# Text Block (132) | Precedents by Fion Fong

Planting Green Roofs and Living Walls, by Nigel Dunnett and Noël Kingsbury
Timber Press, Inc. 2008
-Why build green roofs? (page 41)
-Has the potential to increase biodiversity and habitats for animals in an urban environment.
-Green roofs help to manage storm water: plants absorb some of the water and transpire it back into the atmosphere while the soil absorbs the majority of the rest. This can greatly help to combat the negative effects of heavy runoff water like flooding.
-Trees and large vegetation in urban areas can filter out fine airborne particles like those produced primarily by vehicles to reduce air pollution.
-Foliage can also absorb gaseous pollutants, sequestering the material in their tissues.
-Carbon sequestration: capturing CO2 from the atmosphere or capturing anthropogenic (human) CO2 from large-scale stationary sources like power plants before it is released into the atmosphere. Once captured, it is put into long term storage.
-Terrestrial Sequestration: using plants to capture CO2 from the atmosphere and then storing it as carbon in the stems and roots and the soil.
-Geological Sequestration: putting CO2 into long-term storage in geologic zones deep underground. This method is not as sustainable or practical.
Above referenced from: http://www.undeerc.org/pcor/sequestration/whatissequestration.aspx
-Green roofs have the potential to be carbon sinks for the city.
-Green roofs also help to reduce increased temperatures in urban environments that can potentially lead to smog formation. This is done through the natural process of evapotranspiration, (the movement of water through the plant from the roots to its release to the atmosphere from the leaves as water vapour), and the evaporation of water from the soil and vegetation surfaces. The energy is then retained within the water vapour and is prevented from being converted into heat at the surface directly improving climate quality.
-Hard urban surfaces tend to reflect sound while green roofs can absorb sound and reduce noise pollution.
-Green roofs provide cooling in the summer and insulation in the winter for a building, improving energy efficiency and reducing cost.
-Research: Fairmount Hotel, Vancourver, Canada. Rooftop garden provides all the herbs for the hotel saving an estimated C$25,000 -$30,000.
-Research: New York City's Earth Pledge. Horticulture challenges.
-Herb species perform best in free-draining soils in sunny situations. Alpine strawberries will grow in shady /damper areas at the base of a sloping roof.
-Rooftop crops will require 30-40cm of depth in soil and irrigation. As well, the weight bearing capacity of the roof will have to be considered.
-Lightweight media, rooftop greenhouses and hydroponic techniques can widen the potential for rooftop 'farms'.
-Constructing Green Roofs (page 91)
-The load-bearing capacity of each modified rooftop must be taken into consideration. The load of a green roof should be measured when they are most compact and moist as these saturated weights of the materials will indicate their maximum loading.
-Testing, Germany: standard test is applied that involves moistening a sample of the material, compressing it into a mould, soaking for 24 hours, draining for 1 hour, and then weighing. This saturated moisture content is usually around twice as high as normal field capacity, giving a clear indication of the maximum weight of the material.
-Additional columns, beams and braces can be placed in existing buildings to bear the weight of the green roof. On smaller structures, a framework can be constructed around the building the enables the green roof to sit clear of the existing roof.
-Each country or region will have its own building standards, catered specifically towards their weather conditions, that determine the minimum load-bearing capacity of a rooftop.
-Newly constructed buildings can factor in the weight of a green roof/urban farm into the design.
-The maximum possible slope of a rooftop is controlled by the friction coefficient between the two most slippery materials in the green-roof profile. Most green roofs have membrane interface at root barriers or sheet drains at which the slippage will occur the most. With additional stabilization measures, (horizontal strapping, laths, battens, or grids), it is unwise to design on roofs steeper than 2:12 (9.5 degrees).
-To combat wind, a strip of gravel, stones, or pavers around the edge of the roof can prevent such wind damage. These strips can also act as vegetation barriers, preventing damage by plants to the edges of roofs.
-Irrigation methods:

1. Drip and tube method pegged to the surface or buried in the substrate. Sub-irrigation systems direct the water only to plant roots, lose less water to evaporation and are not visible. Buried pipes are more efficient that surface pipes. If the substrate surface is kept dry, weed seeds are less likely to germinate.
2. Capillary systems are porous mats that deliver water to the base of the substrate and are ideal for shallower systems (20cm or less in depth). Water introduced at just a few locations can be distributed evenly through the use of a capillary mat.
3. Stranding-water systems maintain a layer of water at the base of the roof. These systems can be self-regulating, being filled by percolating rainfall. They can also be maintained by floating control devices.

-Green roofs reduce the risk of fire compared to a traditional biomass/bitumen roof, as tested in Germany.
-Layers of a green roof: roof deck, thermal insulation, waterproof/root-proof membrane, drainage layer, filter mat, substrate/growing medium.
-As with all construction, a life cycle cost analysis and environmental impact assessment will document and analyze the environmental impacts of the fabrication, transportation, installation, operation, maintenance and disposal of a green roof system.
-Types of waterproof membranes:

1. Single-ply membrane: rolled sheets of inorganic plastic or synthetic rubber material that are overlapped at the joints and sealed with heat. Seams and bonds between the sheets and tiles can form weak points that may be exploited by plant roots that may lead to leaks.
2. Fluid-applied membrane: available in hot or cold liquid form that is sprayed or painted on to the surface of the roof and forms a complete seal when set, eliminating joint problems.

-Root protection barrier made of rolls of PVC that performs multiple functions, is long-lasting, easily recycled and eliminates the need for additional materials and costs. As well, the sheets are welded together leaving no gaps to be exploited by plant roots.
-Drainage layer:

1. Granular materials: gravel, stone chips, broken clay tiles, clinker, scoria (lava rock), pumice, expanded shale, or expanded clay granules contain large amounts of air or pore space between them when packed together in a layer or space allowing water to move into it from the vegetation and substrate layers above. Very low-tech but not always even in terms of drainage. Gives more room for plant roots to grow.
2. Porous mats: sponge-like mats constructed from a range of materials (ex. Recycled materials like clothing and car seats) that absorb water. Can be too absorbent, sucking water from plants.
3. Lightweight plastic or polystyrene: thinner than 2.5cm, differ greatly in design and appearance. They provide permanent free-flowing light weight drainage layer beneath the planting medium. Some are designed to store water.

-Filter mat: a semi-permeable polypropylene fabric laid loosely over the drainage layer to prevent fine materials from being washed into the drainage and blocking pore space.