetated roofs dotting urban landscapes that require care. Creating these green roofs is not only an esthetic choice, but also a major maintenance commitment. Some municipalities and cities, such as Toronto, require rooftop maintenance plans in order to obtain a vegetated roof permit, and some cities even require rooftop inspections twice a year. Many green roof system warranties are tied to performing maintenance and filing reports. Reports should include health of plants, recent weather conditions, and inspection check list of what was found and what was done. Ultimately, maintenance keeps vegetated roofs alive, looking well and performing as they should.

The extent of maintenance depends on design, plant selection, depth of growing medium, the roof’s exposure, and weather. While it is hard to predict weather, a designer can opt for a system with low-maintenance features to reduce overall labour and cost.

Technological advances offer plant options that do not require loose growing medium. Saturated water retention layers are a fraction of the weight of wet growing medium. Replacing all or part of the growing medium in a green roof with water retention materials such as fleece and hygroscopic mats can achieve equal or better water storage capacity while keeping he system weigh low. These lightweight systems can achieve desired stormwater management goals and are easy to install, and retract. Without additional growing medium, they invite fewer roof-damaging weeds such as thistle, and require less manual labour. Aggressive roots or rhizomes can compromise the roofing membrane by seeking out fissures and discontinuities in the surface or seams of the membrane. The roots will then grow into these imperfections and cause a breach in the roofing membrane

Plant selection
Locally grown, hardy, and drought-tolerant plants such as sedum and moss are ideal for rooftop designs. Maintenance personnel should be familiar with green roof plants and the owner’s green roof esthetic preferences, as some ‘weeds’ might be tolerated.

Irrigation plan
Access to water with adequate pressure for supplemental irrigation during the establishment phase and drought periods is critical. Irrigation options range from hose and sprinkler to more sophisticated permanent irrigation systems.

Fertilizing plan
An annual application of slow-release fertilizer, usually in spring, helps feed plants nutrients over a three-to-nine-month period. Care must be taken to avoid a nutrient supply on plants preparing to go into dormancy in the fall season.

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