With so much recent talk of biodiversity loss and climate change, green roofs have forged a reputation as one of many strikingly simple ways to push back. But like all the best wheezes, green roofs are in fact nothing new. Traditional turf roofs were commonplace in many Nordic countries and the go-to roofing choice of Vikings!
Today they have taken on a new importance, transforming angular, jarring buildings into structures of beauty. Green roofs bring colour and movement from birds, butterflies and bees. They help to filter out air pollutants and they slow rainwater runoff during downpours, reducing flood risk.
On a garden scale they can make a stunning feature. And, with a little forethought, can present yet another opportunity to grow more food. So if you’ve an outbuilding, shed or wood store with a bare roof, why not give it a makeover?
Planning a Green Roof
Green roofs are made up of several layers, typically a waterproof/root barrier layer to protect the roof, a drainage layer, water retention mat, then the growing medium and vegetation. When saturated these layers can weigh in at more than 100kg per square metre, or 20lbs per square foot. This substantial weight is likely to be too much for your typical garden shed, so consider whether extra support is needed. This may necessitate building a rooftop frame, supported by sturdy corner posts to bear the additional load.
Roofs with a pitch of more than 20 degrees are best divided up into multiple boxes or compartments to stop the growing medium from simply slipping off or sloshing away in wet weather. Slopes greater than 30 degrees are generally unsuitable for green roofs.
The simplest, most affordable and lowest-maintenance green roofs comprise a carpet of sedums. These incredibly tough succulents are both drought and deluge resistant and their blooms are much-loved by butterflies and bees. A step up in size and cost from sedums is wildflowers, especially hardier, lower-growing natives, alpine plants and bulbs.
Roofs that are easy to access can be suitable for growing hardy herbs such as thyme, prostrate rosemary or chives. You can grow shallow-rooted crops such as salad leaves and strawberries on an edible green roof too, though these are perhaps best saved for the most accessible areas of roof.
Green Roof Construction
Green roofs of creeping plants and herbs need some sort of frame around the perimeter to hold everything in place. This can be made out of timber, while off-the-shelf installations are typically metal.
Think of a green roof as a lasagne of ingredients. First down is the waterproof/root barrier layer, which will protect the surface of the roof from water damage and leaks. On a small scale, butyl pond liner or builder’s plastic is a good option. Use a single piece of material if you can, or be sure to thoroughly seal any overlapping edges. You can attach this waterproofing layer directly onto the roof, but make sure to secure it in such a way that doesn’t puncture as this will of course compromise its effectiveness! Alternatively, wrap a sheet of plywood with the waterproofing material then secure this onto the roof.
For larger roofs you’ll next require a layer of drainage. This is usually in the form of a purpose-sold plastic honeycomb-style material. For smaller green roofs like those shown in the photos you needn’t bother with this layer. Next up is a filter sheet to hold the growing medium in place. If you’re using a drainage layer it will also stop the finer particles from clogging up the drainage layer.
Now comes the growing medium, which for a sedum roof should be at least 8cm (3in) deep and for a roof of wildflowers, herbs, salad and strawberries at least 15cm (6in) deep. A multipurpose soil-less, peat-free potting mix is fine for growing herbs and other edibles, but mix in plenty of perlite to both improve drainage and save on weight. For sedums and wildflowers you actually want a very poor growing medium that is low in nutrients. Expanded clay granules and potting mix in equal parts is a good combination to achieve this.
Planting a Green Roof
Finally it’s time for the plants. Ready-to-roll carpets of sedum are easiest but you should also be able to find plug trays of sedums and other plants sold specifically for green roofs, which will work out cheaper. Taking cuttings or sowing seed to grow on and transplant will of course be cheaper still.
Avoid using just one species of plant across the entire roof. A mix of species helps to make the community that emerges more resilient – and more interesting.
Ensure plants establish properly by watering for the first year after planting. Fleshy leaved herbs and salads may need watering often in hot weather. Be aware that plants towards the apex of the roof will dry out quicker than those further down. Bare patches in mature green roofs will need re-planting as necessary.
Setting up a green roof is well worth the initial investment of time and money. Once filled out it’s a thing of real beauty – a special place away from the busy comings and goings of down below, somewhere for wildlife to hang out, and a verdant backdrop that’s a pleasure to admire.