Complete guide to green roofs

Extensive green roofs

Extensive green roofs have a growing medium depth of 5-15cm, that includes lightweight materials such as perlite, leca, sand, rockwool and crushed tiles or concrete. Planting choices include ready-grown sedum mats, which are shallow-rooted and don't usually need watering or weeding.

Semi-extensive green roofs

Semi-extensive green roofs have a growing medium depth of 10-20cm, and can support deeper-rooted perennial plants, but not shrubs or trees. Suitable waterproofing, root barrier, drainage and filter layers are necessary. Planting choices can be varied, but all perennials will need watering until they become established, and weeding is usually necessary, too.

Intensive green roofs

Intensive green roofs have a growing medium depth of at least 30cm, much of which needs to be organic matter. These are usually designed as raised beds that sit over roofs and other constructions, such as bin stores. They are heavier than extensive and semi-extensive green roofs, so need to be sturdy to take the weight of the compost and plants. An irrigation system may be necessary to keep plants hydrated.

What is a green roof system?

A green roof system involves the following:

• A waterproofing layer to protect the roof
• A root-resistant membrane to prevent plant roots from penetrating the roof
• A water draining system to prevent the roof from becoming waterlogged
• A filter layer to stop the growing medium entering the other layers
• A lightweight growing medium that doesn't become heavy when wet
• Suitable plants, both for the growing medium but also the weight, structure and location of the roof

How do green roofs work?

Green roofs work to improve air quality, provide wildlife habitats and prevent flooding by holding on to water. With the correct green roof system, the roof will hold on to enough water without becoming waterlogged and heavy, support diverse plant life and wildlife, and improve air quality.

Advantages of a green roof

Green roofs can replace the loss of green space in urban areas. They help to prevent flooding by slowing the flow of water into the sewerage system, they help to clean the air and lower urban temperatures, they provide habitats for wildlife and improve the local environment. In gardens, they look good and can help to blur hard boundaries or hide unsightly structures, improving the look of the space.

Disadvantages of a green roof

If done well, there are no disadvantages to a green roof. However, if the correct drainage and water-proofing have not been used, a green roof can lead to water leakage, and the roof itself can collapse with the weight of the growing medium if it's too heavy.

Are green roofs for wildlife?

Green roofs can be made with a focus for wildlife. This can include using a variety of plants that benefit pollinators and adding bee hotels or sand banks for solitary bees, as well as growing plants that produce seed, for birds. However, there are many additional benefits to adding a green roof, including reducing urban temperatures, reducing flood risk and improving the look of a garden.

Green roof ideas

Put a green roof on your shed or garage roof, bin or bike store, or other structure in your garden. If you're having a flat-roof extension, consider a green roof to replace the loss of green space in the garden. Many garden offices or other outbuildings can be designed with a green roof, which can enhance their look, prevent flooding and help them to blend into the space better than they would otherwise.

Planning and designing your green roof

When planning your green roof, think about whether you want an extensive, semi-extensive or intensive design. Intensive green roofs are best for bin stores and small sheds, while semi-extensive is best for a flat roof, garage or garden office. Make sure the structure you're planning the green roof for is reinforced to take the extra weight of the green roof – a garden shed, for example, may need additional reinforcements in the corners, especially if you're choosing an intensive design. Think about the plants you want to grow and the depth and type of growing medium required to grow them.

Choice of plants for your green roof

You can buy bespoke green roof matting for extensive and semi-extensive designs – these include sedum matting and wildflower turf, both of which are low maintenance and lightweight. Mosses and low-growing grasses are also a good option, as they thrive in shallow soil and require very little maintenance.

If you're planning to grow deeper-rooted perennials, choose those that require little maintenance, and which will thrive in a shallow, nutrient-poor growing medium and a drought-tolerant environment.

Maintaining your green roof

The most well-designed green roofs require little maintenance, but you may need to replace dead plants or remove weeds every now and again. It's also important to check that drainage systems don't become clogged up, and that any waterproofing membrane does not become damaged.

Frequently asked questions

Can you turn any roof into a green roof?

Most roofs can be adapted to become green roofs, as long as reinforcements are made to cope with the extra weight, and sufficient waterproofing, drainage and filter systems have been put in place. For buildings and large surfaces, it's best to seek advice from a professional, who can advise you on what is needed to transform your roof to a green roof.

How long will a green roof last?

Green roofs can last forever if they are installed correctly. Some intensive designs, which are essentially raised beds sitting on top of a structure such as a shed, may be subject to rooting if the structure is made from wood. Otherwise drainage systems, waterproof layers, plants and growing medium will all last indefinitely, if they are maintained regularly to ensure they don't degrade.