Gardens

Green roofs – an expert guide to growing a living roof

Green roofs aren't just attractive, they have a whole host of eco benefits, too

Green roofs
(Image credit: Getty Images)

Green roofs can be a fabulous roofing alternative for a new home, creating a verdant view over windows that look over it, a way for a new house to blend seamlessly with the landscape around it, and of course a wildlife haven.

You needn't be building or putting on an addition for this eco home improvement to be part of your plans. A green roof can be added in some cases to an existing house, a garage or backyard building. But these planted roofs are far more than a great way to improve the view. 

‘Green roofs are the most biophilic roofing system there is,’ says Karl Harrison, Trex (opens in new tab) decking pro and founder of Karl Harrison Design (opens in new tab). ‘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 roofs 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 rainwater harvesting and storm water retention, increasing biodiversity, adding insulation, and increasing the lifespan of your roof.

Insulation

'For residential green roofs, one of the core benefits is insulation, both from heat and also from the cold, so you energy bill goes down,' says Riane Hunt of Recover Green Roofs, LLC (opens in new tab), a green roof design and installation company in Somerville, Mass. 'Houses lose a lot of energy through the roof, and they intake a lot of energy on the roof, too. Green roofs retain a lot of water, which reduces the ambient temperature in the air around it.'

Some studies have also found that green roofs can also make solar panels more efficient by keeping the panels cool in the summer months.

Better ecosystems

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. 

 'If you have a roof that can handle a middling amount of weigh and you can add native pollinator plants, you can create an oasis for bees and insects that would otherwise have had to leave the area or travel a long way to find what they need,' says Hunt. 'In cities, bees can have to travel 5 miles to find the plants they need, when in the past they would only have had to travel 30 feet. A green roof adds an oasis for bees, so it’s a huge thing for pollination and bee health.'

Storm water retention and rainwater harvesting

Another big benefit, especially in dense, older cities, is storm water retention. 

'When rain falls, it rushes into the storm drains, and in older cities like Boston, it then filters down into the sewage system. When these pipes fill up, they get dumped into the waterways, so sewage gets into our waterways,' says Hunt. 'The risks include a polluted water supply and polluted soil, and it also decreases the overall health of any eco system the water touches.'

Because green roofs absorb rainwater, they help prevent the overwhelm of city drainage systems. In suburban and rural areas, this translates to less flooding. 

Increased roof lifespan

Finally, a green rood can improve the lifespan of a roof by big margins. 'Generally in the us, a residential roof membrane won't last more than 20 years, but a green roof will last far longer,' says Hunt. 'In Germany, there are some green roofs that have lasted over 100 years.' 

What type of green roof systems are there?

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. 

A green roof for a home can be an intensive, or thick, type; extensive, or thin; or semi-extensive, which is a combination of the two. 

An intensive system uses a layer of soil that's at least 6" deep, and allows a variety of plants to be grown. It also offers the best water absorption. An intensive roof needs to be accessible to allow you to tend to it and you would need to dedicate time to doing so. It also offers good insulation.

An intensive system also 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 Harrison. ‘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 or soil is shallow, which means it works for structures that can't withstand the weight required for an intensive roof. 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 if you're looking for an entry-level way in to creating an eco house

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.

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 shingles, 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.

Once the roof is installed, it will also need some sort of maintenance, but the level will depend on the type of roof you install, i.e. intensive or extensive. 

Because a green roof does not have a replenishing soil source like a garden on the ground, you will periodically need to add more soil to the green roof due to loss from wind or rain water. You'll also need to pull any weeds that make their way onto the roof.

If your roof is inaccessible or makes it hazardous to do these tasks yourself, you'll want to hire a professional to do it for you. 

Other than sedum roofs, what plants can be used on a green roof?

Green roofs

Photo by Katrin Hauf (opens in new tab) on Unsplash (opens in new tab)

(Image credit: Katrin Hauf/Unsplash)

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 other drought-tolerant plant ideas, such as sempervivum or moss.

‘Specialist companies that supply sedum roofing materials have researched the best planting schemes for just this application,’ says Harrison. ‘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.’

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.

What are the pros and cons of a green roof?

Like most home improvements, green roofs come with pros and cons. Among the pros we listed earlier, green roofs can help trap rainwater, provide a layer of insulation, create a habitat for fauna displaced during the building process, and contribute to improved air quality.

Still, there are a few cons. For one, green roofs cost more than standard, shingled roofs. They also require more maintenance. Finally, a green roof can increase the potential of roof leaks. 

Can you have a green roof in any climate?

If you live in a place where the weather gets extreme and are worried about the viability of a green roof, don't be.  

'Climate is not a problem, it’s just a design consideration,' says Hunt. 'In Arizona you design for drought conditions. In Boston, we design roofs that are able to withstand the snow each year.'  

One thing that's important to note, says Hunt, is that, if you live in an area with lots of rain or snow, you need to make sure that your roof can withstand both the weight of the green roof material, and the water it will absorb.

'The thing you really need to consider is if the roof can take 2-3x time the weight from the snow and rainwater. It's the engineering gate that determines whether you can put a green rood on your building, and what type it is,' she says. 

The design company you work with will be able to suggest plants and a roof style that make sense for where you live. you choose your green roofing materials, be sure to choose options that can withstand colder temps. Generally, the sedum varieties used for green roofs are hardy enough to withstand both extreme drought conditions and winter temperatures.

Sarah Warwick
Contributing Editor

Sarah is a freelance journalist and editor. Previously executive editor of Ideal Home, she’s specialized in interiors, property and gardens for over 20 years, and covers interior design, house design, gardens, and cleaning and organizing a home for H&G. She’s written for websites, including Houzz, Channel 4’s flagship website, 4Homes, and Future’s T3; national newspapers, including The Guardian; and magazines including Future’s Country Homes & Interiors, Homebuilding & Renovating, Period Living, and Style at Home, as well as House Beautiful, Good Homes, Grand Designs, Homes & Antiques, LandLove and The English Home among others. It’s no big surprise that she likes to put what she writes about into practice, and is a serial house renovator. 

With contributions from