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Green Roofs

Due to a general lack of parking spaces in urban areas, new office and residential developments create parking areas for their employees or residents. This area would ordinarily be an unattractive, ground level, open space taking up more valuable land. However many new developments have created below ground car parks, with the roof deck being turned into a green space. This has the obvious advantage for the users of the building who would look out over a landscaped area or could use the green roof as a place to sit and relax in their leisure time.

Icopal Green Roofs are part of the Icopal EcoActiv product range of environmentally benefitial products.

There are numerous benefits associated with green roofs, which include:

Aesthetics – Urban greening has, for a long time, been seen as a way to make towns and cities look more pleasing to the eye.

Improved Air Quality – A green roof will filter the air moving across it. 1m2 of grass roof can remove 0.2 Kg of airborne particulates from the air every year. 1.5m2 of uncut grass produces enough oxygen per year to supply 1 human with their yearly oxygen intake requirement.

Temperature Regulation – Through the daily cycles of condensation and evaporation, plants are able to cool cities during hot summer months. In the process of evapotranspiration, plants use heat energy from their surroundings when evaporating water. This process reduces the ‘Urban Heat Island Effect’ in the summer, which is the difference in temperature between a city and the surrounding countryside.

Rooms under a green roof are at least 3 – 4 oC cooler than the air outside, when temperatures range between 25-30 oC.

Water – Water is stored by the green roof substrate and then taken up by the plants from where it is returned through transpiration and evaporation into the atmosphere. In summer, green roofs can retain 70-80% of the precipitation that falls on them; in winter they retain 25-40%.

Green roofs reduce the amount of storm water runoff and also delay the time at which runoff occurs, resulting in decreased stress on drains and sewers at peak flow periods. Green roofs also filter the water as it passes through the profile of the system.

Wildlife Habitats – Rooftop habitats can provide a ‘green link’ connecting natural pockets of habitat with each other. They can also provide isolated habitats, which aren’t disturbed by other green areas at ground level. In Germany it has been shown that green roofs can harbour 10-40 different insect species and have been found to support nesting birds.

Recreation/Leisure – Enclosed green roofs can be developed around residential housing to provide children’s play areas or recreational spaces, solely for the people who live there.

Roof Protection – Vegetation on a roof deck protects the roof surface from the extremes of weather, and ultra-violet radiation, prolonging its life.

What is a green roof?
A green roof is created by a planting scheme growing within a substrate having an impermeable base, which is formed by the roof of a building. The roof can occur at different levels ranging from many storeys up, or in the case of an underground car park, at ‘ground level’.
The green roof can be designed as an intensive or extensive system: -

Extensive Systems.
Extensive systems generally have a shallow profile (approx 100 – 150mm deep), and are much lighter weight than intensive systems.

Extensive green roofs are created purely as design features, and are generally intended to be viewed from another location. Therefore they often have limited access for maintenance only, and are not primarily used as a recreational space to be walked through or sat in.

Often the planting types are low growing drought tolerant species such as sedums. As sedums are not very competitive plants, within a relatively short space of time they can become overgrown with weeds, therefore they do need a regular maintenance programme.

Intensive Systems.
These systems are designed to be used as recreational spaces and often involve many features similar to traditional ground level gardens, which can include paving, water features, lawns, shrubs and trees.

These ‘roof gardens’ are soil based multi layered systems and require a much higher level of maintenance than their extensive counterparts.

Intensive systems are much deeper than extensive systems (generally around 200 – 900mm), and because they are soil based create a much greater weight loading on the roof deck. Therefore it is imperative that the roof deck can withstand not only the saturated weight of the fully planted system with paving etc, but also any people or cars which are designed to be present.

Insulation
It is important that warm roofs are adequately insulated so that no heat can escape from the room below the roof garden and reach the roots of the plants. This could be a particular problem in the winter months when any heat escaping from a roof can cause plants to produce new growth which, could subsequently be destroyed by frost, and therefore potentially seriously damage the plant.

Water Requirements
Greater wind speeds, reflected heat from surrounding buildings and shallower soil depths may cause green roofs to dry out more readily than conventional terrestrial gardens. Therefore, it is generally recommended that an irrigation system be installed for intensive systems.

Once extensive systems are established, they very rarely need irrigating as the planting scheme is designed to be drought tolerant.

Invaluable components such as the “Fytocell” water reservoir layer, present in the Icopal Intensive Green Roof system, holds around 80% of its volume with water. This reservoir layer is situated just below the soil, and the plant roots grow into it and tap out the available water and nutrients held there. This means that there is a much greater water buffer within the rooting zone, and therefore water consumption and the need for irrigation is greatly reduced.

Soil Mixes.
It is important to have a good quality soil mix which is specifically designed for green roofs. The soil must be: -

Lightweight – Standard topsoil may weigh around 1,600 – 1,800 Kg/m3 when saturated. This is a major weight element in any green roof system and will obviously add a significant loading onto the roof deck when installed. By using a specially designed lightweight soil mix, saturated weights can be reduced to 1,200 Kg/m3 (Icopal 60/40 mix) or even 1,000 Kg/m3 (Icopal 30/30/40 mix). By reducing the soil weight, the roof deck does not have to be as load bearing, so reducing costs of construction.

Free Draining – It is important that green roof soils are free draining so that “ponding” of excess water does not occur in times of heavy rainfall. Any standing water on the surface of the soil is not only detrimental to the planting, but also adds a greater weight loading on the roof deck. Therefore it is important that water filters through the green roof system quickly to the drainage layer below, where it then moves across the roof deck and off via the drainage outlets.

Water Retentive – Due to the often increased evaporation rates from green roofs, it is important that the soil mix used is able to retain water to supply the plants when needed.

Slow to Breakdown – Roof garden soils which contain high amounts of organic matter, e.g. peat, will breakdown more quickly, therefore topping up of planting beds may be necessary. This may be logistically quite difficult on roof decks. Therefore by using more mineral based soils, which only break down very slowly, the need for topping up will be reduced if not eliminated.

Contain Nutrients – Fertilisers can be added in the production of the soil mix, or after it has been installed on the roof deck. The fertiliser will need to provide at least a base level of nutrition to help the plants establish and grow on in the early stages. Subsequent applications will then be required throughout the life of the green roof.

Guaranteed Waterproofing Integrity

Underlying the landscaping is a high quality roof waterproofing and insulation system capable of long term in-use performance providing a homogenous and continuous waterproof layer under hard and soft landscaping. Icopal has introduced modified membranes that have root retardant qualities. In this system the Rootbar cap sheet top layer, a two layer system, combines the elastomeric properties of SBS modified bitumen with specially formulate root inhibitors.  High quality polyester reinforcement gives puncture resistance and dimensional stability.  Similarly Rootbar underlay provides a secondary layer of performance waterproofing.  The membrane is fully bonded to the substrate  thus preventing water tracking beneath in the event of damage. This also avoids the possible cost of landscaping removal to carry out repairs. The application of a waterproofing layer enables the system to meet certain key criteria such as resistance to root penetration, high resistance to mechanical and chemical damage, aging and imposed stresses. Companies such as Icopal guarantee their waterproofing systems for up to 20 years.

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