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Green roofs: design guidance

December 5, 2013


Use

The function of the roof will effect its design. For example, a green roof designed to retain storm water may look very different from one whose main purpose is to brighten a hospital courtyard. Along with the question of aesthetics are inherent differences in the required depth of growing medium, the ongoing maintenance programme, and overall cost.

Location

The location of the green roof plays an important role in the design process. The height of the roof above the ground, its exposure to wind, the roof’s orientation to the sun and shading by surrounding buildings during parts of the day will have an impact. The general climate of the area and the specific microclimate on the roof must also be considered. Views to and from the roof may also determine where certain elements are located for maximum effect.

Consultants

Structural Engineer – to calculate loading capacity of the roof
M&E – to calculate the heating and cooling implications and to integrate existing and proposed mechanical equipment and drainage needs.
Landscape architect – to design the layout of the planting areas and specify the planting.
Health and Safety consultant – to advise on maintenance issues.

Structure

Additional loading is one of the main factors in determining both the viability and the cost of a green roof installation. If a green roof is part of the initial design of the building, the additional loading can be accommodated easily and for a relatively minor cost. However, if a green roof is installed on an existing building, the design will be limited to the carrying capacity of the existing roof, unless the owner is prepared to upgrade the structure, which can be a significant investment.

Loading guide (saturated weights):

· Extensive Green roof: 60–150 Kg/m2 – Intensive green Roof: 200–500 Kg/m2

Access and Exits

Access to the green roof site is crucial – not only for installation and ongoing maintenance, but also for bringing up materials, soil and plants. If the green roof is designed for use by tenants or the general public, then questions of access and of exiting are taken to another level altogether, from mere convenience to strict standards of safety and security as regulated by Building Regs.Roofing

One of the most important components of the green roof system is the waterproofing/roof membrane. For an existing building, the membrane should be carefully inspected to determine if it needs to be repaired or replaced before the installation. Many manufacturers of green roof systems will not provide a warranty on the green roof system if new membranes are not applied. The normal 10-15 year reroofing cycle provides a window of opportunity to investigate the potential of applying a longer lasting green roof. Green roofs can be applied on inverted or traditional roofing systems. If the existing system is inverted, then one needs to determine whether the insulation can be replaced by an equivalent R- value of growing medium. If the insulation is to remain, then good drainage must be ensured to prevent continuous contact with water and subsequent damage.

Although the green roof will retain much of the rain that falls on it, maintaining proper drainage on the roof is still very important. Parapets, edges, flashing and roof penetrations made by skylights, mechanical systems, vents, and chimneys must be well protected with a gravel skirt and sometimes a weeping drain pipe. If the drainage layer is too thin or if the routes to the roof drains become blocked, leakage of the membrane may occur, due to continuous contact with water or wet medium. The growing medium itself may sour, causing the plants to drown or rot. On a roof slope greater than 20 degrees, the green roof installer needs to ensure that the sod or plant layer does not slip or slump through its own weight, especially when it becomes wet. This can be prevented through the use of horizontal strapping, wood or metal, placed either under the membrane, or loose-laid on top. Support grid systems for green roofs have been designed by some green roof manufacturing companies specifically for this application.

Plants

Location, wind, rainfall, air pollution, building height, shade and soil depth are all factors in determining what plants can be grown and where. The ability of plants to survive on a green roof is directly proportional to the amount of maintenance time and budget allocated to the project, particularly in the first two years when they are getting established. Climatic conditions on a rooftop are often extreme. Unless one is willing to provide shading devices, irrigation and fertilization, the choice of planting material should be limited to hardier or indigenous varieties of grasses and sedums. Root size and depth should also be considered in determining whether the plant will stabilize in 100mm or in 600mm of growing medium. It is vital to know where the plants were previously grown and if the growing conditions were comparable to the ones on the roof to ensure their ability to adapt and flourish. Typically, extensive green roofs rely on a mixture of grasses, mosses, sedums, sempervirums, festucas, irises and wildflowers plants that are native to drylands, tundras, alvars and alpine slopes. On an intensive green roof, with few exceptions, the choices are limitless.

Construction / Installation

Be aware of installation issues. The installers should have experience with green roof systems. In fact, it may be preferable to have one company handle the whole project, from re-roofing to planting, thus avoiding scheduling conflicts and damage claims between the various trades. It will also bring single point responsibility post-construction. Methods for getting the materials up to the roof should be discussed to determine cost and potential equipment rentals. Timing is also important.

Depending on the local climate, planting in high summer requires extra irrigation to get the plants through the heat before they are established. Autumn planting will depend on the availability of suitable plant stock and time enough to allow for the plants to get established before the cold weather sets in. Some plants can be installed while dormant. If you prepare everything in the Autumn for Spring planting there may be some erosion of the growing medium through winter wind and runoff. Covering the roof with burlap or some other material could reduce this problem. Compartmentalisation of the green roof into sections may allow for easier access to the membrane and the roof drains, for inspection and maintenance, without having to pull up the whole installation.

Maintenance

Both plant maintenance and maintenance of the waterproofing membrane are required. Depending on whether the green roof is extensive or intensive, required plant maintenance will range from two to three yearly inspections to check for weeds or damage, to weekly visits for irrigation, pruning and replanting. To ensure continuity in the warranty and the upkeep, it is recommended that the fees for three to five years of this service be included in the original tender, and that maintenance contracts be awarded to the company that installed the green roof, or to an affiliate. Intensive systems typically require more maintenance than extensive systems due to the greater diversity of plants.

Maintenance and visual inspections of the waterproofing membrane can be complicated by the fact that the green roof system completely covers the membrane. Although the green roof protects the membrane from puncture damage and solar radiation, doubling its lifespan, leaks can still occur at joints, penetrations and flashings, due more to sloppy installation than to material failure. Regular maintenance inspections should be scheduled as for a standard roof installation, especially just before the warranty period expires. Some companies are recommending the incorporation of an electronic leak detection system between or underneath the waterproofing membrane to pinpoint the exact location of water leaks. Access strategies include keeping the sensitive areas free of plants and growing medium (gravel skirts, etc), and dividing the green roof into distinct compartments for ease of removal. Eventually, after 30-50 years, the membrane will have to be replaced. Depending on the roof size, building height, type of planting, and depth of growing medium, the system will either be removed and reinstalled over the new membrane, or replaced entirely. If the green roof can be removed and stored on the roof while the membrane is being replaced in sections, then the additional cost is “labour” only, and comparable to the original installation cost; if the green roof has to be moved off the roof, and then brought back up, costs will increase accordingly, and the arguments for starting afresh with new growing medium and plants, become more convincing.