Designing vegetated roofs to survive winter

April 16, 2018

Photo courtesy Alvin Curling/Next Level Stormwater Management

By Cristina Senjug
High above the ground, vegetated roofs mimic nature, helping clean the air, cooling down temperatures, keeping rainfall onsite, and alleviating pressure on urban stormwater systems. Vegetated roofs, also referred to as a green roofs, eco-roofs, or living roofs, typically include a vegetation layer, growing medium, retention layer, drainage, and a root barrier. They are present throughout North America in almost all climate zones and can be installed on various roofing assemblies.

Green roofs are classified as either intensive or extensive. Intensive roofs feature a variety  of plants in a heavy and deep substrate layer of250 mm (10 in.) or more. These green roofs can incorporate many sizes and types of plants foundon ground-level, including shrubs and trees. Typically installed as a rooftop amenity space, intensive roofs may include planter boxes, benches, ponds, and fountains in park-like settings.

Extensive roof systems are lighter in weight because of a shallow substrate—20 to 150 mm (3⁄4 to 5.6 in.)—and they typically feature vigorous, low-growing, drought-tolerant plant species such as sedums and mosses. These systems are an economical stormwater management solution; are easier to install, remove, and repair; require less maintenance than intensive systems; and may be retrofitted on existing buildings.

All vegetated roofs—whether intensive or extensive—experience cycles of growth, flowering, and dormancy like plants on the ground. When vegetated roofs are properly designed, correctly installed, and appropriately maintained for the four-season temperate climate, they have a greater probability of survival in the harsh, cold Canadian winters.

This article focuses on the best autumn and winter practices for extensive vegetated roofs, which are more dominant in urban centres. (For more information, read the “2015 Annual Green Roof Industry Survey” by Green Roofs For Healthy Cities.) These roof systems are prevalent throughout Canada as a green infrastructure technology. However, many questions arise as to their esthetics, performance, and survivability in the winter. Some question whether vegetated roofs die in the cold climate, if snow damages the vegetation, can the roof be walked on in winter months, or if vegetation needs to be re-seeded in the spring. This article seeks to provide answers.

Sedum reflexum or sedum rupestre, also known as reflexed stonecrop or “blue spruce” is in dormancy under the cover of snow—the plant’s natural reaction to adverse environmental conditions. In spring and summer, this vigorous sedum species will produce yellow star-like flowers.
Photo courtesy Next Level Stormwater Management

Surviving winter
Extensive systems predominantly use hardy sedum species that can withstand harsh rooftop conditions year-round. Sedum are drought-tolerant and thrive in harsh conditions. The best-suited rooftop plants are pre-grown outdoors locally, for at least several months and usually over winter, to make certain they are acclimatized to the local weather conditions. Ensuring the vegetation is mature upon installation offers a higher probability of survival. In colder plant-hardiness zones (e.g. Ottawa, Edmonton, and Winnipeg) or in areas where there are extreme fluctuations in temperature (e.g. Calgary due to the Chinook winds), vegetated roofs require a deeper depth of engineered growing medium to help mitigate the effects of intense temperature and moisture changes. Additional growing medium acts as a thermal mass, helping moderate the temperature and further hydrating the plants, thereby reducing significant die-back in the winter or during hot summer droughts.

The growing medium for green roof systems is typically engineered to be a lightweight composition of locally and sustainably sourced ingredients.

Most growing medium employed on North American green roofs is designed based on the German FLL guidelines and is tested to ASTM standards. It has consistent chemical and physical characteristics of nutrients, bulk density, air space, water retention, and hydraulic activity or drainage. Depending on what is locally available, and on the specifications required for the regional climate and roof type, growing medium is blended using horticultural aggregates—which may include sand, lava, expanded clay, and crushed slate, depending on availability—and organics (e.g. peat, humus, wood chips, and high-quality compost from grass clippings and mulched leaves).

Pre-vegetated systems provide instant green upon installation and protect the growing medium from wind erosion at higher levels. Ensuring the vegetation is mature upon installation offers a higher probability of the green roof’s survival during winter.

In autumn, vegetated roofs enter the dormancy cycle. As rooftop conditions are harsher than at the ground-level, the exposed vegetation enters into the dormancy cycle earlier than its counterparts on the ground. Dormancy is a natural reaction to adverse environmental conditions, a defense mechanism. It can happen in the summer during periods of intense heat and drought, or in the fall in preparation for the cold winter months. It is a state of reduced metabolic activity when plants simply stop growing and developing to conserve energy. (Read Brian Capon’s Botany for Gardeners, published by Timber Press in 2005.)

During dormancy, the retreating plants are not dead. Dormancy synchronizes with the environment and can be predictive or consequential. In the first category, the plant’s dormancy cycle is triggered by a temperature drop, predicting the onset of winter. Consequential dormancy occurs when there are sudden changes in climactic conditions, such as reduction in rainfall and water shortages, causing arrested development with a goal to conserve energy and survive.

When experiencing predictive or consequential dormancy, vegetated roofs lose their flowers and change colour, creating a beautiful landscape of reds, browns, and deep purples. During winter dormancy, coniferous sedum plants retreat to form a dense mat of glossy and fleshy leaves, while the leaves of deciduous sedum species completely fall off.

Installation tips
Many vegetated roofs are installed in the autumn due to the construction schedule and a desire to finish the project. In such cases, it is critical to have mature and well-established plants as they have greater survivability on rooftops than immature seedlings. Pre-cultivated systems should have a minimum of 80 per cent coverage upon installation to ensure there is blanket coverage and minimum erosion of growing medium. The pre-cultivated vegetation may have been disturbed during harvest, transport, and installation. Therefore, after installation, it is imperative to keep the vegetated system hydrated during the establishment period for the plants to recover, take root, and prepare for the coming winter.

Extreme temperatures
Once cold temperatures set in, a well-hydrated vegetated roof goes into dormancy. The pre-cultivated vegetation protects the growing medium from the strong, cold winds and potential erosion. Snow accumulation is ideal as it insulates the vegetation. A blanket of snow shelters the vegetated roof from strong winds and helps the plants retain moisture. If there is little or no snow accumulation, and the vegetated roof is subject to high winds, extreme fluctuations in temperature, or a particularly severe ice storm, plant desiccation or “winter burn” can occur. This type of scenario, where plants dry out and die, may expose areas of growing medium to erosion by high winds. If such damage occurs to the vegetated roof, a straightforward procedure for repair can be implemented the following spring. This procedure also applies to other vegetated roof repairs necessitated by excessive foot traffic or construction materials left on the plants for extended periods.

Repairs are done in the spring using a variety of sedum clippings sprinkled, where needed, to replace damaged or eroded sections. Succulent plants such as sedums are easy to propagate. Many can be rooted from a single leaf; others will root quickly from a stem cutting. For areas that suffered erosion, additional engineered growing medium should be added, as required. After the initial planting of clippings, frequent, light irrigation is required to keep the vegetated roof moist and help ensure the plants take root.

Autumn maintenance should include inspection of all drainage paths.

Fall and spring maintenance
Year-round maintenance is key to ensuring long-term survivability as well as the stormwater performance of vegetated roofs. One last maintenance check should be done in the autumn as part of the yearly routine. Vegetated roofs are also known for their ability to protect the underlying roof membrane. However, circumventing maintenance and not taking proper preventative measures can lead to unforeseen problems.

Autumn maintenance must include removal of debris and inspection of all drainage paths. Large commercial and residential high-rise flat roofs, especially those with parapet walls, are catchalls for debris, including leaves, twigs, and plastic bags. In addition to potentially damaging the building, blocked drains and pooling water can cause the roots to rot and the plants to drown, leading to the failure of green roofs. Regular inspections can avoid costly repairs in the future. (For more information, refer to the 2013 Roofing Preventative Maintenance Manual by Canadian Roofing Contractors’ Association [CRCA].)

In autumn, it is best to remove fallen leaves and twigs from the green roof. Thick layers of leaves and twigs can also stick together to form an impenetrable mat and may lock in too much moisture, potentially causing rot or damage to the plants.

One must weed before winter sets in. Every effort must be taken to capture the seed pod intact. It must be tucked away for disposal to avoid blowing away seeds. Some weeds may be tolerated. Therefore, maintenance personnel should be familiar with green roof plants and the owner’s esthetic preference for the roof.

Keeping the vegetated roof hydrated in the autumn months helps the vegetation avoid winter’s freeze-drying effect, which is caused by low temperatures and high winds.

If there is an irrigation system, it needs to be winterized around October as frost typically sets in soon after in many Canadian cities.  (Visit[5] for frost and hardiness maps.) Maintenance crews typically use an air compressor to blow out the water from the irrigation system.

As rooftop conditions are harsher than at the ground-level, the vegetation enters into the dormancy cycle earlier than its counterparts on the ground.

On ground-level, perennials tend to be cut back. On the rooftop, however, tall grasses or wildflowers are ideally cut back the following spring. This encourages re-seeding and the stalks protect bees and other fauna during winter. Snow-covered stalks also offer a pleasing visual esthetic.

Extensive vegetated roofs require a minimal amount of slow-release fertilizer, which is best applied once, ideally in the spring at an application rate of 80 g (2.8 oz) per m2. This allows the plants to uptake most of the nutrients without much excess to go into the runoff. Over-fertilization can also cause plants to burn.

Slow-release fertilizer should not be employed on vegetated roofs after August because they may stimulate growth, compromising the plant’s hardiness. Left-over fertilizer should be discarded, or if the shelf-life extends another year, stored properly for the following spring.

Owners must assess the green roof’s performance in the autumn and prepare for spring. It is a good idea to document the visit with photos and record weather conditions for future reference as well as potential warranty claims.

In cold climates, particularly when temperatures drop below 0 C (32 F), foot traffic over vegetation must be avoided as it can prove lethal and leave foot-shaped patches of dead plants. The damage will be obvious in the spring. Recovery may take several months or may not happen at all. If pedestrian traffic is unavoidable, protective measures must be taken to diminish the impact of damage. Materials like plywood sheets and insulation boards may be laid over the vegetation temporarily to help distribute the weight on the green roof, and removed at the end of each day to allow plants to recover.

Finally, as maintenance workers may make frequent, short visits to vegetated roofs throughout the year, it is important they have appropriate working-at-heights training to conduct the work safely. In Canada, any person working at elevations is required by provincial law to be trained in fall protection. In the absence of appropriate parapets, or a permanent rail system, permanent or temporarily installed roof anchors are crucial to protect maintenance workers. Designing and building vegetated roofs with height safety in mind and providing workers with the necessary training is crucial to prevent falls from rooftops and subsequent death or serious injury.

In cold climates such as Canada, properly designed vegetated roofs, installed with hardy and mature plants that are well hydrated before winter, have a good probability of survival. If in the following spring, there is evidence of damage—caused by extreme temperature fluctuations, erosion due to high winds, or plant desiccation because of foot traffic—a basic repair is possible to help these sections quickly recover.

Cristina Senjug serves as communications manager at Next Level Stormwater Management. In addition to researching and writing articles, she organizes vegetated roof tours and technical and educational seminars for designers, engineers, and installers. Senjug can be reached at[7].

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