Blue-green factor

BLUE-green factor


The blue-green coefficient is a tool for checking yard plans. It is used to estimate:

  1. the amount of vegetation and surfaces on a plot/block
  2. the quality of vegetation and surfaces on a plot/block
  3. an indicative estimate of how much the site's vegetation, surfaces, and possible stormwater structures delay stormwater

The blue-green coefficient breaks down the green efficiency of the plot/block, i.e., the ratio of vegetation and ecologically useful surfaces to the built-up area.

Operating principle and use

In practice, the blue-green factor is a three-step spreadsheet:

  1. The plot and planned construction data with target values are entered as initial data.
  2. The amount of vegetation, surfaces and possible stormwater structures are described on the actual calculation tab.
  3. The result is the numerical value of the green coefficient. In addition, the proportion of impervious surface area and the volume of stormwater retention structures are also shown.

Options for vegetation and structures can be re-examined to achieve the goals.

The blue-green coefficient is a checking tool for designers who make yard plans in an urban area. The blue-green factor separates the components of the yard's greenery and sums them up into a numerical value of the green factor, which indicates the green efficiency of the plot. The blue-green factor guides you to work on yard, storm water and HVAC planning together, but it does not replace them. In the services of the city of Turku, the blue-green coefficient is a tool primarily in urban planning and construction supervision, but also in facilities services, for example. Green coefficients are also used in other largest Finnish cities.

Green efficiency in urban planning and building regulations in the city of Turku

The City Council decided on 8.2.2021 § 70 on green efficiency in urban planning. The decision concerns the application of green efficiency and target levels in the new site plans. It defines the site plan markings to which the green efficiency goals are applied, as well as the values of the green coefficient using the blue-green coefficient method:  

  • Areas of housing and downtown activities AK, AR, LPA, AL, C 0,8 In housing, the application starts with terraced houses, regardless of the purpose of use of the site plan (AR or AP block area). Detached small house areas (AO and AP) are excluded from the application.
  • Services and administration areas P, Y 0,7
  • Commercial and office building areas K 0,6          
  • Industrial and warehouse areas T 0,5     

The decision applies to plan proposals to be made available after March 1, 2021.

The target levels of green efficiency in the valid planning areas were decided in connection with the new building order (Section 8a). The city council approved of the building ordinance on 15.2.2021 § 29. The fulfillment of the goal regarding green efficiency (the value of the green factor) must be demonstrated in connection with the building permit application when it is a new construction or a comparable repair or alteration work or a measure that significantly changes the use of the yard or the stormwater system.

Building regulations of the city of Turku 1.3.2021 onwards (in Finnish) (pdf)

Basics for controlling green efficiency

A green and pleasant city and decentralized management of stormwater in the built-up area are goals to be aimed for by controlling green efficiency and the blue-green coefficient method. In addition to general green and street areas, courtyards are important in urban areas: for example, in Turku, approximately 45% of the urban area is in blocks.

For residents, greenery is, above all, a quality factor in the surrounding environment. However, a green yard also has other advantages, especially in adapting to climate change.

  • vegetation balances small and local climates, i.e., heat, windiness, humidity, and air quality
  • green yards support the diversity of urban nature and the connection of green areas
  • large, long-lived trees act as carbon stores
  • along with the decentralized management of stormwater, the quality of discharge water bodies is also nurtured

Stormwater is rainwater and meltwater in built-up areas. Vegetation and permeable surfaces support distributed, delayed management of surface runoff. The management challenge has increased because the water cycle has changed and at the same time the opportunities for delaying stormwater in the terrain are decreasing: the amount of rain, spikes of heavy rain and mild winters have increased with climate change. On the other hand, large cities grow mainly inwards as supplementary construction. The consequence of this is that the unpaved surface that delays stormwater and the vegetation cover decrease. More stormwater runoff is formed because impermeable surfaces and stormwater drains, the so-called gray infra, do not retain, evaporate, absorb, or delay stormwater in the same way as vegetation and permeable surfaces. The risks of stormwater floods have increased.

It's still worth noting that the blue-green factor is a plot- and block-level planning tool, and stormwater is also managed in many other ways: more on the matter on the stormwater website in Finnish.

About the backgrounds and feedback of the blue-green factor

The blue-green factor was worked on in the international iWater project 2016-2018. The City of Turku partially customized its own calculation method, but at its core, the blue-green coefficient is very similar to the green coefficient of the city of Helsinki, which was involved in the project. The principles of the coefficients were already created in the ILKKA project 2012-2014.

The CANEMURE project of climate-resilient urban planning continued the development of a version of the coefficient and its introduction into systematic use, primarily in urban planning and construction supervision in Turku. The project coordinated the preparation of green efficiency decision guidelines, and a new 2.0 version of the multiplier was also worked on in January 2021. User support was offered in 2020-2021. In the fall of 2021, Canemure also experimented with a regional green factor in the City Garden's site plan area. Project designer Ilona Rantola's report on the experiment can be found below in additional information sources.

The production of this page has received funding from the LIFE program of the European Union. The content of this page represents only the views of the Canemure project and EASM/the Commission is not responsible for the possible use of the information contained on the page 



Planning instruction and additional information

Blue-green coefficient table as a Strict Open XML file(xlsx)

An experiment of the regional green factor in the area of the City Park's site plan(pdf)

RT CARDS / Rakennustieto

  • Storm water management, 2015, RT 89-11196
  • Green roofs and roof and deck gardens, 2016:
    • principles, RT 85-11203
    • structures, RT 85-11205
    • vegetation and growing medium, RT 85-11204
  • Storm water structures, 2018, RT 103006
  • Vegetation of storm water structures, 2018, RT 103007

Other instructions and examples in Finnish

International design guidelines for green roofs

  • Gröna tak handboken / Vinnova 2017
    Swedish, comprehensive, and prepared in extensive R&D cooperation. Note the website is running very heavy, so you should download the publications in pdf format to your computer.
  • Green Roof Guidelines 2018 / FLL
    The updated, fundamental, and widely used international design guidelines of a German research institute.

Studies and other background materials

Instructional video for blue-green factor

Blue-green factor’s quick reference. The duration of the video is about 3½ min. The video is silent.