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Green-Roofing: How and Why

Updated on April 27, 2017

The rooftops of buildings and houses are spaces that usually go unused. Green-roofing the whole terrace is a new trend. Here, all or part of the roof is converted into a conventional garden by covering its surface with a layer of soil and planting, just as you would a back yard. Living roofs (or eco-roofs as they are also called) not only look beautiful, but offer many advantages. In nations like France, Switzerland, and Canada, laws have been enacted for commercial and residential buildings to have at least a partly green rooftop. There are companies sprouting up all over the world to help you make the transition.

Below, we'll first take a look at the benefits of living roofs, then discuss installation.

Landscape planners will have the opportunity to make sculptured roofscapes, so that cities appear to be verdant hills and valleys. Streets will become shady routes carved through the undergrowth. Roofs will become mountain tops. People will become ants.

— English architect and garden historian Tom Turner in his book, City as Landscapes

Psychological Benefits of Green Space

During a break at work, you go to a nearby cafeteria to relax over a cup of coffee. Imagine what you would do if you had a garden on the topmost floor of the building. You would take the elevator up to enjoy some tranquil moments there.

According to studies, contact with nature improves your mental health, level of concentration, and productivity. You will come back to resume work with a sense of happiness. Therefore eco-roofs on office buildings can improve the productivity of employees working inside.

Plants' Effects on Air Quality

The pollution in cities is always increasing. The air is filled with harmful gases, smog, and dust particles. Eco-roofs can play a vital role in mitigating greenhouse emissions. It can also reduce atmospheric deposition. But these benefits are only possible if most buildings in the cities have living roofs. One or two will hardly have an impact.

A Vegetated Roof Is Economical

Greenery will make your roof last longer. Factors such as ultraviolet radiations and severe temperatures adversely affect the lifespan of roofs. The layer of soil and the plants on the rooftop are effective barriers against these and will reduce the production of waste in the long run that otherwise accumulates due to frequent repair and replacement of roofing.

To install one of these roofs, you will need to make certain changes on your rooftop terrace. One of these is waterproofing the roof membrane to disable the water from permeating into it. This will increase the longevity of your roof.

Garden Rooftops Increase Energy Efficiency

Rooftops gardens provide great insulation, retaining heat in winter and keeping temperatures cool in summer. The verdure will keep the floors underneath cool. This translates into lesser use of heating, ventilation, and air conditioning systems.

Rainwater Cachement

Rainwater harvesting has been a very popular way of using the rainwater that would otherwise flows through your rain pipes from the roof and go into the drains. To use the rainwater, it is made to flow through the gutter and downspout into a tank in the ground. But the tank can only capture 5 to 6% of the water that flows down your roof. So why not use it all?

The living roof is a great way to use the rain water. In summer, rooftop gardens can retain up to 80% of rainfall while in winter this can be up to 40%. The garden modifies the temperature of the run-off water as well as filters it. It delays the time of run-off and lessen the pressure on the sewer system. It can prevent local flooding.

Control Urban Heat Island (UHI)

Green roofs abate the effect of urban heat island (UHI). Cities have more surfaces covered with buildings, roads, and other infrastructure than vegetation. Unlike open land, these concrete surfaces are impermeable and dry. Moreover, the presence of too many people, tools, and factories that are constantly burning off energy contributes to heat. They make the urban islands warmer than their rural counterparts. And one way to control heat is to have more and vegetated rooftops.

Added Commercial Value

Green roofs on buildings have commercial value. They can be used as public garden, restaurants, and children’s park. They can be effective to regulate the ever-growing infill projects in urban areas.These rooftops can be used as sites for urban agriculture. This will reduce a community’s urban footprint through the creation of a local food system. Uncommon Ground, a restaurant in Chicago, is growing some of their ingredients on the roof above. Sky Vegetables, as plans to build and operate rooftop greenhouse gardens on supermarket rooftops. Living roofs on educational facilities can be useful to teach students and visitors about biology.

Control Noise Pollution

Loud music, sounds from televisions, people chatting on phones, traffic, and even barking pets are quite common noises in urban areas. These things have their consequences such as causing hearing problems, disruption in sleep, cardiovascular disorders, etc. The combination of soil strata and the plants helps to absorb, reflect, or deflect sound waves. Green roofs can also reduce the level of electromagnetic radiation that is rampant due to mobile phones and other electronic devices.

Foster Wildlife

Green rooftops are great stopovers for migrating birds. Migrating bird populations are on the wane due to stress and lack of food and habitat along the way. Cites with waning greenery are uncongenial for birds.

Take Javits Center, situated in Manhattan’s West Side, as an example. The building is a glass behemoth on the bank of Hudson. The birds flying along the Atlantic flyway collide against the glass windows of the building and meet their death. The building was written off as an avian death-trap, so management installed bird-deterring windows that brought down the death rate by 90%. The terrace of the building has been transformed into a eco-roof. Javits Center now is an avian haven. According to a study conducted in 2014 by New York City Audubon Society and researchers from Fordham University, almost 524 birds were spotted hanging on the roof.

Shrubs and bushes planted in the back yard are often prone to attacks by vermin. Moving these to elevated areas will reduce the invasion of your garden and house by vermin. Living roofs have an aesthetic appeal. Having many of these in a community can increase psychological well-being of people.

Types of Green Roofs

Living roofs can be classified in to three categories: extensive, intensive, and semi-intensive.

The soil layer in extensive roofs is about four inches thick. The plants that are grown are less demanding and small in size such as herbaceous plants, small grasses, and sedums. They need less maintenance.

Intensive roofs are fully landscaped rooftop gardens. The growing medium is between 8 and 12 inches thick. They can have diverse plants and trees. In case of semi-intensive roofs the thickness of soil layer is somewhere between 4 and 8 inches. They are appropriate for growing wildflowers and grasses.

Besides these there are bespoke and sloped eco-roofs. The former is made with a special attention to aesthetic appeal and detailing. These make use of modern technologies, integrated planter bed, green walls etc. Sloped roofs offer greater visibility. They are pitched at an angle ranging from 5 to 15 degrees to the roof surface.

Insulation and Root Barrier

The roof membrane needs to be covered with a sheet of insulation so that it does not get wet with water dripping from the soil above. The insulation is usually made of ethylene propylene diene monomer (EPDM) which is rubbery in texture. Roll out the insulation on the roof and cut to size.

The root barrier would sit just on the insulation. The root barrier prevents rhizomes, stolons, and invasive roots of trees and shrubs from growing into building. It is impermeable, tough, flexible, lightweight, and easy to install. However, check its compatibility with your water-proofing material. It should be resistant to humic acid which is produced when plants decompose. If your waterproof membrane is certified as root-resistant, you may not need a root barrier.

How to Build an Eco-Roof?

You can transform the roofs of your commercial building, house, garden shed, or garage into living roofs. The roof must be strong enough to carry a growing medium and the plants that will grow in it. Remember, a living roof can weigh up to 100 lbs or more per square foot. Consult a structural engineer to ensure that your terrace is strong enough to withstand an eco-roof. Also find out whether there are regulations stipulated by any local council or municipality about making a eco-roof. The roof surface should have slope to allow the water to drain naturally.

An eco-roof is made by assembling six layers. They are:

  • layer of insulation
  • root barrier
  • drainage layer
  • filter sheet
  • growing medium
  • plants.

Drainage

Drainage is required to allow excess water run off to the storm sewer through appropriate outlets and the downspout. There are a few options in drainage systems:

  • The first is a drainage mat. This is easiest to install and would make your eco-roof assembly thin and light. The mat is unrolled on the root barrier fabric and the openings are cut for roof drains However, they have limited water storage and drainage capacity and are suitable for sloped roofs.
  • The other option is granular drainage. In this a network of slotted plastic pipes assembled over the root barrier layer. These are connected to roof drain access boxes. A granular layer made of heat-expanded rock, gravel, and other particles is laid on the pipes. It is lightweight and has high water storage capacity. Granular drainage offers more root space and allows for better water and air distribution compared to drainage mats.
  • Installing drainage plates is also an option where you place a waffled plastic sheet on the root barrier layer. The sheet has waffles on the upper side which retains water. Excess water spills over the sides of the drainage plate and is drained away.

Filter Sheet

This layer is situated just between the substrate layer and the drainage system and prevents the soil layer from dropping into the drainage system. The filter sheet helps to retain small particles, humus, and organic materials in the soil instead of allowing them to flow away with water. It also prevents the drainage system from getting clogged with silt. Filter layers are usually made of woven or non-woven materials. Non-woven materials are preferable as they are resistant to penetration of roots. Filters sheet are not weatherproof and can get damaged due to long exposure to sunlight. Therefore unroll it immediately before making the soil bed.

Sections of  a green roof with waffled drainage.
Sections of a green roof with waffled drainage. | Source

Growing Medium

Now it is time to put in the soil layer. You can make a lightweight soil by mixing together expanded shale and slate with an organic soil. Pre-blended lightweight soil can also be purchased from landscaper shops. Such a mixture soaks in enough water, drains well, and would not become sticky and clayey. Once you are done, it is time to start planting. You can buy plants from nurseries. Choose plants tolerant to strong sunshine and wind and that need minimal irrigation. You can plant succulents, grasses, wild flowers, aromatic herbs. Planting is best done during autumn and winter so that the plants establish themselves before summer. Water them the first few months to help them settle on your rooftop, unless it is raining.

The soil layer should be quite deep for trees to protect them from blowdown. Trees need to be planted in pits that are sufficiently deep and wide for the roots to grow. The anchorage or bracing that is used to protect against wind throw should be checked from time to time to ensure they are not interfering with tree roots.

Other Ways of Growing Green Roofs

There are two other ways of growing eco-roofs as well.

  • You can place trays on the roof space. These trays are filled with soil and 95% covered with vegetation. They are wrapped in plastic and shipped to your house on trucks. They are moved to the roof with a forklift. These are placed in rows to cover the rooftop. However, you should cover the roof with a layer of insulation before you place these trays.
  • The other way is to go for vegetated mats. These mats are sedum sods that have 80% plant coverage. They are rolled out on the soil laid on your rooftop. They need a thin layer of soil, since most of the soil comes with the mat.

Summary

The green roof technology evolved in Germany in 1970s. It is engineered to survive in various climatic conditions around the world. It is a solution tailored for modern homeowners who live in a concrete jungle with marginal greenery. Eco-roof has made the old notion - ‘garden stops where the home begins’ obsolete indeed!

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