Garden Advice

Green roofs: Types, installation and plants

If you’re wondering “what is a green roof?”, it’s a green roof system that, as the name implies, is ultimately “green”. It means that you build a supporting structure onto your roof which contains vegetation. It’s one of the ways to create a sustainable home, so much so that London has introduced regulations requiring buildings of a certain size and function to have green roofs. Although there are some criticisms of these living roofs or eco green roofs, there are numerous benefits to installing one in your home. It’s not only aesthetically pleasing, but they also absorb heat and carbon dioxide. They also absorb water from rain, frost and humidity, provide some insulation to the building underneath it and finally, they change the ecology of the building itself, making it an eco-roof.

Different types of green roof systems

Green roof construction is not as straightforward as it might appear at first glance as the different types of green roofs constructed will depend on the supporting structure underneath them. This will also depend on the weight of the actual living roof systems, whether they need a supporting structure such as a metal or wooden frame as well as whether the supporting structure can withstand the weight. Living roof construction comes in three main types. These are intensive, extensive and semi-extensive. 

  • Intensive green roof: The soil thickness of this type of green roof is between 50 mm and 200 mm+. It enables you to grow a variety of grasses, herbs, flowers and shrubs on it. Although they provide a valuable habitat for wildlife, they place significant weight on the building and need considerable roof support. Another feature of this type of green roof is that they offer insulation and better water absorption than the thinner types. However, they do require more care and maintenance than extensive green roofs, as one of the main differences between intensive and extensive green roofs. 
  • Extensive green roof: The general soil thickness of extensive green roofs is usually more shallow than the intensive type with about 20 mm  to 100 mm of substrate planted. You can grow low-growing, stress-tolerant grasses, mosses and sedum, although the latter adds sedum roof weight to the structure. Relatively little maintenance is required and the insulation quality is relatively low.
  • Semi-extensive: The soil thickness of these semi-extensive green roofs is between 100mm and 200 mm). They can allow for a greater diversity of plants and are relatively low-maintenance..

How to make your DIY green roof

Ready to find out how to build a green roof? Here are the steps.

  1. Check your structure for safety

    A typical DIY green roof will weigh between 60 and 150 kg per square metre and this weight will only increase when there’s snow or rain added to the mix. This is why you need to check that your building is sturdy and waterproof, and that it can also take the weight of your green roof. For the structural loading capacity of your structure, if uncertain, consult a structural engineer or architect before embarking on this project.

  2. Determine the angle of roof

    Green roofs can be created on flat or angled surfaces and in the best case scenario, a green roof will go on a slope between 10° and 30°. However, when proceeding with how to make your own green roof and the slope is over 20°, it’s a good idea to create a frame to prevent the structure from slipping.

  3. Add a layer of waterproofing

    Adding a layer of waterproofing is the next step in the process of how to make a green roof. Your best option is to use material like heavy-duty rubber pond liner to prevent water or roots from getting through.

  4. Add a protective fleece liner

    In an effort to protect the pond liner, add a plant protection fleece.

  5. Building the frame

    The frame that you will place on top of your structure should be made of either rot-proof wood or lightweight metal. Ensure that the depth of the frame is the same as that of the depth of the soil. Create your frame easily by nailing the corners together or by using L-shaped brackets at the corners. This frame will not only hold the soil but will also ensure that rainwater can drain out, preventing water logging. To ensure effective green roof drainage, add holes to the side of the frame which are closest to the ground. 

  6. Add pea gravel for flat roof structures

    If your roof structure is flat (or up to 5°), add a thin layer of pea gravel over the plant protection fleece. This will ensure water can drain without blockages.

  7. Place the soil on top

    For your DIY sedum roof, use lightweight soil to minimise the weight that will be applied on the structure. In addition, consider using extensive lightweight green roof substrate designed for sedum or wild flowers. These usually require between 70-200 mm depth of substrate to root. This will help you determine how to make a sedum roof. 

  8. Time to plant

    You have two options when it comes to planting – either purchase pre-vegetated mats or plant directly. Sedum plants, herbs, wildflowers and grasses are the best option as they are more resilient and can still attract wildlife. 

Choosing the right green roof plants

The location of your green roof creates an interesting dilemma when it comes to selecting your plants. This is because the plants need to be able to withstand wind and frost, be drought-resistant, tolerate living in poor soil and be relatively maintenance-free. What’s more is that you’d like your sedum roof construction to be attractive in addition to attracting wildlife. With that being said, there are some plants well-suited for your grass roof construction. We take a look at each one in turn.

Gold sedum – Sedum Kamtschaticum

Sedum roofs wouldn’t be complete without Sedum kamstschtiucum, a honeybee’s friend. Your green roof construction will greatly benefit from this addition as it provides excellent ground cover, is hardy and easy to maintain and it grows well in a shallow layer of soil. This reduces the amount of pressure placed on the roof structure.

White stonecrop – Sedum album

Your green roof plants need the addition of a Sedum album. When grouped together, they produce an effect of candyfloss with star-shaped white flowers during summer. They are hardy and easy-to-grow, creating a mass of mat-like stems and leaves. Sedum album thrives in thin, dry soil. It also grows low, providing excellent cover, insulating a roof and offering food and shelter for wildlife.

Widow’s cross – Sedum pulchellum

The flower head of the Sedum pulchellum is made up of many small pink star-shaped flowers. This is a drought-resistant plant which is wildlife-friendly and adds an intensity of colour to your sedum green roof during spring and summer. 

Meadow saxifrage – Saxifraga granulata

The white flowers of Meadow saxifrage appear early in the year, generally from April to June. The plant has green leaves which contrast with the white of the flowers. They can grow up to 50 cm in height and are a wonderful way of providing more volume to a wildflower roof.

Two row stonecrop – Sedum spurium

The Sedum spurium blooms from late summer into autumn and thrives in well-drained, poor soil. It offers a mass of green succulent leaves and produces clusters of star-shaped flowers in vivid pink and red throughout summer.

Birdsfoot trefoil

Another friend of the bees and butterflies is Birdsfoot trefoil. This is a low-lying plant that is part of the pea family and produces a mass of yellow flowers in summer. The flowers then develop into seed pods.

Houseleeks

Houseleeks is another type of living roof plants and while it is usually found living in pots indoors, this plant is hardy, evergreen and offers a mass of rosettes and spiral foliage bearing attractive flowers in summer.

Yarrow

Yarrow is easy to maintain and smells sweet, producing clumps of white or pink flowers. It is also attractive to wildlife and because it has medicinal properties, it can be eaten. Consider getting a 100 mm growing medium to support your yarrow as it can increase in weight. It may also require watering during dryer seasons.

Sea thrift

Sea thrift is usually found in coastal areas and favours dry, sandy soils. It is a grassy plant that produces long stems of pink or white flowers in summer.

Oregano

Oregano thrives in well-drained soil and needs little maintenance. It also provides good ground cover and produces pollen-rich flowers that attract insects.

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Takeaways

  • Check that your structure can support the extra weight
  • If your roof is slanted at around 30 degrees, make sure you build a frame for it so that it doesn’t fall off
  • Line your roof with the right materials including pea gravel, fleece, and waterproofing
  • Choose the right plants to create your green roof
  • And that’s it! You’re ready to enjoy the results of your labour!

Did you find this article useful? Let us know in the comments if you have any other questions!

Image source: shutterstock/ Ellyy

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