Green roof
See also Tips what to do against Heat Islands
Greenroofs are vegetated roof covers constructed atop and across a roof deck, and with engineered soil and plants taking the place of bare membrane, gravel ballast, shingles or tiles. They sometimes are called living roofs, ecoroofs, sky gardens, vegetated rooftops, or skyrise gardens. Greenroofs are the result of a complete underlying roof build-up system, providing continuous, uninterrupted layers of protection and drainage. The number of layers and the layer placement vary from system to system and greenroof type, but at the very least all greenroofs include a single to multi-ply waterproofing layer, drainage, growing media and the plants, covering the entire roof deck surface. There are two main types of greenroofs – extensive or intensive - although a greenroof is often designed with features of both and then are referred to as either semi-extensive or semi-intensive. The classifications indicate maintenance requirements, ranging from virtually maintenance free to quite elaborate. The limiting factors for greenroofs include: the roof loading capacity or maximum dead and live weight loads, determined by a structural engineer; the slope of the roof and the budget. The difference between a greenroof and a roof garden is that vegetation is planted directly into the soil, not in planters or containers. Greenroofs do not require particularly high-tech design. It is important to understand, however, that these are engineered systems consisting of various material layers which must work in tandem to perform correctly and allow growth of plants on rooftops.
Greenroof plants should ideally be high drought, wind, frost, and heat tolerant. Monoculture, such as the use of sods, is not advised as they are open to plant disease, or an insect infestation could wipe it out. Grass or sod needs to be watered, fertilized, and cut regularly, so they would incur greater costs and be considered high maintenance. Generally, green roof systems are not deep enough for growing vegetables and the tools of traditional gardening would damage the green roof and the underlying waterproofing. However, incorporating container gardens or modular elements can accommodate an owner's horticultural or agricultural desires. Vegetation selection is crucial to the long term success of the project. It takes an experienced plantsman to know what perennials will perform well in this unique, often extreme microclimate. Greenroofs can be warranted for up to 20 years while plant materials are usually warranted for one growing season.
Extensive greenroofs, also referred to as eco-roofs, roof meadows, and low-profile greenroofs, Extensive greenroofs are built when the primary desire is for an ecological roof cover with limited human access. The minimum growing media or soil substrate starts at about 2 1/2” to 6” at most (although vegetative mats can actually have even less than 1" of growth media); the engineered soil media contains 70 – 80% inorganic or mineral material (or higher) to 20 – 30% organic (or less). Low growing, horizontally spreading root ground covers with general maximum plant heights of 16 – 24” are ideal. Plants for extensive greenroofs must be tolerant of high heat, drought, wind, and frost. They must also be self-regenerative in nature, and have low maintenance requirements overall. Plants include sedums and other succulents, flowering herbs, certain grasses and mosses and perennials. Fully saturated weights range from a low of about 10 – 50 lbs/sq. ft. Thus, they are lighter and less expensive systems. Extensive greenroofs can be constructed on slopes up to 30° (the recommended maximum slope) and steeper ones can be installed with raised grids or laths to hold plants and soil media in place. Roofs with a slope of 20º or more require steps to prevent shearing and erosion, and it is usually necessary to install additional support with cross battens (ZinCo's "The Green Roof Planning Guide"). Moreover, roofs with strong wind uplift or with slopes 15° and higher should be protected during establishment with an erosion control net in the form of jute or other natural biodegradable fiber. Cost for this type of roof may range from $14 to $25 per square foot. It is the most likely option for existing buildings since they are less likely to require structural enhancement of the underlying roof.
Intensive greenroofs, or high-profile greenroofs, look like traditional roof gardens because a much wider variety of plant material can be included. This is because the growing media starts from about 8 - 12” and can range up to 15 feet or more, depending on the loading capacity of the roof and the architectural and plant features that the building owner desires. The engineered soil media usually contains about 45 - 50% organic material to 50 - 55% mineral, and fully saturated weights range from about 80 -120 lbs/sq. ft. and up. Architectural accents such as waterfalls, ponds, gazebos, etc. are possible and these greenroofs provide recreation spaces and encourage interaction between people and nature. Maintenance requirements are also more intensive, and of course, these roofs are relatively flat. The cost ranges from $25 or $40 and up.
There is also what is called wildlife roofs, also known as biodiverse roof. These greenroofs are designed either to replicate specific habitat needs of a single or small number of species, or to create a range of habitats which can maximise the array of species which inhabit and use the roof. Installing a wildlife roof is often considered where the budget for installing a green roof is being squeezed. Wildlife and biodiverse roofs are sometimes assumed to be less costly to install, due to the recycled materials they may include. However, in reality this is seldom the case, a wildlife roof that will both deliver genuine habitat replacement and compliance with good green roof practice will cost approximately the same as an ‘off the shelf’ green roof.
Cost per square foot depends on many factors: the size and slope of the roof, depth and complexity of the system, height and accessibility from the ground, cost of labor, and need for specialized elements, such as drains, railings, pavers, slope stabilization measures, etc. When using recycled materials on green roofs, there is a three-way interaction among quantity, cost, and performance. Currently, components containing recycled materials are often more expensive than those made from virgin materials. In some instances, accommodating materials with recycled content would compromise performance. Moreover, many sythetic components are made in Europe.
About the soil, they are engineered. It is mostly light-weight mineral material, with a minimum of organic material. It should be designed to meet established FLL guidelines for both water retention and drainage. Regular garden soil is heavy, can contain pathogens, undesirable insects, and weeds while greenroof “soils” need to be lightweight to conform to roof loading weight restrictions, drain properly and yet retain a certain amount of rain water, and uniform in character. Some designers will approve a certain amount of topsoil to reduce costs, but great care needs to be taken with this option. Very lightweight media often contains high amounts of organic content (i.e., pine bark, coir) resulting in media that is inconsistent with the industry standard.
Soil has two components. The inorganic component of the soil/media refers to a high porosity natural mineral element such as expanded slate, shale, extruded clay, rock wool, lava or pumice, etc. They maintain void or air space necessary for the plant roots to breathe and for the excess water to drain properly. Thus, they also prevents total compaction of organic matter through settling over time. The organic component are well-rotted humus material (hen manure, guano, mushroom compost, etc.) augmented with organic fibrous material and a small amount of clay particles. This mixture holds and slowly releases essential trace elements necessary for the health of the soil community.
Watering of green roofs is necessary for intensive greenroofs as plants here require more water than succulents and herbs in extensive greenroofs, which may be sustained by rain only. Both definitely need water during there first years and during periods of dry spells. As the risk of erosion is greatest after the initial planting, soil must be achored with either an erosion control mat or hydromulch at this early stage. About irrigation systems, if to be installed, subsurface irrigation methods is advised. Surface irrigation does not deliver water efficiently to the roots and this system tends to have the roots develop near the surface, making the plants dependent on irrigation and vulnerable to lapses in watering schedule.
Use of gel paks is not advised.
Waterproofing of the building requires greenroofs to possess several layers of protective materials to convey water away from the roof deck. The waterproofing membrane (or membranes) is the single highest cost item of a new roof. World-wide, polyvinyl chloride (PVC) and polymer modified bituminous membranes are the most common. Many of these installations have now been in place for over 30 years and continue to perform as designed. PVC, EPDM and thermal polyolefin (TPO) are, in most cases, inherently root-resistant; other common waterproofing materials require a root barrier between the waterproofing materials and the vegetated cover. Starting from the bottom up, a waterproof membrane is followed by a root barrier, a layer of insulation (optional), a drainage layer, a filter fabric for fine soils, the engineered growing medium or soil substrate, and the plant material. Usually some form of a biodegradable erosion or "wind blanket," such as a jute or coco liner-type mesh, is placed over the new plants to stabilize establishing roots.
About the use of fertilizers, it is advised to use encapsulated slow release fertilizer twice during the first year of establishment in the early spring and fall, and then yearly thereafter for the next 4 years or so. Soluble N fertilizer may just get into the runoff. Due to the return of organic matter to the soil, additional fertilization is not usually necessary.
As seeds from the wind or from birds may come into the roof, protection from roof problems, such as growing roots, is needed. If your roofing membrane is organic in nature – asphalt, asphaltic bitumen, etc., you need a root barrier. It needs to be a dense inorganic material that inhibits root penetration, like polyethylene. This protective layer can be a heavy duty pond liner (EPDM, etc.) or other non-organic element that contains an injected root repellent, such as a copper element. As copper films are recent development with no proven track record, it is recommended to use thermoplastic membranes. Also, many North American drainage products bypass a separate root barrier layer and now incorporate a root-repellent ingredient in their synthetic filter fabric, placed atop the drainage layer.
About the possibility of being at risk of fire, extensive greenroofs can be designed to follow European Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau. e.V (FLL) guidelines:
- substrate should be at least 30mm deep and contain a maximum of 20% organic material,
- the forms of vegetation must constitute a low fire load (i.e. they provide a low volume of combustible matter that can act as a fuel to feed a fire),
- 0.5m wide gravel fire breaks should be provided around any openings penetrating the roof surface and around the edges of the roof, and
- for larger roofs gravel fire breaks should be provided at 40m spacing.
These standards and guidelines are comprehensive, and include industry standard tests for the weight, moisture, nutrient content, and grain-size distribution of growing media. FLL also certifies laboratories to conduct critical tests, such as the root penetration resistance of waterproofing membranes. These guidelines are available in English translation directly from the FLL or through Roofscapes, Inc. The American Standard Testing Methods (ASTM) has convened a Green Roof Task Force to create green roof standards.
Green roofs are not included in U-Value calculations as they are considered to be continually saturated. The U-Value will be taken to be the same as the roof without the extra green roof layer. The roof will however benefit from increased protection from heat in the summer and some protection from cold in the winter.
Green roofs have been built most widely in temperate climates, but special techniques allow them to thrive in semi-arid, tropical, and even windy coastal areas. Greenroof technology requires an appreciation and detailed knowledge of plant biology, hydraulic engineering and architecture. These are thoroughly engineered systems which address all the critical aspects of design, including: the saturated weight of the system and load bearing capacity of the underlying roof deck; moisture and root penetration resistance of the waterproofing membrane; resistance to wind shear; management of drainage; and the suitability of the proposed plant material. Moreover, no one system is suited to all roofs—there are too many variations in pitch, drainage, climate, and exposure, to name but a few . An installer who has not been part of the design process may—for reasons of cost, convenience, or unfamiliarity with the process—make changes that undermine the integrity of the system. This is especially common with landscape contractors who have had no or little green roof experience. Also, there are often aspects of a project that only become apparent on-site and that if your designer does not supervise the installation, s/he cannot make appropriate adjustments.
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[edit] See also
- Sod roof, Traditional roof in Scandinavia
[edit] Private benefits
[edit] Public benefits
- Public Economic Benefits
- Improved Air Quality
- Temperature Regulation
- Conserving Water
- Social Benefits
- Preservation of Habitat & Biodiversity
