Green roofing design and specification

green roofing


More and more people are seeing the environmental benefits of intensive and extensive green roofing as providing environmental benefits and bringing a level of biodiversity to urban areas. In this article we discuss the two main types of green roofing, the overall advantages they offer and how to combine green roofing with solar panels.

Advantages of green roofing:

  • Green roofing takes pressure off draining systems in urban areas, as it reduces the need for rainwater drainage as plants absorb excess water that would otherwise require draining.
  • Green roofing attracts insects, bees, butterflies and other pollinators, creating precious ‘nature reserves’ on city roofs, which brings a level of biodiversity to urban areas.
  • Green roofing reduces the amount of heat gain in the building fabric. A building without green roofing absorbs heat, while a green-roofed building makes for a more comfortable environment, simply because there is less heat gain.

Intensive versus extensive green roofs

Although green roofs come in many shapes and sizes, a general distinction can be made between intensive and extensive green roofs. Extensive green roofs are lightweight and by far the most common type of green roof, combining up to 150 millimetres of substrate with shallow rooted plants, like sedums, mosses and grasses. Intensive green roofs can feature any plant that requires depth, such as trees and shrubs, thanks to their substrate being more than 150 millimetres deep.

Designing green roofs

1.     Layers

Extensive and intensive green roofs have quite the similar build-up. With extensive green roofs, reducing weight is obviously key, so this variant calls for some specific modifications that are not necessary for intensive green roofing. Both types of roofs consist of the following layers:

  • The first layer is made up of the roofing system
  • On top of that is a root inhibitor or root barrier
  • The next layer contains a drainage or reservoir system
  • A growth medium, like lightweight soil, is added on top of the reservoir system. SM-50 synthetic substrate panels are often used on extensive green roofs, where only minimal roof loads are possible. These special mineral wool panels weigh three times less than soil.
  • The final layer is where the sedum blanket or green roof mix is laid.

2.     Types of plants

When selecting the plants for your green roof, speak with an expert to ensure the species chosen are suitable for the area you are in.

3.     Perimeter detailing and perimeter ballast

On standard roofs wind tends to create suction and in the worst-case scenario, the roof rolls back and potentially blows away. A green roof is an open-celled system, which means the wind simply passes through. When installing a green roof special special attention is taken with perimeter detailing and perimeter ballast to keep the sedum in place.

4.     Sloped green roof

Finally, when designing a green roof, taking the slope into account is key. The standard recommendation is; If the roof has a slope of more than 15 degrees, a specialist design is crucial to keep the sedum material in place. However, at Soprema we recommend a specialist design even with a slope of 5 degrees or over.

More and more ZEBs in the EU

The Energy Performance of Buildings Directive, which came into force on 9 July 2018, requires all new buildings in the EU to be as close to zero-energy as possible by the end of 2020. For commercial projects the Directive calls for a large quantity of solar panels leaving no space for a green roof to be installed, Since some local authorities specify that a certain percentage of any roof must be green, it makes complying with both the local regulations and the EPBD difficult.

Soprema take an approach with these regulations involved the use of brown roofs underneath solar panels. A brown roof has a green roof build-up, yet has no sedum to help meet fire requirements.