Home Research Articles Affordable Housing Building Construction and Installations of Green Walls and Green Roofs Part-I

Building Construction and Installations of Green Walls and Green Roofs Part-I

689
Green infrastructure
Dr. A.N. Sarkar
Ex-Senior Professor (International Business) & Dean (Research), Asia-Pacific Institute of
Management, New Delhi

Most cities and townships in the world emphasize upon creation and development of green and sustainable infrastructure. Green infrastructure includes trees, parks, water sensitive urban design (such as wetlands and rain gardens) and green roofs, walls and facades. Green walls, roofs and facades are undergoing constant innovations paving the way of adding green infrastructure to cities in an eco-friendly manner. Walls, Roofs and facades can be installed on existing buildings, or built into new structures. A Green roof is a vegetated landscape that is installed on a roof surface with esthetics and installed scientifically. Green facades are created by growing plants up and across the face of a building. Plants are either rooted in the ground or grown from containers installed at different levels on the face of the building. Green walls are plants grown in vertical systems that are usually attached to internal or external walls. Green walls differ from green facades in that plantings are made across the entire vertical structure, as opposed to planting at the base of the structure to enable vertical and horizontal growth. In a green wall, plants, growing medium, irrigation and drainage are incorporated into the system. The paper discusses the various methodologies and techniques used in construction and installation of Green walls and Green Roofs.

1.0. Green Walls Building Construction and Installation

A green wall designed for urban agriculture can provide a multitude of benefits such as providing the basis for better community interaction (community gardening), improving access to fresh food; and reducing the environmental impacts associated with traditional food production and distribution (http://www.growinggreenguide.org/wp-content/uploads/2014/02/growing_green_guide_ebook_130214.pdf).

Green walls can be considered as vegetation growing on or against a vertical surface, both outdoors and indoors. Green walls can be installed at the point a building is initially constructed or can be retrofitted to existing buildings. Green walls provide many benefits similar to those generated by green roofs. However as visual access to green walls is much higher, there have a greater aesthetic value and social benefit. There are two main categories of green walls which could both be considered for installation on campus (http://documents.manchester.ac.uk/display.aspx?DocID=33158). In order to maximise the benefits and sustainability of green roofs / green walls, it is important that a coordinated approach is implemented.

1.1.Design considerations

With good design you can make the most of underused roof and wall spaces. The ideas given below may not all apply to every building, but they will help you to think about different aspects of your green roof or wall design (https://greenroofsaustralasia.com.au/sites/default/files/sponsors/files/Green-Roofs-DA-and-Design-Advice_CoS_0.pdf).

1.2. Structures and components for green wall systems

Hydroponic green wall systems can be either modular containers or large panels. The systems are installed via brackets that sit out from the load-bearing wall (or a stand-alone structure) to create an air gap between the wall (or other structure) and the backing sheet of the green wall system. In a hydroponic system, an inert growing medium is provided to which the plants physically anchor, such as horticultural foam, a mineral fibre or a felt mat. These materials can act as a water retentive sponge, although the more they soak up the heavier the system becomes. The advantage of the hydroponic system is that there is no structural decay of the growing medium, no salt build up from fertilizers and nutrients are supplied in a precise and controlled manner. Over time, plant roots grow and ramify through the entire system to create a very robust network.

Substrate-based systems use substrate-holding containers made of plastic or metal. The substrate is packed directly into the empty container or placed in a water permeable, synthetic fibre bag. The containers are connected together and anchored to the wall or to an independent, structurally secure metal rack or framework. Alternatively, plastic or metal growing containers can be hung on a metal grid fixed to the wall. Individual growing containers can be removed for maintenance or replanting. Most substrate based systems are designed for automatic irrigation, just like the hydroponic green wall systems. The growing medium in these systems provides a structure to support the plant and facilitates water, air and nutrient access, decreasing the need for constant management associated with hydroponic systems. However, over time the reserve of nutrients will be exhausted and there can be a buildup of salts in the growing medium. Traditional potting mix is not a suitable substrate for green walls. A specialist green wall provider will advise on the most appropriate growing medium for a particular green wall design.

Drip trays are used to capture excess irrigation water from the growing medium as well as water droplets that drip off foliage. The size of the drip tray should be sufficient to hold an entire irrigation cycle’s water volume (before draining away prior to subsequent cycles starting). Drip trays may not be necessary if the run-off is intended to irrigate vegetation below the green wall. If drip trays are not used, ensure run-off does not create slip hazards, damage the building fabric or provide excess moisture or nutrients to ground-based plantings below. Water captured in the drip tray or reservoir at the base of the planting system can be pumped back to the top of the wall for re-use rather than being wasted, provided that it is treated to prevent build-up of nutrients. Drip trays should have a drainage pipe of sufficient diameter to empty the drip tray or manage water capture sufficiently to mitigate overflow of the tray. Fascia treatments may be added to conceal the edges and functional elements of the green wall system, such as the irrigation system and drip tray.

1.3. Waterproofing

Waterproofing is project-dependent; in some cases there will be a sufficient air gap between the back of the planting system and the wall, making waterproofing treatment unnecessary. The air gap prevents movement of water between the wall and the planting system, and air-prunes plant roots to reduce the risk that they will directly contact the wall and provide a path for movement of moisture. Provision for an air gap between the planting system and the building wall will also prevent growth of mould. Where waterproofing is necessary, it will prevent damage to the wall from moisture and dissolved salts from fertilizers. In some cases the supporting wall might be considered waterproof as is; for example, a preformed concrete wall may be thick enough to be rated as fully waterproof, or a wall constructed from marine grade plywood will have some degree of waterproofing from the glues used within the ply. Consideration must be given to waterproofing penetrations to the wall as well as junctions between surrounding fascia and junctions between wall waterproofing and drip trays. Roller-applied liquid waterproofing treatments can be used on internal and external green walls. When considering waterproofing for any green wall, seek advice from a waterproofing consultant to ensure the most suitable treatment is chosen.

1.4.Irrigation and plant nutrition

Green walls cannot be sustained without irrigation. Interruptions to the water supply are a common cause of plant failure on green walls. Systems designed with inbuilt irrigation should mitigate plant losses due to inconsistent moisture management, although errors can still occur. Automated, remotely controllable irrigation systems are used for walls in high profile locations, or in situations where access is challenging. Note that the quality, design and costs will vary between different systems. The most sophisticated systems enable the maintenance supervisor to keep track of the automated performance of the system, including the volume of irrigation delivered, its frequency, substrate moisture content, as well as pH and nutrient levels in the water supply. The settings can be overridden if needed; for instance, the frequency or duration of irrigation cycles may be increased on hot days. In hydroponic systems, plant nutrition is delivered by a fertilizer injection system that releases controlled doses of fertilizer into the irrigation system (fertigation).

Management of fertigation systems and rates of delivery requires specialist knowledge, as it is more complex than fertilising soil or growing media. Hydroponic systems require continual monitoring of pH, water hardness and total dissolved solids (TDS), and adjustment of these parameters where necessary. For hydroponic green wall systems, the fertigation system may apply 0.5-20 litres of irrigation solution per square metre per day. Internal green wall requirements are at the lower end of this range, and external green walls at the higher end. Irrigation cycles typically last a few minutes and will be required several times a day. Keeping irrigation volumes low minimises waste and reduces run-off. Irrigation run-off may be captured in a tank at the base of the wall and recycled back through the green wall system. Green walls that use a high quality, water-retentive growing medium, and are not in an exposed or particularly hot location, may thrive on a weekly watering regime. In most simple, soil-based systems, including DIY systems, controlled release fertilizer is mixed in with the growing medium, rather than using a fertigation system (http://www.growinggreenguide.org/wp-content/uploads/2014/02/growing_green_guide_ebook_130214.pdf).

1.5. Selection of Vegetation

The size of plant materials used in green walls will depend on the required look and finish of the wall at project completion. Planting densities can be as high as 25 to 30 plants per square metre. Decorative patterns can be designed by repeat planting across an area; however, these patterns can be difficult to implement if shadows and light across the different parts of the wall have not been considered. Systems incorporating smaller plants at the outset will take longer to fill out than those using well-established container plants. The dimensions of the planting module will determine the size of the planting stock. Different green wall systems will be better for different types of plant growth habits, from those that grow upright to those with a clumping growth habit, scrambling, cascading or creeping.

1.6. Special Consideration for Green walls

Lighting is often required for green walls positioned in low light exposure areas. Many green walls are installed within non-lit areas. Lighting green walls is highly specialised, requiring the services of a lighting designer or engineer. Plants require very specific lighting quantities and qualities to photosynthesize, grow, flower and develop appropriately. Another consideration for green wall installations is air movement around the foliage. This is important to help prevent fungal growth, and additional ventilation may be needed to ensure sufficient air movement for indoor walls. Outdoor walls usually create their own microclimate that creates enough air movement, but in very sheltered positions attention should be given to this issue.

1.7. Wall Protection and different Facade treatments

Waterproofing treatment of the wall is not required for green facades. It is important to select plant species with a growth habit that will not damage the fabric of the wall. Some species with adventitious roots or scrambling stems can damage the building fabric over time, such as Common Ivy (Hedera helix). However, self-clinging climbers are exceptionally well suited to many vertical building surfaces, particularly old stone structures or those with minimal exposed mortar, and have lasted a long time without negative impacts on the building fabric. Plants can damage buildings by physical and chemical means, over timescales of centuries. Damage can be superficial, causing only aesthetic changes to the facade, or more structural damage may result, usually over much longer time periods. If in doubt, choose a green facade where the plants are grown on a support system that is installed separately from the building. Support systems for facades involving plants that have tendrils or twining stems may be made of plastic, timber, metal, or stainless steel cables or cable mesh.

Design of the support system must consider the intended lifespan of the facade, the growth habit of the plant species, and how spacing and offset from the wall can help to provide the desired end result. For containerized systems, plant species choice and the spacing and volume of containers are critical for establishing effective facade coverage. Specialists in green facade design and installation can provide advice on the most suitable system and the best construction approach. Wooden trellises are prone to damage by weather and plant growth and many plastics become brittle over time with ongoing exposure to UV light, heat and cold. Metal systems have the longest lifespan and require less maintenance. Stainless steel cables and trellis are low maintenance and have a long lifespan and probably offer the greatest flexibility to suit a variety of plant species and wind loads. Steel nets and mesh provide additional options, with closer ‘weaves’ than horizontal and vertical cabling.

1.8. Soils and Other Growth Media

Lighting is often required for green walls positioned in low light exposure areas. Many green walls are installed within non-lit areas. Lighting green walls is highly specialised, requiring the services of a lighting designer or engineer. Plants require very specific lighting quantities and qualities to photosynthesize, grow, flower and develop appropriately. Tropical and subtropical green wall installations can generally survive in lower light conditions than Mediterranean, temperate plantings. Extensive knowledge of horticulture and design of green wall systems is needed to choose the right species for the light levels available on-site. Another consideration for green wall installations is air movement around the foliage. This is important to help prevent fungal growth, and additional ventilation may be needed to ensure sufficient air movement for indoor walls. Outdoor walls usually create their own microclimate that creates enough air movement, but in very sheltered positions attention should be given to this issue.

1.9. Drainage and Irrigation Systems

Garden beds, or at-ground planter boxes used for climbing plants for facade greening, should have drainage appropriate for the plant species selected for use. Container systems placed at elevation on the face of a building should have a free-draining growing substrate to avoid potential water logging in the event of prolonged periods of wet weather. The potential for ponding of water above the top of the growing substrate should be minimised by providing overflow drainage holes in the sides of the container, just higher than the level to which the container is filled. In most cases, run-off through the base of the growing containers will simply run down onto the ground beneath, but drip trays can be installed to collect water. The vigour of many climbing plant species means that irrigation will be required to maintain high-density foliage cover and long-term performance of the green facade. In ground plantings in domestic settings will need irrigation at least during the hotter months, if not year-round. Harvested, recycled water should be used for irrigation wherever possible. Irrigation frequency will depend on the plant species selected, the type of growing medium used, and how exposed the facade is to sun and drying winds. At-ground plantings can be irrigated by automatic systems or manually with a hose. Surface or sub-surface dripper systems are suitable automatic systems. Irrigation supplied to elevated planters requires appropriate water supply pressure from tank, recycled and mains water supplies.

1.10. Maintenance Planning

A maintenance plan should include a clear description of:

  • Maintenance objectives – created based on the design intent, or the landscaping or environmental objectives that were the basis for the roof, wall or facade development
  • Performance targets, such as the time frame for complete coverage of an area by plants and foliage
  • Responsibilities of various personnel involved in operating the building, outlining the type, scope, duration of task and occurrence
  • Training requirements (such as Working at Heights certification) and safety equipment
  • Resources available Maintenance planning should also incorporate risk management, with the aim of reducing or eliminating the likelihood of failure that could result in property damage or personal injury.

For large projects, maintenance planning is often based on ‘asset management planning’ where the whole life of the asset is considered, including design, construction, establishment, operation, maintenance, renewal and demolition/replacement. For some green roofs, walls or facades, particularly those located on commercial premises, maintenance will be undertaken by someone other than the building owner. A maintenance agreement with the installation company or with a recommended third party may be the most economical way to ensure the best long-term performance of a green roof, wall or facade to either new contractors or back to the building owner. If a maintenance contract is used, it is important to be clear about the duration of the maintenance agreement, the scope of maintenance responsibility, and the need for handover at changeover. A supervisor may be designated to oversee the overall and ongoing management of maintenance activities, and can provide direction to maintenance staff and assess that work has been carried out satisfactorily.

 

2.0. Building Construction and Installation of Green Roofs

Green roofs can help regulate a building’s internal temperature, reduce storm-water runoff, and mitigate the urban heat island effect. Green roofs offer significant economic benefits, including a longer roof life and heating and cooling energy savings. Green roofs also provide an opportunity for urban food production, and increasing urban biodiversity (Figure 1). If well-designed and cared for, green roofs can offer people the psychological benefits of nature (https://www.asla.org/ContentDetail.aspx?id=43536).

2.1. Layers of Green

A green roof contains plants that are grown in a layer of substrate varying in depth from a few centimetres to well over a metre. The growing substrate is usually composed of a high proportion of mineral particles with a long lifespan, with a small proportion of organic matter. A filter sheet retains the substrate and prevents washout into the underlying drainage layer. A protection mat and a root barrier may be installed to prevent damage to the underlying waterproofing membrane that covers the roof deck (Figure 2).

A typical green roof system comprises of multiple layers, each providing a separate function. Figure 3 sets out the configuration of these layers and briefly describes the functions they provide (http://documents.manchester.ac.uk/display.aspx?DocID=33158).

In the UK there are 3 main categories of green roof, all of which could potentially be implemented across campus.

Brown roofs or ‘biodiversity roofs’ are usually similar in structure to an extensive green roof with an ecological focus. These roofs are designed with aims such as encouraging local biodiversity and recreating suitable habitat on the roof. Objects such as log piles and stones can be introduced to encourage insects on the roof. ‘Rubble roofs’ can be created using brick rubble and other man made materials. There are also a few additions to roofs which can be made without any large scale changes and investment. Plastic tubs can be placed on roofs to collect and hold water, small log piles can be constructed and stones placed on the roof. These small changes can help to reduce surface water discharge and they will provide benefits to wildlife such as shelter for invertebrates or a source of water (http://documents.manchester.ac.uk/display.aspx?DocID=33158).

2.2. Roof Deck

Green roofs can be installed on roofs made of concrete, timber, sheet metal (usually clip-lock, or corrugated galvanised steel) and a range of other materials. However, it may be difficult to obtain insurance for green roofs installed on roof decks made from materials other than structural concrete or metal. Green roofs are most commonly installed on concrete roof decks because of structural integrity, ease of design, durability and amenity when complete. Slopes and wind protection for specific considerations relating to pitched roofs. Greening a tiled roof requires special treatment. A green roof cannot be installed directly on a tiled roof. If the building has adequate load capacity, or if structural reinforcement and a support system can be incorporated into the design, a green roof may be constructed as a self-contained waterproof module that sits above the existing roof. Drainage from the green roof must connect directly into the lower level roof drainage system. It may be more economical to replace the roof to make it suitable for a green roof. The components installed on the roof deck will be either loose laid over the roof surface or installed as modules that connect together to form a continuous effect. Individual containers that are separately placed on a roof deck are considered a roof garden rather than a green roof, and because they do not cover a significant proportion of the roof they do not provide most of the benefits associated with green roofs. It may be necessary to remove or relocate existing infrastructure on roof decks, or the green roof can be built around the equipment.

2.3. Waterproofing

A watertight roof is critical to successful green roof construction. While some roofs are intrinsically waterproof when built, most will require some form of treatment to prevent water entry into the building. Waterproofing treatment must provide a strong but flexible layer that allows expansion under physical or thermal movements of the building structure, without compromising water tightness. Vegetation generally should not be installed over areas such as expansion joints, where regular inspection of the waterproofing will be necessary. Advice should be sought from a specialist waterproofing manufacturer to find the most suitable type of waterproofing treatment for the roof structure and the proposed green roof design.  In Australia, waterproofing is likely to be installed by a third party waterproofing contractor. The involvement of an independent contractor means that a clear agreement between all parties must be established for responsibility of the waterproofing membrane once it has been installed and certified as watertight. An independent leak detection specialist should test the waterproofing after its installation, and again after the green roof build-up is installed, prior to handover.

The following tables contrast the two major types of waterproofing – liquid applied treatments and preformed sheets:

  • Liquid applied treatments can be composed of bitumen emulsions, modified bitumen, polymer cement systems, polyurethane, polyurethane modified acrylic, acrylic or two part polyurethane hybrid elastomers that require mixing prior to application.
  • Preformed sheets are asphalt-based or comprised of thermosetting polymers or thermoplastic polymers.

Preformed waterproofing may suit green roofs with gentle slopes and large uninterrupted areas. A green roof with many fixings onto the roof deck or penetrations, such as for lighting, power or ventilation, may be more suited to liquid applied waterproofing that is sprayed on or rolled on to form a cohesive single layer. Waterproofing membranes must be protected from physical and chemical damage. This includes cuts and tears, the action of invasive roots and rhizomes, and exposure to the elements. All membranes will become brittle over time, and this is accelerated by exposure to cold, heat and UV rays from sunlight.

A green roof will shield the membrane from damage and can significantly lengthen its life. Some preformed membranes have a surface coating that provides additional protection. Ensure that the waterproofing material is certified root resistant, suitable for the substrate, and installed by experienced, trained and certified professionals. Root resistance may be built into waterproofing membranes either by the addition of root-inhibiting chemical treatments, or because the composition of the membrane provides an impenetrable barrier to root growth. Root resistant waterproofing is quicker to install than separate waterproofing and root barrier layers, but can be more costly. Examples include certain types of ethylene propylene diene monomer thermosetting, thermoplastic PVC and thermoplastic polyolefin membranes; however, the root resistance of a product must be confirmed with the manufacturer, with certification provided.

2.4. Protection Layers

A watertight roof is critical to successful green roof construction. While some roofs are intrinsically waterproof when built, most will require some form of treatment to prevent water entry into the building. Waterproofing treatment must provide a strong but flexible layer that allows expansion under physical or thermal movements of the building structure, without compromising water tightness. Vegetation generally should not be installed over areas such as expansion joints, where regular inspection of the waterproofing will be necessary. Advice should be sought from a specialist waterproofing manufacturer to find the most suitable type of waterproofing treatment for the roof structure and the proposed green roof design. The involvement of an independent contractor means that a clear agreement between all parties must be established for responsibility of the waterproofing membrane once it has been installed and certified as watertight. An independent leak detection specialist should test the waterproofing after its installation, and again after the green roof build-up is installed, prior to handover.

The following tables contrast the two major types of waterproofing – liquid applied treatments and preformed sheets:

  • Liquid applied treatments can be composed of bitumen emulsions, modified bitumen, polymer cement systems, polyurethane, polyurethane modified acrylic, acrylic or two-part polyurethane hybrid elastomers that require mixing prior to application.
  • Preformed sheets are asphalt-based or comprised of thermosetting polymers or thermoplastic polymers. Preformed waterproofing may suit green roofs with gentle slopes and large uninterrupted areas.

A green roof with many fixings onto the roof deck or penetrations, such as for lighting, power or ventilation, may be more suited to liquid applied waterproofing that is sprayed on or rolled on to form a cohesive single layer. Waterproofing membranes must be protected from physical and chemical damage. This includes cuts and tears, the action of invasive roots and rhizomes, and exposure to the elements. All membranes will become brittle over time, and this is accelerated by exposure to cold, heat and UV rays from sunlight. A green roof will shield the membrane from damage and can significantly lengthen its life. Some preformed membranes have a surface coating that provides additional protection. Ensure that the waterproofing material is certified root resistant, suitable for the substrate, and installed by experienced, trained and certified professionals. Root resistance may be built into waterproofing membranes either by the addition of root-inhibiting chemical treatments, or because the composition of the membrane provides an impenetrable barrier to root growth. Root resistant waterproofing is quicker to install than separate waterproofing and root barrier layers, but can be more costly. Examples include certain types of ethylene propylene diene monomer thermosetting, thermoplastic PVC and thermoplastic polyolefin membranes; however, the root resistance of a product must be confirmed with the manufacturer, with certification provided.

2.5. Drainage Layers and Filter sheets

Good drainage is critical for green roofs and ensures that large amounts of water are not retained on the roof, compromising both the structural integrity of the building and plant health through water logging and oxygen-depleted substrates. Sub-surface run-off must drain efficiently from the substrate, into the drainage layer, off the roof surface, and into drains to the storm-water or rainwater collection. The growing substrate must be kept separate from the drainage layer with a filter sheet. Older green roofs often used a permeable layer of rock aggregate (such as scoria or gravel) for drainage. The clay and silt content of materials used in a rock aggregate drainage layer should be =10 per cent by mass. The rock aggregate should also have a suitable pH and be low in soluble salts to ensure plant growth is not adversely affected. This form of drainage is heavy and does not allow for air pruning of roots, which is now achieved with plastic drainage cells. However, in some situations, rock aggregate drainage does a better job controlling peak flow.

On modern, lightweight, green roofs, plastic drainage sheets or boards are the preferred drainage materials. Plastic drainage layers may be rigid, open mesh structures that allow unrestricted drainage of water, or they may have a cup-style, ‘egg carton’ design that enables water to be stored at the base of the profile (see image below). The advantage of the latter is that water can be stored and used later by the plant. The volume of water that can be stored varies with each product and the size and packing density of the cells. Drainage is installed as a continuous layer over the entire surface of the green roof. Cup-style drainage sheets should be overlapped to eliminate the possibility of gaps being created between sheets. Other plastic drainage types should have adjacent sheets butted together.

Very rigid drainage layers should be installed in trafficable areas of the green roof to avoid compression of the layer. A layer of filter sheet will prevent substrate dropping into the drainage layer. Enough filter sheets is set down over the roof so that at the vertical edges of the planting beds the sheet will be the same height as the substrate.

An eggcup-shaped drainage layer retains some water. Image: KHD Landscaping Engineering Solutions A filter sheet acts to retain the growing substrate, by preventing wash-through of the substrate particles into the underlying layers, and to prevent clogging of holes in the drainage layer. Filter fabric is sometimes referred to as geo-textile fabric. Considerations in the selection and use of filter sheets include:

  • The expected flow rate of water as it drains through the system
  • Substrate type – if components of the substrate have sharp edges, the filter sheet should be strong
  • Vegetation type – the filter sheet must allow penetration of roots, and certain roots will be more or less aggressive (for example, herbs versus trees roots) The filter sheet can be either a woven or non-woven material.

 

The Part II will be continued in the next edition

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