Living roofs - Why are green roofs a key part of urban greening?

Soil and vegetation are known to be effective at improving air quality – it’s nature’s way. In areas of more greenery, the air quality is better.

Applying this principle to our buildings, taking soil and vegetation onto the roofs of our buildings, means we can reap the same benefits, in spaces that often need the most help. With roofs mostly being wasted spaces in our cities, they present the perfect opportunity to incorporate greening.

But how can living roofs improve air quality?
Green roofs can actively work to reduce air pollution through the plants photosynthesising: absorbing carbon dioxide and releasing oxygen. Soil plays a key role in improving air quality too, as it acts as a sink of air pollutants and supports plant growth, which plays an important role in regulating air quality.

It’s important to bear in mind that for a green roof to improve the air quality, the health of the soil and plants needs to be maintained.

With the prevalence of concrete, steel and glass in cities, nature is confined to small pockets of green space. Vast areas of roof space are left bare.

By adding biodiverse green roofs to our cities, we can massively increase the habitats available to birds, bees, butterflies and insects.
Specific soil and plant mixes can be used to bring local plants back to the environment and other features such as log piles, recycled aggregates, rocks, bird baths, insect hotels, rope coils and beehives can be included to create additional micro-habitats and to boost the positive ecological impact of the green roof.

In order to create a successful green roof for improved biodiversity, you need to focus on getting the design right: using native species, pollinator planting, sources of food (nectar, seed and fodder), natural soil and other materials will significantly improve the biodiverse benefits of your roof.

As soil and plants retain moisture, they have a cooling effect and through the process of evapotranspiration, they remove heat from the air, reducing temperatures of the roof surface and the surrounding area. They also provide effective insulation, keeping the building cooler in the summer and warmer in the winter, thus reducing the buildings energy requirements.

In fact, tests have shown temperature differences of up to 17C between hard and vegetated surfaces in the same location. A green roof with approximately 100mm of soil equals a 15% reduction of heating required in winter, and a 25% reduction of cooling needs in summer.

On a summer day, the temperature of a gravel roof can increase by as much as 25°C (77°F), to 60 - 80°C (140 - 176°F). Covered with a green roof, the temperature of that roof will stay at 25°C (77°F), saving energy costs.

Green roofs are a type of natural flood management. In urban areas roofs make up large areas of hard standing and runoff can be huge. The plants on a green roof use up a small amount of water, however the substrate (soil) beneath them will soak up a significant amount of water. The saturated soil will slowly release water off the roof, but the green roof slows the process down reducing the risk of a flash flood.

Living roofs alone would not prevent flooding, however as part of a wider rainwater management scheme they can be a useful contribution.

Combining a living roof with a blue roof concept increases the ability of a green roof to store rainwater and slow any stormwater runoff. A blue roof means an additional layer or layers in a green roof build-up which provides storage for rainwater. This can be used to mitigate runoff impacts or provide storage for re-use (for example, irrigating other green infrastructure on-site).

Did you know that plants are naturally gifted with acoustic benefits? Compared to hard surfaces where sound waves bounce off, plants break up the sound waves and deaden the sound.
Soil, leaves, branches, and woodier stems absorb sound effectively, and the plant leaves refract noise and eliminate echoes as the sound waves do not bounce off.

Whilst the growing medium tends to block lower sound frequencies, the plants tend to block higher frequencies. The effectiveness of a green roof for sound insulation is dependent on the depth of the substrate.

Bear in mind that the deeper the substrate (soil), the better the sound insulation. A green roof with a 120mm substrate layer can reduce sound by 40dB (decibels) and one of 200mm by 46-50dB.

Most flat roof waterproof membranes will come with a guarantee for a number of years, however this is limited due to the membrane breaking down from the sun’s ultraviolet (UV) rays, acidic rain, puddling and freeze thaw weathering where water that sits in puddles on the roof freezes and expands breaking down the roof membrane.

By installing a green roof these weaknesses are mitigated: the plants absorb the sun’s rays and use them to grow rather than letting them break down the waterproofing beneath. By using an extensive green roof (like the GrufeKit), you can increase the lifespan of a roof from 30 years (typical single ply roof) to 40-50 years and more in some cases.

Green roofs present a great opportunity for introducing other systems, work on other benefits or providing opportunities for social interaction.

Biosolar is one good example, where you can integrate solar panels onto a green roof. Usually installed upon extensive green roofs, where plants do not grow too high to obstruct the sunlight from the panels, research has also shown that the energy performance of solar panels increases by 3.3 - 5.3% when combined with a green roof, as the transpiring vegetation helps keep the solar PVs (photovoltaics) at an optimum temperature.

Green roofs also hold potential for urban farming, bringing in the local community to nurture, grow and harvest produce, using the soil on a green roof.

The science of biophilia tells us that we’re drawn to nature and feel better around it. When we need to de-stress or relax, many of us turn to nature.

Being wasted spaces in urban areas, where natural spaces tend to be scarce, roofs are the perfect opportunity to re-introduce nature and provide spaces that building occupants, or the local community, can use.

For recreational use, intensive green roofs tend to be the most suitable, with a deeper substrate allowing for huge scope in design and the ability to create a space that has a garden-like feel. Seating, pathways and water features can be integrated to improve accessibility, make it easier for the people to interact with nature and create a welcoming green space.

Green roofs present an opportunity to design towards building standards such as LEED and BREEAM, or meet environmental regulations like Biodiversity Net Gain (BNG) or the Urban Greening Factor (UGF).

It’s all down to design, as at this stage you can focus on designing your green roof for well-being standards, for improving your biodiversity net gain or for a specific goal, such as improving energy performance.