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Can biochar be used to improve green roofs

TonyC 18/03/2023

Can biochar be used to improve green roofs

Can biochar be used to improve green roofs?

The quick answer is a huge yes. 

Let's look in a bit more depth at how biochar is used in green roofs

What is a green roof?

A green roof (also known as a 'living roof) is a roof on a building that is covered with plants growing in a growing medium. This usually sits on a waterproofing membrane and may include other layers such as a root barrier, drainage, and irrigation pipework.

Why add biochar to a green roof growing medium?

Biochar delivers several benefits

  • It fundamentally improves plant growth (see our biochar tests results)
  • It retains water - preventing 'thin' living roof layers from drying out as quickly, but also helps prevent waterlogging
  • It has a low bulk density - 250Kg/m3 dry, compare with compost 500 Kg/m3 or soil 1700 Kg/m3. The extra weight of a living roof is a substantial design challenge

SoilFixer's 0-8mm Biochar weighs just 250 Kg/m3 dry, 400 Kg/m3 as supplied, and 700 Kg/m3 saturated. Biochar can be sourced in fine granules (0-2mm), medium (0-8mm), and coarse (2-8mm).  We recommend no bigger than 10 mm as larger particles tend to have a detrimental impact on properties like 'aggregation' which makes the soil fall apart too easily.

How much biochar is added to green roof soil / growing media?

  • For base layers, use 100%, ie just biochar
  • For the growing media, we recommend 10-30%

More than 30% will help with weight, but it tends to tip the balance away from strong plant growth

Green roof types and construction

The experts describe two types of green roofs

  • Intensive roofs, which are thicker and require more maintenance
  • Extensive ones, which are lighter, have less vegetation and require less maintenance.

They come in many forms from slim-line to very thick. 

(Rooftop designers have precise maths and engineering formulas for loading and safety - the above is a rough approximation to give gardeners an appreciation of the challenge)

The addition of biochar to green roof soil / growing media is only a few years old -  as more roofs are built and tested, the advice will become more precise.

If you have a shed, garden office or outbuilding, or even wheelie bin storage, you can create your own DIY living roof by adding a water-proof membrane and a barrier to keep everything in place and plant your own.

Best plants to use for green roofs

Shallow-rooted varieties are the best options for planting living roofs such as various species of sedum, succulents, wildflowers, and grasses. Be mindful of the positioning of the roof and how much or how little exposure to the sun it gets, choose plants depending on the conditions.

Benefits of living roofs

Creates a habitat and food for wildlife 

Contributes to improving air quality

Provides insulating and soundproofing properties

Improves drainage

Adds a touch of nature to an otherwise unnatural surface

How is biochar added to the green roof soil/medium?

Usually, this is added at the start, but it can be added when the growing medium is being 'repaired/topped up (see degradation below)

Sometimes the biochar is added as a granular base layer and other times it is integrated into the growing medium (see SF40)

Biochar offers multiple benefits but some of these benefits have to be balanced with other challenges. The route chosen depends on the specific challenges the roof installer is looking to address (see below).

What are some of the other criteria a green roof designer might consider when utilising biochar?

  • Weight and density of the growing media layer, safe structural loading
  • Hydrological  - desired drainage, stormwater runoff, water holding capacity
  • Nutrients – how much, amount, slow-release, nutrient runoff
  • Minimise long-term degradation – eg media items such as peat, coir, and compost decompose over 1-5 years 
  • Creating a natural habitat or “urban wilderness”
  • Filter pollutants and CO2 from the air
  • If a company building, contributing to Corporate CSRs goals
  • Enhancing building aesthetics
  • Sustainable wastewater management
  • Outdoor water use reduction
  • Design for Enhanced Resilience
  • Local food production
  • Green power and carbon offsets
  • Rainwater management
  • In high urban densities, reduction of the city’s average temperature during the summer.

From Wiki….

Green roofs improve and reduce energy consumption.[10] They can reduce heating by adding mass and thermal resistance value also can reduce the heat island by increasing evapotranspiration.[11] A 2005 study by Brad Bass of the University of Toronto showed that green roofs can also reduce heat loss and energy consumption in winter conditions.[12] A modeling study found that adding green roofs to 50 percent of the available surfaces in downtown Toronto would cool the entire city by 0.1 to 0.8 °C (0.2 to 1.4 °F).[13]

A green roof reduces cooling (by evaporative cooling) loads on a building by fifty to ninety percent,[14] especially if it is glassed-in so as to act as a terrarium and passive solar heat reservoir.

There is a mass of DIY information on making green roofs. If you are looking at a whole building and want to gain all the thermal benefits, we suggest you approach a green roof structural design expert who will work with your architect. 

Are there any experts in green roof tech? Yes 

Google 'Green roof design and installation'. Although we have not worked in partnership with any particular installer, we did like the information presented on  GreenRooftech  website and Green roof training.

Green roofs with biochar: carbon offsetting & positive climate impact

We predict a huge uptake in biochar use for green roofs. All the above benefits are great - but biochar has a trump card - it is now widely recognized as the leading carbon-offsetting technology. Carbon Credits (see PuroEarth, Verra) use a certified methodology that demonstrates biochar-carbon is locked away for at least 100 years and hence offsets CO2 emissions. This marketplace is now growing at +50% year on year.  

Not only will biochar be added to agricultural soils and horticultural growing media it will be a number one choice for corporate buildings with green roofs.

The benefits of adding biochar to green roofs also apply to " Living Walls"

Living Walls are popular where space is limited or the building has a restricted floor space for soil/growing media. Adding biochar to living walls can help improve plant growth and health by providing a range of benefits, including improved water retention, higher nutrient availability, and better root development. 

Image: British_Horse_Society_Head_Quarters_and_Green_Roof.jpg. Please note: Our use of the open-source image is not meant to imply any association with or use of SoilFixer Products. It is just a really nice green roof!  Image attribution:  Sky Garden Ltd, CC BY-SA 4.0 <>, via Wikimedia Commons

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