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Getting the specification right

by Matt Downs

Nick Day, Managing Director of green roof specialists Optigreen, discusses the importance of selecting the right products when implementing green roof systems.

The benefits of green roofs are now widely recognised, and they are becoming a familiar inclusion on building developments, with the requirements of specifiers when it comes to roof greening increasingly varied. 

Nick Day, MD at Optigreen.

As with other parts of the building fabric it is important for the green roof specification to be correct to ensure long-term performance. Whether the goal is to increase biodiversity, manage rainfall or improve air quality, it’s important to select the right products and systems and to remember that there’s no “one size fits all” when it comes to green roofs. This article examines some areas of the green roof build-up where correct product selection is vitally important so that subsequent performance issues can be avoided.

Illustration shows green roof build-up.

System approach

At Optigrun we have nearly 50 years’ worth of experience developing green roof products and systems, and regularly advise specifiers and contractors regarding specification and roof design. We have found that the system approach to product choice works best using a combination of layers that have been developed and tested to work together in the same way that the waterproofing and insulation system beneath the green roof will have been. The green roof system chosen needs to reflect the type of vegetation that will be required, the level of water storage required and the type of use that the roof will receive. Within each green roof system there are key product areas that need to be considered.

Optigrun FKD 25 drainage board – for use on standard sedum roof system build up.

Drainage layers

Every vegetated green roof needs a reservoir function to store water for irrigation purposes during extended dry periods, but also good drainage. Poor drainage provision can risk waterlogging of the substrate layer with consequent root damage and potential failure of the vegetation. A good green roof system will provide this at different layers within the roof make up, one of which is within the drainage layers.

The drainage layers on a vegetated green roof will typically consist of a protection fleece – unless the roof is of an inverted design – drainage and reservoir board and filter layer. With regard to the drainage and reservoir board, this will typically be manufactured from a recycled plastic, and it’s important that it has good reservoir function on its upper face with individual storage cells or chambers with overflow provision so that excess water can drain away below the board. 

The underside of the board should facilitate good drainage so that water cannot back up through the board and risk waterlogging of the substrate layer above. For example, at Optigrun we have different drainage and reservoir boards within our systems, ranging from 20mm to 60mm in depth, with different performance characteristics so that the correct product can be chosen for each project. The filter fleece that’s laid above it should  have good mechanical filtration properties but also allow a sufficient rate of water flow. The current Green Roof Organisation GRO Green Roof Code (2021) sets out minimum performance characteristics for this layer.

Pitched roofs

Pitched green roofs are being specified in ever greater numbers and these present their own set of requirements when it comes to product selection. The general rule here is that it’s best not to improvise with the specification, but instead to use a system designed for the purpose. The “system approach”, mentioned previously, definitely applies where pitched green roofs are concerned.

Generally, a pitched green roof is considered to be one that has a pitch in excess of 5 degrees. Beyond this figure the green roof specification will need to be completely different as the physics of the roof change. For example, transferred shear load and slippage risk have to be considered and the method of accommodating this will vary on each roof depending on, for example, pitch, slope length, waterproofing membrane being used and roof design – i.e., mono or duo pitch, perimeter detailing etc. The method of providing water storage and drainage will also change. I have surveyed several failed pitched green roofs in the past where an improvised anti-slip arrangement has been used, or in one case none at all, creating costly mistakes which could have been avoided if the correct system had been selected originally

For pitched green roofs , therefore, a range of solutions is needed, and an engineered solution is required to provide a bespoke specification suited to the individual project. 

Optigrun substrates are specifically engineered to provide the correct organic content needed for each green roof system.


For the substrate or growing medium for a green roof, topsoil or recovered site spoil are not suitable products. Instead, a blended and engineered soil replacement should be used with a specified organic content. In this way the performance characteristics of the material, such as weight, water storage capacity, organic content and porosity are known. The GRO Green Roof Code sets out performance characteristics  and also recommends that substrates selected should have been tested according to BS 8616:2019 Specification for performance parameters and test methods for green roof substrates.

Technical support

Finally, it may seem obvious but make use of the technical services that your green roof supplier provides. They should be able to provide a bespoke specification for your project which should include technical data for the products included. If you are a contractor tendering for a green roof, this is particularly important as the specification that came with the enquiry may have been cut and pasted from a previous project and be entirely unsuitable for the project in hand. There is also useful independent advice available from GRO (The Green Roof Organisation) who are the body that represents the green roof industry in the UK. The latest edition of their Green Roof Code, introduced in 2021, and mentioned in this article, is full of useful information and design guidance for green roofs. You can download a free copy at www.greenrooforganisation.org


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