Guide
Extensive or intensive green roof: choosing the right system for your project
Before you invite tenders for a green roof, you need to choose the system. Extensive and intensive are the two basic types, but commercial flat roofs often also suit stormwater retention roofs or solar green roofs. The right choice depends less on your ideal design than on three practical factors: roof slope, spare structural load capacity and how you plan to use the space.
The four system types at a glance
Extensive green roof
A low-maintenance system planted with sedum, herbs and grasses. The build-up is around 8 to 15 cm deep and weighs roughly 60 to 150 kg/m² when saturated with water. Suitable for roof slopes up to 10 degrees, it is the standard choice for commercial and office roofs where the roof area itself is not intended for people to use.
Intensive green roof
A roof garden with perennials, shrubs or lawns. The build-up starts at around 25 cm, with a weight of roughly 200 kg/m² or more, rising significantly where shrubs or trees are planted. Suitable for roof slopes up to 5 degrees: on steeper roofs, erosion and water runoff become difficult to manage in the deeper growing medium. Typically used for accessible roof terraces and roof gardens where people can spend time.
Stormwater retention roof
An extensive green roof combined with a flow-control device that holds back rainwater and releases it gradually. It weighs more than a conventional extensive green roof, depending on the planned water storage volume. This option is particularly relevant where you need to reduce pressure on the sewer network or meet a stormwater management requirement.
Solar green roof
An extensive green roof combined with photovoltaic panels on raised supports. Vegetation occupies part of the area, while the rest accommodates the solar panels. The two uses work well together. On commercial buildings, this combination is increasingly becoming the norm rather than the exception.
What really decides your choice
Roof slope
An intensive green roof becomes technically challenging from around 5 degrees. Above 10 degrees, an extensive green roof also becomes more demanding and needs additional measures to stop the growing medium from washing or sliding away.
Spare structural load capacity
The available load capacity often decides whether an intensive green roof is possible before you even consider the design. Where an existing building has little capacity to spare, a lightweight extensive system is often the only practical option.
Planned use
If you want people to access and use the space, an intensive green roof is usually the right choice. If the roof stays a purely functional area, an extensive system is generally more economical and needs less maintenance.
Stormwater management
If you need to retain rainwater, whether to meet a local authority requirement or to reduce stormwater fees, a retention roof is a better fit than a simple extensive green roof. In Germany, these fees cover the cost of draining rainwater from a property into the public sewer network.
Your system checklist
- Establish the roof slope before choosing a system.
- Check the spare structural load capacity, or have it assessed, before considering an intensive green roof.
- Decide whether people will need to access and use the roof area.
- Check whether any stormwater retention requirements apply.
- If you are also planning solar panels, design the green roof and the panel layout together rather than commissioning them separately.
Frequently asked questions
Can I combine extensive and intensive areas on the same roof?
Does a retention roof cost more than a conventional extensive green roof?
Which system is most likely to work on an existing building?
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