Green roof systems – a guide to installing a sedum roof at home

When it comes to creating a green roof, size doesn't matter. You can brighten up a bird house as a garden idea or add an entire layer to the top of your home; the choice is yours.

If you do the latter, you'll add more insulation to your home (more cooling in the summer, than heat in the winter) and increase the biodiversity of your garden. Not only will it look lovely, a green roof will also absorb carbon dioxide, heat from the sun, a large proportion of rain water and even noise.

Living Roofs explains more, 'The combination of soil, plants and trapped layers of air within green roof systems can act as a sound insulation barrier. Sound waves are absorbed, reflected or deflected. The growing medium tends to block lower sound frequencies whilst the plants block higher frequencies.'

Critically though, they also encourages biodiversity, says Living Roofs. 'Green roofs can provide important refuges for wildlife in urban areas. Research in Switzerland and the UK has demonstrated that green roofs can provide important refuges for rare invertebrate populations.'

The benefits of a green roof are indefinite, from creating natural areas for wildlife to thrive, to reducing your carbon footprint and adding visual beauty to an area.

So if you fancy getting green-fingered and sprucing up your outdoor space, we'll take you through everything you need to know.

What is a green roof?

Also known as a ‘living roof’, a green roof is a roof of a building that is partially or completely covered with vegetation, which is most often sedum.

They have become a popular way to add another dimension of green space to a garden, particularly in urban gardening and new-builds that aim to blend with the surrounding environment.

What are benefits of a green roof?

Living roofs offer so many advantages, including increasing a roof’s lifespan by protecting it from the elements, combating heat loss and saving on energy, improving the local air quality, creating water attenuation, minimising maintenance, reducing sound transfer through buildings and providing an aesthetic appeal.

1. Green roofs last three times longer

Tom Mckinna, designer at Resi says, 'Living roofs have shown to almost triple the life expectancy of the roofing below. This is thanks to the underlying structure being protected from mechanical damage, ultraviolet radiation, and extreme temperatures. You’ll also find those extra layers can help reduce noise pollution, as well as provide a higher level of fire resistance!'

2. Minimal maintenance

‘Extensive' roofs require minimal upkeep and have plants that need little watering. Usually a sedum mat is placed underneath the vegetation; these are wind, frost and drought resistant which makes them an ideal foundation for your plants. Extensive roofs are easier and cheaper to install than ‘intensive' roofs.

3. Provide insulation

Green roofs can also add a layer of insulation to your home, giving you extra warmth in the winter months. They provide sound insulation too, due to the combination of trapped air within the soil and plants absorbing the frequencies.

4. Eco-friendly benefits of a green roof

Just like a living wall idea, the ecological benefits of roof gardens are endless. They provide additional oxygen and contribute to the reduction of the `UHI' or Urban Heat Island effect, reduce solar radiation and help to decrease air pollution.

'As well as benefitting your home, a green roof really comes into its own in terms of environmental impact. First, the added greenery helps remove dangerous CO2 from the atmosphere. It’s been estimated that a 1m2 green roof can take up to 5kg of emissions each year, which is the same amount as a regular car will emit during an 80km drive,' explains Tom Mckinna, designer at Resi.

'Alongside this, you’ll provide a natural habitat for local wildlife, especially if your choose flowering plants - a favourite of the struggling bee. And with your roof helping to filter rainwater, you’ll be giving the critters in the garden below better water quality too.'

5. Green roofs add beauty

Lee Evans, Founder of Organic Roofs promotes the aesthetic benefits of a green roof.

'Nothing expresses your principles quite like a green roof. Instead of an expanse of mineral felt, the view is one of native wildflowers, grasses and sedum and the bees, butterflies and birds that will make it home. Designing-in a green roof will transform the feel of your home and garden.'

What are the disadvantages of a green roof?

One of the only real disadvantages is the cost of the installation. A green roof will be more expensive to install than a traditional flat roof. Mainly because the underlying roof structure may need strengthening to cope with the extra load.

Things to consider

Before you plan your green roof, make sure you talk to a specialist. There are guidelines set out by The GRO Green Roof Code which will help you to establish which kind of planting scheme suits your roof and your lifestyle.

As many roof structures haven’t been designed to bear a heavy load, it’s important to ask a structural engineer to design your green roof so load capacity is taken into consideration. You’ll also need to consider waterproofing, a root barrier, protection mat, drainage layer, filter sheet and planting.

Tom Mckinna, designer at Resi explains, 'Green roofs, also known as living roofs, fall into two basic categories: turf and sedum roofs. In order to add turf to your home, you’ll need to add at least 15 cm of soil to your roof.'

'This adds a lot of extra weight when compared to its sedum counterpart. Because of this added load, special considerations will need to be made for the weight - meaning you’ll need to consult your structural engineer during the building regulations stage.'

Sedum roofs, on the other hand, are so light they can usually be built on most properties. Rather than using grass, sedum roofs are made from succulents - a much more robust plant, which can survive easily in a drought.”

‘Intensive' roofs have deeper soil levels and generally require more attention; plants on an intensive roof garden will need frequent watering and looking after, though they offer more flexibility for a wider range of plant life. Make sure your green roof can be structurally supported under the added weight; it may need reinforcing.

We recommend taking a look at Living Roofs’ DIY guide to green roofs, £12.75.

What are the three types of green roofs?

Before you begin, you need to determine what kind of green roof you want, there are three main types:

• Extensive green roof: generally a shallow layer of stress-tolerant grasses, mosses and sedum; these require little maintenance and are nearly self-sufficient
• Semi-Extensive green roof: slightly deeper, but similar to extensive green roofs; these also require low maintenance
• Intensive green roof: a thick layer of soil with a variety of grasses, herbs, flowers and shrubs; these require a lot of care.

Choosing plants for your green roof

Have a think about the purpose of your living roof. It's a great wildlife garden idea, if you want to provide a safe haven for bees, insects and other wildlife, then plant flowering species. Or if you’re concerned about the surround eco-systems and habitats, consider native plantings instead.

Gregg Setherton CSSW, Technical Manager at Permagard says, 'You can plant a variety of plants on a green roof. Grasses, mosses and sedum are common for extensive green roofs. Certain types of wildflowers are a nice option for attracting wildlife.'

'Sedum green roofs are one the most popular options because this alpine plant can tolerate our varying weather conditions. You can also easily and cheaply pick up pre-grown sedum mats ready to install - providing low growing, instant greenery to whatever sized roof. You can also get other suitable pre-grown mats suitable for green roofs.'

Which plants can you use for a green roof?

The Royal Horticultural Society (RHS) has detailed more specific information on suitable plants for green roofs:

• Suitable plants for extensive green roofs: mat-forming species of Sedum, Sempervivum and moss. Ferns such as Polypodium vulgare and Asplenium trichomanes are suitable for dry shady conditions.

Lee Evans, founder of Organic Roofs adds, 'Succulents which grow in around 60mm of substrate (growing medium). Usually supplied in pre-grown rolls but also available in plugs, cuttings. Sedum roofs are good nectar sources during their flowering season but this tends to be quite short. Can be augmented with wildflowers.'

• Suitable plants for semi-extensive green roofs: dry habitat perennials and ornamental grasses such as:
  • Rudbeckia
  • Achillea
  • Potentilla
  • Armeria
  • Dianthus
  • Helictotrichon sempervirens
  • Stipa tenuissima

'Native UK wildflowers often make great green roof plants and are particularly good for providing food sources for invertebrates for longer periods of the year,' explains Lee.

'Flowering from March till November, these roofs require deeper substrate, which is undulated to encourage diversity. Usually tolerant of shade and longer dry periods, these plants will brown off in extended drought conditions but will revive again when the rains return.'

• Suitable plants for intensive green roofs: The RHS recommends drought tolerant plants that are both sun and wind tolerant.

How expensive is a green roof?

If you don't fancy installing the green roof yourself, the cost of the green roof installation will depend on the contractor. It's worth getting a few different quotes to compare prices before deciding on one. And do ask lots of questions around the design and how long it will take.

'Green roof costs can vary dramatically based on the type, size and construction method. As an approximate guide, you can have a fully functioning 20m2 extensive wildflower green roof for a total cost of between £600 - £1400,' says Gregg Setherton CSSW, Technical Manager at Permagard.

Do you need planning permission for a green roof?

Green roofs on sheds and other separate outbuildings are usually fine without planning permission. However, if the building is occupied or is attached to one that is, you should contact your council’s planning department to make doubly sure you're on the right side of the law.

The Renewable Energy Hub explains, 'In general, a replacement roof or retrofit does not need planning permission before it can be carried out and installed.'

'Having said that it is always a good idea to check with your local council before you go ahead and carry out any major external renovation, like a small house extension idea, particularly if you live in a conservation area or own a listed building. It might also be polite to run your plans by your neighbours if you are on good terms with them.'

'Incorporating a green roof into the plans for any new build can nowadays ease the approval process as it is seen as desirable aspect for many councils.'

What drainage does a green roof need?

Drainage is super important to incorporate into your green roof installation. The reason is because you need to be able to effectively drain away large volumes of rainwater, while still providing sufficient watering so the plants on the roof thrive.

Andrew Mckenzie of Bauder explains, 'The soft landscaping on a green roof will retain a large percentage of the average annual rainfall, as for example an intensive green roof can retain up to 90%.'

'However, the UK’s National Annex to BS EN 12056 does not permit the use of a coefficient to factor down the drainage infrastructure to take account for factors such as the retention performance of green roofs.'

Gregg Setherton adds, 'The green roof drainage layer is usually a HDPE membrane. These can feature cavities or cups that collect water. Water is retained in these little reservoirs until it is required when it is drawn back up by the vegetation.'

'Any excess water such as from heavy rain, passes through perforations in the HDPE membrane. The water then travels through to a drainage space below where it can be safely evacuated via the drainage outlet.'

'This prevents too much water accumulating and potentially damaging the roots and causing harm to your green roof.'