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What are the first considerations to undertake when designing a Green Roof?

February 26, 2013

When you are in the design stages of your green roof there are a few main things to be aware of to make the build a success.

Outlets and Drainage Points

When designing your roof having at least two outlet points is very important just in-case one of the outlets becomes blocked. We suggest that all of the outlets are covered by a cobbled edge to stop any growth around these points.

The green roof actually enhances the drainage system because it holds up to 87% of the rainwater and slowly releases it back into the drainage system. The Peak Flow changes dependant on the amount of rainfall.

Structural Considerations

The structural loading of a green roof will depend very much on the type of roof you choose. Different plants require different levels of substrate and therefore each individual type ha a different mass per Meter Square.

Extensive roofs are the lightest on the market and intensive planting schemes can be very heavy indeed. It is very easy to allow for these weight loadings in design stage the joists may just need to be slightly bigger.

Plant and Machinery

Many roofs will have machinery and plant on the roof. Some of these will need servicing. When designing your roof take into account a potential walkway and also a clean edge separating the green roof to stop it growing to close to the plant.


Most extensive green roofs will not require an irrigation system. Sometimes on a pitch we encourage irrigation to reduce colour changes across the roof. More intensive roofs and planting schemes will need regular irrigation.

Solar Panels

Solar panels and green roofs are a great partnership. Solar panel work most efficiently at 25 degrees, every degree over 30 degrees the solar panel will lose half a percent of output. A green roof keeps the roof temperature at between 23-28 degrees in summer months.

Biodiversity and BREEAM

When gaining planning permission bio-diversity can be a deciding factor. A green roof can offer different levels of BREEAM points. A brown roof for example gains more points than a Sedum Roof due to the level of species of wildlife it attracts.

Waterproof Membrane

When you design your green roof you should also consider the waterproofing layer. You always want to make sure that you have ‘picked’ the most suitable type of waterproofing membrane. We offer a range of different membranes for different applications.

Management of the Design

We offer a design team where we help you decide on the right choice for your unique project. We feel each client has their own requirements so we like to ‘run through’ different types of green roof to meet their individual needs. We encourage you to use this free service to ensure you’re total satisfaction.

Here is a quick look checklist with the requirements of a green roof New –build and Retrofit.

New Build

Structure design to take the increased loadings of a green roof

Access is safe

Waterproof membrane is suitable for application

The roof is designed to drain freely without any ponding (pools of water).

The plants are picked in accordance to the amount of direct sunlight the roof has.

The substrate is designed to aerate the system.

Firm up-stands are in place to contain the roof


Is the structure strong enough to hold the extra weight of a green roof?

Is the waterproof membrane in good condition?

Is the roof free draining?

Is the roof ponding?

We have experts who are more than happy to assist your enquiries please use us. We are dealing with projects on a daily basis and are the right people to talk to!

I hope this has been an informative read