What is a Green Roof?

Green roofs are systems that use vegetation to cover the surface of a flat roof. They are an addition to the existing flat roofing membrane and provide aesthetic as well as insulation and protective benefits. Research is also showing that surrounding ourselves with greenery will reduce our stress levels.

What is sedum? 

Sedum is a vegetative plant founding mountainous regions around the world. You might find it in Snowdonia in North Wales or the Mendip Hills, or even on a flat roof in Bristol. 

It is incredibly durable and robust; it survives the cold, icy conditions found in mountains regions as much as the hot, dry spells found during summer. 

green sedum roofing layer in Bristol                            close up of a green sedum flat roof in Bristol

Who installs sedum green roofs?

Sedum green roofs can be installed by trained professionals. Green roofing services that operate throughout Bristol, Bath, Chippenham and the wider South West are available from Strandek Flat Roofing.

Benefits of green roofing

There are many benefits of green roofing – some of these are outlined in the video below!

How easy is it to grow sedum?

Sedum based green roofs are almost always pre-grown at specialist sites. The sedum itself is grown on a recycled support substrate which provides a robust platform.

This means that the sedum can be transported to the desired location and readily installed.

Maintenance is generally simple, requiring annual or bi-annual weed removal and adjustment. 

In hotter climates, more watering would is usually required during summer, but for the most part, sedum installed in areas around Bristol and other areas is expected to be quite robust. 

Because green roofs come with 8-12 different types of sedum, they are resistant to disease.

Sedum Roofing Systems

In general, a green roofing system of the extensive sort comprises:

  • Protection fleece and a root-resistant barrier
  • Drainage structure or reservoir layer
  • Filter fleece
  • Growing medium or crushed aggregate
  • 8 – 12 different species of sedum vegetation

How much carbon dioxide can a green roof remove from the atmosphere?

The amount of carbon dioxide that green sedum roof can remove from the atmosphere depends on multiple factors:

  • Pollution
  • Oxygen and carbon dioxide levels
  • Temperature
  • Rainfall
  • Maintenance 
  • Sedum species
  • Sun exposure
  • Length of time since planting

In general, the amount of carbon dioxide removed by a green sedum-based roof can vary considerably, according to the measurement method used and the aforementioned factors.

It depends on the type of vegetation, the atmospheric conditions, the type of green roof (a thin extensive one or a thicker “intensive” one).

A variety of studies have attempted to quantify carbon dioxide removal. The precise method used depends on the experimental method and in our opinion could vary considerably.

Causes of pollution in Bristol

  • Solid fuel-burning appliances like wood burners
  • Vehicle exhaust fumes (particularly diesel)

A range of harmful pollutants exists in the air of an urban environment like Bristol. The most common types are ozone (O3), sulfur dioxide (SO2), nitrogen dioxide (NO2), as well as particulate matter (PM2.5 and 10) according to the World Health Organisation (WHO), which also provides guidelines on air quality.

Bristol itself is England’s sixth and the UK’s eighth most populous city. Recent estimates place it to exceed annual objectives for nitrogen dioxide according to a recent report by the council. In 2017, an investigation into nitrogen dioxide emissions from vehicles found that the majority (96 %) came from vehicles using diesel fuels, of which cars, buses and light goods vehicles accounted for 40, 23 and 22 % respectively.

Levels of Air Pollution in Bristol

The legal limit for air pollution in 2018 was 40 micrograms per cubic meter (ug/m3).  In 2018, an estimated 45 locations in Bristol exceeded this limit based on pollution measurements throughout the city, ranging from 41.14 to 65.17 ug/m3 based on intermittent measurements.

In 2019, continuous pollution monitors were used in selected areas. These captured around 55 % continuously. Table 1 displays this data based on Bristol City’s Air Quality dashboard.

Table 1. Summary Data for Continuous Pollution Monitoring Sites in Bristol in 2019 (ug/m3).

Site  Mean NOx Mean NO2 Mean NO Count Data Capture %
Colston Avenue 214 67 96 4,861 55
Bristol Temple Way 83 38 29 4,782 55
Fishponds Road 79 42 24 4,522 52
Bristol St. Pauls 38 23 10 4,817 55
Wells Road 79 31 31 4,782 55
Parson Street 75 34 27 4,867 56
Brislington Depot 56 29 18 4,400 50

How does Bristol plan to improve its air quality?

  • Reducing the number of vehicles on the road: West of England Partnership. (2011). West of England Joint Local Transport Plan 3 2011-2026
  • Development of a Clean Air Plan: Joint Air Quality Unit
  • Promotion of electric vehicles via GoUltraLowWest
  • Adoption of renewable technologies like solar and eco-friendly roofing systems

How can green roofs remove air pollutants?

Pollutants can be removed from the air by green roofing vegetations via a range of different mechanisms, from absorption to deposition and entrapment by the sedum leaves.

What are the major pollutants in Bristol?

Nitrogen dioxide

Several studies have reported on the potential for green roofs to remove nitrogen dioxide from the atmosphere by 0.6 – 2.55 g/m2 or roof covering. Another study demonstrated that 1675 kg of air pollutants could be removed by a total of 19.8 ha of green roof annually, with nitrogen dioxide accounting for 27% of this estimate.

Sulphur dioxide

Sulphur dioxide is present in Bristol city centre at relatively high levels.

Ozone

Ozone accounted for 52% of total atmospheric pollutants removed from a 19.8 ha of Chicago’s green roofs.

Particulates

The amount of particulate material removed by a green roof has been removed to range from 0.42 to 3.21 g/m2 from 500,000 square meters of green roofing. For 2,000 square metes of grass that had been allowed to grow on a green roof, 4,000 kg of particulates were estimated to have been removed.

PM10

Several claims have been made regarding PM10 and green roofs. The amount of PM10 that can be removed from a green roof varies according to publication. However, annual removal can be summarised as:

PM2.5

Estimating how much pollution Bristol can remove from its atmosphere

Using NO2 (nitrogen dioxide) as an example, it is possible to extrapolate and visualise how pollution can be removed or offset through the introduction of a sedum-based green roofing system.

Aims

Based on data obtained throughout 2018, Bristol has 45 sites that are believed to exceed the 40 ug/m3 nitrogen dioxide (NO2) legal limits. Green roofs have been shown to remove nitrogen dioxide from the air via a series of natural processes. Strandek® wanted to provide an estimate as to the cost of removing or offsetting these emissions via the installation of a sedum green roof, both based on completely offsetting NO2 or reducing it to the 40 ug/m3 threshold.

Assumptions

  • The cost of a green roof ranges from £60 to £100 per square meter. This figure is an estimate and includes the cost of workmanship and materials. The cost is likely to decline as the size of the project increases.
  • Only extensive green roofs are used
  • NO2 removal levels are based on a range supplied in published literature.
  • NO2 levels will decline with height as the pollutant disperses

What can Bristol learn from other cities and countries?

The U.K. does, however, remain a laggard relative to other countries like Germany, that have adopted a more progressive to green eco roofing. 

Germany, for instance, is the leader in green roofing technology. Estimates place houses to have over 10 % of green roofs installed.

How can Bristol encourage more green roofing installations?

Green roofs are more accessible when government incentives are added. From 2017 onwards, Washington DC installed 4 million square feet of green roofing as of 2018.

As more property owners install green roofs, the overall cost per square meter of green roof will also fall.

Whilst the growing process for a green roof takes several months, which adds to the cost, a cheaper option is to offer ‘brown roofs’. These contain sedum-based seeds that are designed to be grown on site. These systems mean that the extra capacity to support sedum-based growth can be eliminated.

How much would it cost to increase Bristol’s air quality using green roofs?

Firstly, it is worth calculating the available surface area for green roofing in Bristol.

This can be roughly estimated by using the

Option 1: Install green roofs on all of Bristol’s available roofs

Various cities around the world have evaluated the use of green roofs to offset emissions. In Chicago (2008), coverage of  19.8 hectares with green roofing systems would remove 1675 kg per year. This would increase to 2046.89 metric tonnes if all roofs were covered with an intensive green roofing system. However, the cost for coverage of all available space would be $35.2 billion (as of 2008), which would equate to £12 – 16 per square foot, but this price could drop if plants became more available to £7 – 12 per square foot.

However, some of this cost can be reduced when considering that each diesel vehicle causes significant health effects.

Option 2: Install a sufficient number of green roofs to reduce emissions to a safe level

The second option would be to target high-emission areas. This would simply add enough green sedum roofing substrate to offset existing emissions and return the air quality to a normal level.

Alternatives to Green Roofs

Other options to green eco-roofs exist. These centre on moss-based adsorbent vegetation that is equally, if not better at removing pollutants.

Whilst sedum is effective at removing pollutants, moss is considered an excellent vegetative plant

Indeed, it has been commercialised and released as an inner-city bio-filter designed to improve air quality. The CityTree, pioneered by Green City Solutions, was invented in Germany in 2014.

Accessories for Green Sedum Roofing

In addition to your green roof, you could also choose from a range of solar panels, skylights and insulation to make your property brighter, have better insulation and have the capacity to generate its own electricity.