A 2019 report by the European Federation of Green Roof and Walls Associations has shown that the amount of green roof space in our capital has grown exponentially over the course of ten years.[1] To make sure that London remains a pacesetter, the London Plan contains an Urban Greening Factor that requires any new major development to include an element of greening. Yet green roofs are not only spreading across London’s skyline, but across the whole of Britain, and with good reason. Green roof benefits such as the reduction of the urban heat island effect, increased biodiversity, better water management, increased thermal performance and sound insulation are well known. In this article, we list some of the key elements to consider when designing a green roof.
The Saturated Weight Of A Green Roof
Especially in refurbishment projects, ensuring that the structure can take the weight imposed by the green roof is key. Yet weight loading is an important design consideration for new builds as well, where a structural engineer has to account for the necessary calculations. According to the GRO Green Roof Code, you must factor in not only the weight of rain and snow (which saturate the substrate), but also “further imposed service loads” like “pedestrian access loads and point loads from features such as water features and large trees.”[2]
Large trees are a characteristic of intensive green roofs, which typically have a dead load starting from 282 kg/m2, while the overall weight of an extensive or sedum-based green roof system is far less heavy at approximately 30 kg/m2. The difference is not only explained by the type of vegetation involved, but also by the substrate depth required by the plants.
Waterproofing: System, Upstand Height And Falls
While we would not recommend using single-ply membranes beneath green roofs, there is still a wide range of SOPREMA waterproofing options that are sufficiently root resistant, such as reinforced bitumen systems, liquid systems and structural waterproofing. When it comes to root-resistant capsheets for multi-layer waterproofing systems beneath green roof applications, SOPRALENE FLAM GARDEN 250 AF is one of the most robust options on the market.
When designing your waterproofing system, you should anticipate the increased height of the construction. After all, the Building Regulations state that the waterproofing detailing should finish 150 millimetres above the roof surface or, in this case, the green roof surface (not the waterproofing layer!). This evidently influences the design of the upstands and other roof details. Also, be sure to incorporate falls as per the BS 6229- 2018 guidance. To further ensure adequate drainage in a refurbishment project, the existing pipes should be inspected and cleaned as specified. In new builds, the drainage pipes should obviously be adequate as well.
Taking Care Of Vegetation
Finally, green roofs require careful planning, as the sedum mats should be installed immediately upon their arrival on site. If they stay rolled up, the plants will be damaged. Moreover, both extensive and intensive green roofs should be thoroughly watered immediately after installation and throughout the first few weeks – especially in summer. Depending on the height of the building, this can be quite a challenge: a decent water supply with sufficient pressure needs to be provided.
Assistance With Your Green Roof Design
Our experts can help you with advice and calculations for your green roof. Don’t hesitate to get in touch.
[1] Source: https://livingroofs.org/wp-content/uploads/2019/04/LONDON-LIVING-ROOFS-WALLS-REPORT-2019.pdf
[2] Source: https://livingroofs.org/wp-content/uploads/2016/03/grocode2014.pdf