What is Green Roof
A green roof or "living roof" has numerous construction benefits: energy savings as a result of improved insulation; lower ambient temperature and reduction in the effects of urban heat islands, CO2
absorption and particulate matter; and improvement in air quality, deceleration of water outflow and a cleaner environment.
A green roof's unique conformation means it needs a specific and particularly reliable waterproofing system. POLYGLASS has developed two main waterproofing systems: one using prefabricated distillate polymer bitumen membranes and one with prefabricated FPO/TPO (flexible polyolefin) membranes.
Green Roof system with Distillate Polymer Bitumen Waterproofing Membranes and FPO/TPO Synthetic Membranes
A green roof is generally a complex build-up system involving several layers, each with its own specific function. POLYGLASS systems come complete with all the required elements, from primers and vapour barriers to waterproofing membranes or sheaths, and even separating layers and drainage geocomposites are included.
Both solutions (bitumen and synthetic) are CE certified and are resistant to root penetration (EN 13948). They also offer high mechanical strength, which is essential, especially at the garden construction stage.
Advantages and features
Improved thermal insulation
during both the winter months and in summer, resulting in improved occupant comfort
and a considerable saving in energy consumption and bills
to heat and cool interiors.
Improved sound insulation and reduced noise pollution
, due to the mass of the roof and the green roof's dual action of absorbing and reducing the spread of sound waves as it is made up of surfaces that are not level and different materials with impressive soundproofing capability.
Reduction in particulate (including PM10) and smog
, by virtue of the absorption
effect provided by the vegetation.
Reduced CO2 emissions
(reduced greenhouse effect) as a result of a decreased use of air-conditioning in summer and heating in winter. In addition, through the photosynthesis process, the vegetation absorbs CO2, transforming it and releasing it in the form of oxygen.
Most electromagnetic waves absorbed.
Research conducted by the University of Kassel (Germany) has shown that a green roof with a 15 cm substrate can absorb approx. 94.4% of emissions in the cell phone network frequency range from 1.8 to 1.9 GHz. In the range of electromagnetic waves for UMTS frequencies between 1.92 and 2.17 GHz (latest generation mobiles), the research revealed an even greater reduction. For waves in the amateur radio range of 4 GHz, the reduction was as high as 99.9999%.
Decreased "urban heat island" effect
(the term urban heat island is used to describe the phenomenon whereby the temperature generated in urban areas is higher than that recorded in rural areas) as green roofs absorb and retain rainwater and irrigation water, releasing it back into the atmosphere through evapotranspiration (the combined effect of transpiration - through the plants - and evaporation directly from the soil), thus cooling the surrounding air.
Slowed rainwater runoff
into drains and the sewer system due to the optimization of the hydraulic regime by the soil and vegetation, whereby water is absorbed and slowly released.
Prolonged life expectation of waterproofing membranes
, which are protected from UV rays, weather, mechanical damage and temperature variations. On a green roof the maximum surface temperatures in summer are around 25 °C, while on an unprotected roof they can be far higher, especially if the roof is a dark colour.
Increased photovoltaic panel efficiency.
On a green roof, where surface temperatures are lower, photovoltaic panels perform near their peak efficiency, hence producing their maximum energy output. According to published research in this field, as far as optimal temperature is concerned, 25 °C is estimated to provide the classic photovoltaic panel with the best conditions for producing energy. In this regard, a useful indicator is the temperature coefficient, which indicates how much panel efficiency decreases as outside temperature increases.
New amenity and green spaces created
, even in densely built-up urban environments.
New habitats created for flora and fauna
which help protect biodiversity
, beautifying both the urban environment - helping redevelop run-down areas - and the building itself, which also benefits in terms of resale value
, while increasing occupants' wellbeing and quality of life
Key functional layers of a green roof
The typical build-up of a green roof waterproofed with bituminous and synthetic membranes is relatively simple. The key functional layers are described below.
Load-bearing structural support
The load-bearing structural support must be sized to withstand permanent loads and accidental overloading resulting from the weight of the actual garden itself, the weight of irrigation water and the weight of people and equipment required for its maintenance in the event the roof is an amenity space. Various kinds of load-bearing structural support can be used, such as a traditional or prefabricated reinforced concrete deck, a composite steel deck or timber deck. Depending on the green roof design, the load-bearing structure can have a slope varying from steep to shallow, or may even have no slope (in the latter case, the slope will be created with concrete screed or with suitably arranged insulating panels).
The purpose of the levelling layer is to level out and compensate for protuberances and unevenness in the surface the roof is being installed on in order to avoid the risk of the waterproofing membrane being punctured. This layer is produced using geotextiles with a suitable weight and relevant properties.
Root-resistant waterproofing layer
The purpose of the waterproofing layer is to stop rainwater and irrigation water getting inside, as well as protecting the load-bearing structural support from deterioration. This layer must also be resistant to penetration by roots that it might come into contact with.
The purpose of the protection layer is to protect the waterproofing layer from mechanical damage. The waterproofing system is exposed to the risk of mechanical damage both during the building of the actual garden and while it is in service. There are green roof systems that, given the nature of the materials and/or systems, do not require this layer to be built or already have protection built in.
Drainage or drainage/water storage layer
The purpose of the drainage layer is to allow excess rainwater or irrigation water to drain away so that the growing medium does not become saturated, which could compromise the correct development of the vegetation's root system. Many green roof systems feature a water storage layer incorporated into the drainage layer so as to have a reserve of water for "bottom-up" irrigation of the vegetation. The water storage means sprinkler irrigation can be used at greater intervals and, in certain favourable situations, irrigation can be done away with altogether.
The purpose of the filter layer is to stop the growing medium from being wash away and thus clogging and compromising the operation of the drainage layer or drainage/water storage layer.
This is the growing medium, more specifically the natural substrate comprising a blend of soils in which the roof's vegetation can grow and survive.
Vegetation must be chosen carefully, taking into account the environmental conditions and various factors that can affect its development and survival, such as local climate, exposure, the roof's use and maintenance requirements.
Complementary functional layers of a Green Roof
Building a more complete and well-structured green roof involves the use of certain complementary elements, as described below.
Vapour barrier layer
To be included in the building of a green roof with thermal insulation. The purpose of this layer is to control the flow of vapour passing through the structure, from the inside of the building to the outside, in order to avoid the issue of condensation forming inside the roof assembly. It must be applied under the thermal insulation layer.
Thermal insulation layer
To be applied when building thermally insulated green roofs. This layer can be produced using the various usual commercially available insulating materials. Suitable compressive strength is one essential requirement of this layer, which must also be compatible with the waterproofing membrane to be applied on top.
Erosion control/soil-retention systems or layer
Erosion control/soil-retention systems are used when producing green roofs on pitched roofs. Their purpose is to stop the growing medium from slipping and eroding when laid on a slope. Refer to chapter 5 for further information.
The purpose of the irrigation system is to supplement the water that vegetation on the roof is receiving from rainfall in order to ensure correct plant growth and survival. Various kinds can be used, from sprinkler systems - with fixed or oscillating sprinklers - to soaker hoses, drip lines or sub irrigation to water from the bottom up. If used in conjunction with a water storage layer, the irrigation system required may be much smaller or watering times may be reduced considerably.