A green roof can transform an unused flat roof into a space that supports biodiversity, helps manage rainwater and improves the overall appearance and performance of a property.
However, not every flat roof is ready for a green roof straight away. Before installation, there are a few important checks that need to be made around the structure, waterproofing, drainage and ongoing access.
At Western Flat Roofing, we install green roof systems across Cornwall and the South West, including on existing flat roofs as well as new-build projects. In this guide, we explain what we look for when assessing whether a roof is suitable and what may need to be addressed before installation can begin.
Can you install a green roof on an existing flat roof?
In many cases, yes. A large number of the green roof projects we work on involve existing flat roofs rather than roofs specifically designed for planting from the outset.
The key is making sure the existing structure can support the additional load and that the waterproofing and drainage are suitable for the green roof system being considered.
This is why we recommend starting with a roof survey rather than going straight to a quote. Identifying any structural, waterproofing or drainage issues early means we can recommend the most appropriate solution before any planting or substrate is installed.
What checks are needed before installing a green roof?
A proper suitability assessment should cover four main areas: structure, waterproofing, drainage and pitch, and ongoing access. This way, your installer can be confident that your roof is suitable.
Roof Structure
A green roof adds significant weight to a roof once you include the growing medium, drainage layer, vegetation, and water retained after rainfall, making structure one of the first things we check.
The amount of extra load depends on the type of green roof. Extensive green roofs use a shallow substrate and lightweight planting, so they add relatively little weight and can often be fitted to existing structures without reinforcement.
Intensive green roofs use a much deeper growing medium and can support shrubs, small trees or a usable garden, but they’re considerably heavier and usually need a structural engineer to confirm the existing deck and supporting structure can take the load (or that reinforcement is possible).
For most existing flat roofs, an extensive green roof is the most practical option, as it’s lighter, lower maintenance and usually requires less structural work. Intensive green roofs offer greater planting and design flexibility but are heavier and better suited to roofs designed or reinforced for the additional load.
Waterproofing
A green roof sits directly on top of your roof’s waterproofing membrane, so that membrane needs to be in good condition and ideally root-resistant before anything is installed on top of it.
This matters more with a green roof than with most other roof finishes, because once planting and growing medium are in place, replacing a failed membrane underneath becomes a far bigger, more disruptive job.
If your existing waterproofing is towards the end of its life, it usually makes more sense to renew it as part of the green roof installation rather than build on top of it and store up a problem for later.
Drainage
Flat roofs are rarely completely flat. Often, they’re built with a slight fall to move water towards outlets and gutters, and a green roof has to work with that fall rather than against it.
Drainage layers within the green roof system need to allow excess water to escape freely, and outlets need to stay clear of growing medium and root growth. Most green roof systems are designed to work within a specific pitch range, and while this varies by manufacturer and system type, roofs with a steeper pitch may need a different build-up, such as additional battens or a reinforced retention layer, to stop the growing medium slipping over time.
Access and maintenance requirements
Even a low-maintenance extensive green roof needs some ongoing attention, so access is worth thinking about before installation rather than after. This includes safe, permanent access to the roof, appropriate edge protection, and clear routes to reach drainage outlets for inspection.
Once installed, most extensive green roofs need checking around twice a year to clear any unwanted growth, keep drainage clear, and confirm the substrate is settling evenly. Intensive green roofs, being closer to a conventional garden, need more regular upkeep in line with the planting scheme chosen.
Do you need professional green roof installers?
Given how closely a green roof’s success depends on structural capacity, waterproofing condition, and drainage detailing all working together correctly, this isn’t a project suited to a general contractor or a DIY approach. Get any one of those elements wrong and the risk isn’t just a poor-looking roof, it’s a leak that’s expensive and disruptive to trace and fix once planting is established.
As approved, experienced green roof installers, we survey the structure, waterproofing and drainage before we ever quote for a green roof, and we’ve delivered green roof and waterproofing projects on properties including Three Mile Beach.
You can see more of our completed work in our commercial case studies. You can find out more about the green roof systems we install on our systems & suppliers page.
Book a green roof suitability survey
As local green roof installers covering Cornwall and the South West, and now Lincolnshire, we’ll assess your roof’s structure, waterproofing, drainage and access.
Get in touch to book your free green roof suitability survey, and we’ll talk you through your options.
