Green roofs can be a fabulous roofing alternative for a new home, or for an addition to an existing house, a garage or garden building. But these planted roofs are far more than a great way to improve the view.
Green roofs are often planted with sedum, which is ideal for the situation because of its natural characteristics. However, green roofs can be planted with other succulents, cacti, wildflowers and more.
See: Eco home improvements: how to make your house greener
‘Green roofs are the most biophilic roofing system there is,’ says Karl Harrison, Trex decking pro and founder of Karl Harrison Design. ‘They are natural, colorful, biodiverse and contribute to the ecological footprint of any building supporting this type of roof. They attenuate the rain on the roof flowing into the water waste allowing for easier water management, and this is done by absorption. Plants need water to survive and green rooves retain most of the rain that falls on them.
‘The sedum-style plants on a green roof absorb carbon dioxide as well as other airborne particles nearby, and they release oxygen into the atmosphere. They also attract pollinators and are the perfect home for many insects vital for gardens.’
To discover the possibilities for creating a green roof, read on.
What are the benefits of a green roof?
Green roofs have become an increasingly frequent sight on commercial buildings in US cities, echoing the trend in Europe and particularly Germany, which dominates the green roof market.
But roofs planted with dense vegetation are possible for residential properties, too, and bring the same environmental benefits, including reducing the run off of stormwater, cleaning the air, and cooling.
A green roof can absorb up to 70% of the water that falls on it. One of these can also be highly beneficial in hotter regions, keeping down the heat of a home or other building in the warmer months of the year. A green roof will also absorb carbon dioxide, the most abundant of the greenhouse gases and the largest driver of climate change.
If you’re building a home, or adding on to an existing one, choosing a green roof also allows you to replace the ecology the building has taken away with habitats for insects and birds.
‘Green roofs can be situated on most flat-surfaced roofing structures,’ says Karl. ‘In the home you can get green roofs for your garden shed and outbuildings. More homeowners are insisting they are put on their extension roofs.’
What type of green roof systems are there?
A green roof for a home can be an intensive, or thick, type; extensive, or thin; or semi-extensive, that is in between the two.
An intensive system uses a thick layer of soil and allows a variety of plants to be grown. It also offers the best water absorption. One of these needs to be accessible to allow you to tend it and you would need to dedicate time to doing so. It offers good insulation.
An intensive system creates a significant weight which needs adequate support, which should be catered for in the design of a new sustainable home or a new addition to an existing home.
‘An intensive green roof shouldn’t be confused with your normal green roof,’ says Karl. ‘An intensive roof is more like a complete garden on your roof.’
An extensive green roof is often a sedum roof, although it could also be planted with low grasses, mosses and more. The layer of substrate is shallow. Lightweight, one of these is easier to install – as is a semi-extensive green roof – than an intensive system. It won’t provide as much water retention, nor insulation, but it does entail less maintenance.
Extensive green roofs need conventional insulation and rainwater runoff also has to be dealt with.
A semi-intensive system is of slightly greater depth than an extensive green roof and allows more plant diversity. Maintenance needs are relatively low.
See: Rainwater harvesting – how to conserve a precious resource for use at home
What do green roofs need?
For a green roof system to be installed, it’s necessary that the pitch of the roof is no more than 30 degrees.
The structure has different layers. On the surface of the roof is a protection layer. This is because of the impact soil could have on the waterproofing layer. The protection layer is puncture resistant.
Even though a green roof stores rainwater, a drainage layer is also necessary. This could be plastic, but there are natural options as well, including gravel and even shells.
A filter sheet goes above the drainage layer, which prevents soil washing out.
A downspout is also necessary to take excess water away from the roof, but rain chains could be used in the place of conventional guttering.
The perimeter of a green roof generally features an area of shingle, which will need to be kept free of vegetation to allow water to move through it and be taken off the roof.
Finally, there’s the planting that makes the roof a green one.
See: Solar panels – a complete guide
Other than sedum roofs, what plants can be used on a green roof?
A sedum roof can be a great choice. Sedum’s natural characteristics make it a plant that’s eminently suitable for a green roof. It has shallow roots so only needs a thin substrate layer, which makes this a lightweight roof.
Sedum plants, which store water in their leaves, can tolerate drought and are very hardy. Sedum can shut down their gas exchange openings when it’s hot, opening them again at night when it’s cooler and less water will evaporate. Sedums are also easy to care for and resilient when it comes to disease and pests.
An extensive green roof might also be planted with mat-forming species of sempervivum or moss.
‘Specialist companies that supply sedum roofing materials have researched the best planting schemes for just this application,’ says Karl. ‘It’s important to have a mixture of plants in your scheme, and you also have to consider the thickness of the green roof you are creating. The best plants for a green roof with a very thin layer of soil are Cotula hispida, sedums, acaena and sempervivums. Some well drained and thin soil loving plants like thyme work well, too.’
See: The world's most beautiful eco houses – from forest dwellings to city homes
On a semi-extensive roof, think about planting ornamental grasses, muscari, or small species of allium.
If you’re going for an intensive green roof, a wide variety of plants is possible, including shrubs and even trees. Choosing drought-tolerant plants can be a sensible strategy.
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