Urban Sustainability: Harnessing the Potential of Green Roofs

Extensive green roof:

The extensive green roof has a shallow growing layer with a limited diversity of plants.

The shallow growing layer has thin soil and is less than six inches usually with nil or limited irrigation. This creates a condition that is often stressful for plants. So hardy plants are more recommended for this type of roof.

The shallow growing layer is mostly applicable for large areas and for roofs with 0-30 degree slope. The extensive green roof requires comparatively lesser technical expertise and there is no need for a drainage system.

Extensive green roofs are found to be inexpensive and require less maintenance.

Intensive green roof:

The intensive green roof has a deeper growing layer with more soil and hence it supports a wide diversity of plant species which sometimes include small trees. 

Intensive green roofs offer more favorable conditions for plants and hence require higher maintenance and costs. 

Semi-intensive green roof:

A semi-intensive green roof consists of features of both the extensive green roof and the intensive green roof.

Depending on the annual rainfall, plants, stormwater performance requirements, and the structure of the roof, the appropriate depths of the green roof are determined.

There are different roofing materials found on green roofs layers.

Green roofs layers:

• Vegetation
• Growing medium
• Filter membrane
• Drainage layer
• Root repellant/Waterproof
• Roofing membrane support
• Thermal insulation
• Vapor control
• Structural roof support

Green roofs benefits:

Benefits to the community:

• Reduces stormwater accumulation.
• Stress on the local sewer systems is reduced.
• The heat conducted by the buildings is reduced.
• Green roofs reduce the dust and smog distributed over the air.
• Public spaces such as gardens or recreational areas also come under green roofs.
• The increase of jobs and revenue as the amount of work that goes into the construction of green roofs is huge.
• The investment opportunity is increased as it is a means to beautifying the environment.
• Recycled materials can be used in the growing layer.
• Green roofs serve as natural filters for runoff water.
• Reduces greenhouse emissions.
• Green roofs capture airborne pollutants and improve air quality.
• Green roofs can be used as a site for urban agriculture.

Did you know?

Research published by the National Research Council of Canada found that an extensive green roof reduced the daily energy demand for air conditioning in the summer by over 75% (Liu 2003).

Benefits to the homeowner:

• The incoming noise to the house is reduced as green roofs act as a natural barrier.
• The roof membrane lasts longer as it is covered by plants.
• The energy consumption for the house is reduced.
• Safeguards your house by providing fire retardation.
• Waste generated by your house is reduced as green roofs utilize most of the waste products.
• The efficiency of the appliances and property value is increased.
• Green roofs increase a building’s marketability.

Green roofs – Disadvantages:

• Installing a green roof is an expensive procedure.
• By taking potential droughts into consideration, insurance is a must. Plants are flammable.
• Sometimes, the weight of the green roof may cause sagging.
• The damage caused by the plant to the building might result in costly repairs.
  If there is a leak, it is more difficult to repair

Vegetation:

The vegetation provides additional oxygen to the house and brings forth biodiversity to the roof. It absorbs most of the heat and light and therefore keeps the house cool.

Growing medium:

The growing medium may consist of some kind of engineering growing medium or soil depending on the plant profile, designer, and the rooftop. The stormwater load can be retained by the green roof as it acts as a water-retaining material.

Filter Fabric:

The filter fabric is used to remove surplus organic material from the drain and also removes clogs. The suspended particles are filtered through the filter fabric and only the clean water passes through.

Optional water retention layer:

To avoid peak runoff water and slowing stormwater, it is desirable to delay the runoff water from the roof as much as possible. The water in the green roof already travels through vegetation, substrate, and drainage layers. Adding a water retention layer will further delay the process. By absorbing stormwater, green roofs become wet but keep the cities dry.

Drainage layer:

To drain the excess water and to avoid waterlogging, there is a drainage layer in the green roof. Depending on the drain quantity and location, the capacity of the load, desired retention/detention of the stormwater, and the roof slope, the best drainage layer is determined.

Root barrier:

Root barrier is used to prevent the membrane below from getting damaged. It prevents the penetration of roots into the rooftops thus preventing possible damage. Root barriers are made of a flexible material but do not consist of chemicals.

Waterproofing membrane:

This membrane protects the roof from leakage, as the name suggests. There are many different installation methods and formats when it comes to waterproofing membranes.

The other additional green roofs layers include

• Erosion blankets
• Extra insulation
• Irrigation
• Protection boards

Green roof/Rooftop gardens – how are they constructed?

The installation of a green roof has to follow some basic principles.

It is a good and standard practice to cover the surface of the roof with a protection layer that is made of a puncture-resistant geotextile as soils can negatively impact the waterproofing layer.

A plastic eggshell-style drainage layer is used as a traditional practice while there are many natural alternatives.

The filter sheet is placed above the drainage layer to prevent washing out of the soils which also runs across the perimeters. The fine particles in the soil are filtered in this sheet and they allow the water to move through.

Green roofs are installed in roofs that are pitched at less than 30 degrees.

The planting is atop the moisture-retention layer which helps to maintain the water in the soil and prevents it from pooling on the membrane surface.

There is an aqua drain (egg membrane) which is used to carry away the excess water and a waterproofing membrane to prevent leakage as seen above.

A vapor control layer helps to prevent condensation inside and allows the internal water vapor to pass through the roof.

To remove the excess water from the roof, a downpipe is required. Conventional guttering could also be used.

Shingle should be constructed around the green roof perimeter and the outlet to allow the movement of water.

It is advisable to use mesh screens or slatted screens wherever possible as the rooftop gardens may be subject to strong winds.

Green roof/Rooftop gardens – what to plant?

Bulbous plants adapt to extreme conditions of cold and heat.

Bulbs such as crocus, dwarf iris, and non-native alpines can be planted.

A Turkish mountain plant called Muscari could be a great plant for green roofs.

Chives – another bulbous plant that can survive even in drought conditions.

Meadow saffron and cyclamen – autumn bulbs that could bring color.

Native wildflowers can be a great benefit to wildlife such as bird’s foot trefoil and Viper’s bugloss.

Native herbs could be a great addition while basil and wild marjoram could bring the mild perfume of the Meditteranean.

Gold Sedum – It requires only a shallow layer of the substrate. It’s hardy and easy to maintain. It produces yellow flowers.

Widow’s cross – This plant is hardy and drought-resistant and is suitable for green roofs.

Two-row stonecrop – This tough plant thrives in poor and well-drained soil. This plant produces wonderful star-shaped flowers in pink and red. It offers great rooftop fire protection when compared to grass covers.

White stonecrop – This hardy and easy-to-grow plant thrives in dry and thin soil and requires only little maintenance. It produces white flowers, insulates roofs, and offers shelter and food for wildlife.

Meadow saxifrage – This plant grows up to 50 cm and is rich in pollen and nectar. The leaves are kidney-shaped and the flowers are white and hence creates a great visual display.

Sea thrift – This plant is mostly found in coastal areas. It produces pink or white flowers and is suitable for sandy and dry soils.

Oregano – Thrives in well-drained soil, contributes to the roof herb garden. The flowers are oregano are pollen-rich and attract insects.

Creeping thyme – This plant grows in soil that is thin and has sharp drainage. They are nectar-rich. So they attract insects.

Green roofs come with a wide range of benefits including energy consumption, flood mitigation (to check impact you will need a flood risk assessment), habitat for urban wildlife, pollution, and heat mitigation as we have seen above.

According to a study, the green roof market is expanding at a rate of 17% each year.

The vertically built structure which is covered by vegetation is called a green wall.

According to a report by the European Federation of Green Roof and Green Walls, on the title Living Roofs and Walls from policy to practice, the benefits of a green economy using green roofs and walls are outlined.

Benefit – 1: Urban cooling

Urban roofs and walls offer a great deal of evaporation and shade that can be used to cool buildings and reduce temperature.

For example, in Constantine, Algeria, the ambient city air temperature decreased by 1.3 degrees Celsius due to 50 percent coverage of green roofs.

By 2050, it is expected that the average summertime temperatures of more than 970 cities will be 35˚C (95˚F).

It is expected that climate change would cause ~250,000 additional deaths annually, including deaths by heat stress between 2030 and 2050.

Benefit – 2: Biodiversity improvement

Of all wild animals, humans have destroyed 83 percent of biodiversity.

Green roofs and walls provide habitats for plants and animals in spite of their small area.

With green roofs and walls, the important creatures associated are birds, bats, and invertebrates.

The black redstart, in London, uses green roofs as a stepping stone habitat to travel across the city.

Benefit – 3: Carbon sequestration

To minimize the climate change impact and to meet the IPCC target of 2 degrees Celsius, carbon sequestration and carbon use reduction combination is needed.

Vegetation and soils serve as net carbon stores by sequestering carbon as organic matter.

375 g of carbon per square meter can be sequestered by the vegetation and soil of green walls and roofs.

Thus they help to reduce the carbon footprint of the economy.

Integrating green buildings into the economy will help in creating a more sustainable future for individual organizations and businesses.

DIY Kits

If you are looking for products to make your own green roof, this list might be helpful for you.

Green Roof Direct – Produce packs for DIY enthusiasts, installers, and architects.

Sedum Supply – Offers different green roofs like wildflower, meadow, and bio-diverse packs.

Enviromat – Offers ‘living carpet’ of sedum plants.

Grey2Green – Offers substrates, matting, and waterproofing.

Big Green Roof Projects

Sky Garden – Great for small and large projects have their modular system.

Organic Roofs – Design, install and provide off-the-shelf solutions.

Nimrod Environmental Ltd – Experts in removing Japanese knotweed, design and install green roofs.

Oxford Green Roofs – Installs green roofs for businesses and homes.

Pocket Habitat – Modules that can be used on small and grand scales made from recycled PP fibers.

Landmark Living Roofs – Install and supply green roofs.

Frosts Landscapes – Specialize in green roofs.

The Green Roofing Company – Installs sedum, brown, and green roofs for commercial and domestic concerns.

OptiGreen – Provide innovations in green living.

Bauder – Quick installations with a great deal of design element.

Green roof installers

There are no accreditation bodies for green roof companies. However, there is a code of conduct produced by the industry.

It is always worthy to consider the following factors.

• The number of years the company has been installing green roofs
• Customer reviews and feedbacks
• If the company has been associated with any trade association
• If the company has a liability insurance

The installers will carry out a full survey of the roof, have good knowledge of the plants to be grown, and a preliminary maintenance program to make sure that the roof beds down properly.

Code of conduct:

The green roof must have the correct components including proper aeration, good drainage, and adequate sunlight.

Root resistant material must be installed to ensure plants don’t creep into the rooftop.

Protection and moisture retention layers should be installed.

Reservoir layer, filter layer, and a drainage layer to be installed.

The growing medium should be fire-resistant, lightweight, and contain as little organic matter as possible.

The construction weight, the wind effect on installation, and safe access to the rooftop for activities like gardening, etc.

To learn more, visit renewableenergyhub

Green roof designers

In the case of bigger installations, a qualified landscape designer should be employed.

Other companies

Delta Membrane Systems Ltd – Experts in damp proofing and waterproofing.

Plant Plan – Provides a wide range of plants, corporate floristry, and living walls.

Ground Control Ltd – Provide a wide range of services from vegetation management to landscape construction.

Green Roof Examples from around the world

• My neighbour – This is an example of a simple green roof on a bike shelter.
• St Pauli Bunker – Great example of an excelsior green roof (being built)
  5 more examples 
• 25 more examples 

Calculating the cost of a green roof involves many variables like the kinds of plants that are going to be grown on it and not just the accessibility and size of the green roof site.

Factors to consider while calculating the cost of a green roof are

1. Intensive green roofs are more expensive than extensive green roofs due to the increased soil depth.
2. Designing the roof by employing a landscape gardener will cost more especially for large installations.
3. Accessibility of the green roof site plays a huge role in the cost involved. A green roof on a skyscraper will cost more compared to a garden which will involve low cost.
4. To strengthen the roof, the amount of structural engineering required should also be considered.
5. The number of plants to be used, the quality, and other garden materials required should also be taken into account while calculating costs.
6. A brown roof or biodiversity roof might require increased ongoing maintenance when compared to other kinds of installations.
7. Adding to the installation and design costs, certain running costs such as gardening costs and maintenance costs may also be incurred.

Roughly, in the UK, for an extensive roof, one might expect to pay around £100 per meter square and for an intensive roof, one might expect to pay around £150 per meter square.

Green Homes Grant

This voucher from the government can be used for home improvements. The measures are divided into ‘primary’ and ‘secondary measures.

This voucher should be used to install at least one primary measure which consists of insulation and low carbon measures.

Insulation measures include cavity wall insulation, solid wall insulation, pitched roof insulation, flat roof insulation, loft insulation, under-floor insulation, insulating a park home.

Low carbon heating measures include ground source heat pump, hybrid heat pump, biomass boiler, solar thermal, air-source heat pump.

If the voucher has been used for at least one primary measure, it can be used to cover the costs of any of secondary measures that include doors, windows, insulation, and draught-proofing which covers double or triple glazing, secondary glazing, external energy-efficient replacement doors, hot water tank insulation, draught-proofing.

Another secondary measure that the voucher covers is heating controls and insulation which includes measures like hot water tank insulation, heating controls, hot water thermostat.

The voucher covers costs incurred for VAT, labor, and materials.

This voucher covers up to two-thirds of the cost of the improvements with £5,000 as a maximum contribution from the government.

In case of certain benefits, the voucher can cover 100 percent of the cost of the improvements with £10,000 as a maximum contribution from the government.

For newly built homes that have not been occupied previously, this voucher is not applicable.

You are eligible for the voucher if you are the owner of your home, you are the owner of a park home that is on a residential site, or if you are a residential landlord in the social rented sector or private sector.

For more information on the Green Homes Grant voucher, visit  the gov.uk site

Planning Permission for green roofs

The incorporation of a green roof into a new building may ease the process of approval.

It is advisable to get in touch with a reputable installer or your local council to identify

• If planning permission is required for the green roof
• If building regulations approval is needed
• If there are any guides or planning policies related to green roofs

You might also need to get in touch with the mortgage and insurance companies.

Building regulation approval

Building regulation approval is needed

• When new buildings are constructed
• When an existing building is altered or extended
• When loft space is converted or windows are replaced
• When fittings and services are installed into a building

When you are building a green roof, you might have to fill a Building regulation approval notice application. The council will approve someone to inspect the property.