It seems like a strange thing to want to do doesn’t it? Growing plants on your roof. But our ancestors did it centuries ago and they didn’t have the time or the resources to do something for nothing. So why should you grow plants on your roof? Here are just five out of many reasons for green roofing.
These viking houses are built using locally sourced materials – and that includes the turf for the roofs. The forerunners to our modern day green roofs kept the inside of the building warm. I have seen pictures of them being grazed by animals too – but that’s probably not practical on a modern day garden office!
A major benefit of growing plants on the roof – or as we call it – green roofing – is insulation. To grow plants, you need a layer of growing medium. Believe it or not, the substrate used to grow green roof plants is an excellent insulator.
Substrate (we can’t call it soil because it’s very different from topsoil), is a remarkable substance. It’s made up of millions of tiny mineral particles and a small proportion of organic material. Between the particles are air spaces. Some are isolated, some are interlinked to form channels. All that trapped air forms a protective barrier between the atmosphere and the building beneath it. That means it protects the surface of the building from temperature change.
On top of the substrate layer is a vegetative layer – the plants. Provided they keep their foliage in winter, they also trap air close to the roof and help to insulate the building.
In winter, Green roofing works on the same principal as a baby’s shawl or a big fluffy duvet. It keeps the building warm.
In summer, green roofing insulates against heat and helps to keep the building cool. But there’s another natural phenomenon that also helps. On hot days, water evaporating from the substrate layer helps to cool the building. I can only compare it to laundry hanging out to dry on a hot day – the wet sheets, clothes etc always feel cold to the touch don’t they? Same thing.
Plants also lose water through their leaves. That’s called evapotranspiration and it’s why it’s more comfortable so sit on a natural lawn than on a tarmac path.
Green roofs and general ecology
It’s hard to miss all the talk about how our butterflies, birds and bees are in declining in numbers. One of the reasons for that is habitat loss. It’s easy to imagine that in the city there is a lack of wildlife habitat, but there are similar problems in the countryside too. (Actually, some cities are working hard to balance biodiversity so don’t be too hard on towns)
Where I live, in the Norfolk countryside, a beautiful meadow is about to disappear beneath 49 houses. Many of the gardens have been hard-landscaped to allow for extra parking and villagers want all of the road verges etc to be a close-mown monoculture. In the fields, each crop consists of one type of plant and most crops are either wind pollinated or self-pollinating – in other words no use to bees. Some, but not many, farmers grow wild flower margins but wild flowers (aka weeds) are generally discouraged.
Sorry – I’ve digressed. Pretty much, wherever you live in the UK, wildlife habitats are fewer and of poorer quality than they were 100 years ago. Green roofing can address this issue by re-introducing plants but putting them somewhere where they don’t interfere with parking, infrastructure or food production.
Easing drainage issues
Have you noticed how long it takes for water to drain away from some roads and paths these days? Elderly drainage systems are struggling to cope with more runoff than they were designed to take. With more hard surfaces these days, water cannot simply soak into the ground the way nature intended. It’s diverted into drainage systems and its overwhelming some of them resulting in flooding.
But what if some of that water got soaked up before it reached the ground? Yup. You guessed it. Green roofs can absorb rainwater and store some of the excess until it’s needed by the plants on the roof. A well-constructed green roof can also slow down runoff, even if the substrate itself is saturated. Thus relieving pressure on the drains.
This is probably one of the least obvious benefits of a green roof. It’s certainly not one that is shouted about very often. A layer of substrate and plants on top of a building can make for a more peaceful environment indoors. So if perhaps, you work in a garden office not far from a busy road, or worse, under a flight path, you can considerably cut down the noise just by installing a living green roof. I seem to remember it’s something like 7dB reduction for a sedum roof with a 5cm substrate level – but don’t quote me on that figure.
Growing plants on your roof looks good and it’s great PR!
Let’s face it. Looking great is the reason we do lots of things in our lives. Eating healthily, exercising, interior design, garden design, choosing a car. It’s all about personal branding.
Your home and your business premises say lots about your personal brand. They tell people how much you care about the environment, how happy you are to adopt new technologies and how forward-thinking you are.
Not every green roof is visible from the ground, but most of them can be seen from a neighbour’s upstairs window or even from your own windows.
If access is poor, you can always display photographs of your green roof so folks know it’s there.
For me, a green roof is a very selfish thing. I just like looking at it. It makes me feel good.