What are the Structural Requirements for a Green Roof?

As people become increasingly environmentally conscious and seek ways to improve urban biodiversity, reduce energy consumption and increase their property’s aesthetic appeal, the interest in green roofs is on the rise. However, not all roofs are suitable for green roof installations. 

Here, we will explore the structural requirements necessary to support a green roof and discuss how to determine if your existing roof is a good candidate for a green transformation.

Structural loading capacity

The first consideration when planning a green roof is the structural loading capacity of your existing roof. Green roofs can be quite heavy, especially when saturated with water, so it’s crucial to ensure that your roof can support the added weight.

Lightweight sedum green roofs, which consist of low-growing succulent plants, typically require a loading capacity of at least 120 kg per square metre. On the other hand, more complex wildflower roofs with a 150mm depth of substrate need a higher loading capacity of around 250 kg per square metre.

To determine if your roof can support a green roof, it’s essential to consult a structural engineer who can assess your roof’s current capacity and recommend any required modifications.

Waterproofing

Once you have confirmed that your roof can handle the added weight of a green roof, the next consideration is ensuring that your roof is effectively waterproofed. Due to the need for plants to absorb and retain moisture, green roofs require a robust waterproofing system to prevent water ingress, which can cause severe damage to the building structure and interior.

There are several waterproofing options available, including single-ply membranes, liquid-applied membranes, and bitumen systems. The choice of waterproofing material will depend on factors such as your roof’s construction, the substrate type, and compatibility with green roof components. An experienced green roof installer will be able to recommend the best waterproofing solution for your specific needs.

Drainage

Further to the above point, a green roof must strike a balance between water retention and drainage. The roof needs to retain enough water to support the vegetation while also allowing excess water to drain away, preventing damage to both the plants and the building structure.

Water retention layers, such as geotextile fabrics or water retention mats, are designed to hold water close to the plants’ roots, making it readily available during dry periods. These layers also help to filter debris and prevent clogging of the drainage system.

Drainage layers, typically made from lightweight materials such as expanded clay or recycled plastic, ensure that excess water can quickly flow away from the roof surface. Drainage outlets must also be incorporated into the green roof design to prevent water from pooling which can cause structural issues or encourage the growth of harmful mould and algae.

Maintenance and Accessibility

Finally, it’s important to consider the maintenance and accessibility requirements of your green roof. Green roofs require regular upkeep to ensure they remain healthy and visually appealing. This can include typical gardening tasks such as weeding, fertilising, pruning, and monitoring for pests and diseases.

When planning a green roof, you need to be able to easily access all areas of the roof for maintenance purposes. This might involve incorporating walkways or access points into the design. However, this also means you need to account for the additional weight of a person when calculating how much reinforcement your roof needs before installation.

In summary, while green roofs offer numerous benefits, they are not suitable for every building, so it is a good idea to first consult with a qualified green roof installer or structural engineer who can help guide you through the process and advise any necessary structural changes.

By carefully assessing these elements and making any necessary modifications, you can create a beautiful and functional green roof that benefits both your property and the environment.