I have acquired a green roof purely by accident, which isn’t the recommended way to go about it. The roof of a little shed in my garden – it houses the bins, by my garage - had started to leak, so rather than shelling out for a new bit of roofing felt, I used a bit of corrugated plastic roofing that had been left over from some other garden project, and simply lay this on top of the old roof and fashioned a frame to keep it in place. As extra insurance against this lightweight layer blowing away, I decided to fill the gullies with gravel from my pathways. That was lovely, and it looked far nicer with the gravel roof than it had before so I was pleased. However it didn’t stop there.  Nature does, indeed, abhor a vacuum and it took only a very short while – a matter of weeks -  for the gravel to be colonised by a variety of grasses and wildflowers.  After only 6 months it looks as if it has been designed with just this in mind. I think this year I will - deliberately this time -  introduce a few native herbs – creeping thyme, chives and wild marjoram – into the mix and see how they fare.

As I said, this isn’t the way a green roof should come about, but it does illustrate just how eagerly plants will colonise any corner or terrace and how readily they can become established. From the roofs of bird boxes to grand roof gardens, roofs are just empty canvas, waiting to be turned into  little works of art.

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Sedum is, of course, the most well known of all the green roof plants, but from turf roofs –manicured or wildly natural – to complete formal gardens, just about any plant can be accommodated into, or onto a roofing scheme. It is the depth of the substrate - the growing medium - which will determine what you can grow, and that in turn will depend to varying degrees on the strength and angle of the structure that is being built upon.

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My small roof didn’t need any extra support, and it is perfectly angled to shed excess rain water with no extra engineering, but that is an exception rather than a rule. If there is any question about the strength of the structure and if it will support the weight of the substrate, the plants and the extra water burden, an architect or structural engineer should be consulted. A DIY green roof will generally weight in the region of 60-150 kg per square meter...this is within the load bearing capacity of new, well built sheds, but it is always best to err on the side of caution and consult an expert if in any doubt.

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The optimum angle of a roof is determined by its ability to shed rainwater and its ability not to shed its plant community. Too flat and the water won’t drain away without extra help, too steep and the water won’t have a chance to be absorbed. That isn’t to say that flat roofs or steeply angle roofs cannot support a plant community; they can. It is just that they will need a little extra engineering in order to do so.

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The substrate needs to be both lightweight and able to both hold water and drain efficiently...although it sounds complicated just keep in mind that peat and peat substitutes are poor choices as they are deceptively lightweight when dry, but exceptionally heavy when wet, and of course they are often difficult to re-wet once they have dried out completely. The best combination is 70% inorganic material such as crushed bricks, hortag or small gravel and 30% well rotted garden compost. The whole point of a green roof in the scale we are talking about is that it is low maintenance, so any substrate that is overabundant in nutrients will encourage all sorts of unwanted seedlings to colonise. This isn’t bad in itself, but it is a harsh environment up there, and lush growth cannot be maintained through the extremes that these plants will be subject to. For sedum roofs the substrate needs to be about 70 to 100 mm deep. For grass and wildlife roofs you can get away with as little as 150 to 200 mm substrate.

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The profile of a green roof is simple. From the bottom up: the upper surface of the building roof structure, a moisture and root proof membrane (a heavy duty pond liner is just right), and finally, the plants. It is that simple.

Around the edge of the roof should be a clear area of a few inches that will not support plant growth and will allow for efficient shedding of water into a drip line or a gutter.  The ideal material for this edge is large, smooth round pebbles that won’t trap substrate.   Ensure that any drains, gullies or outlets are kept clear and don’t clog with the substrate by placing a wire or nylon mesh over any pipes that have the potential to become clogged.

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Water draining from your green roof can be used around the garden -or even to flush toilets- but if you are accustomed to using rainwater harvested from the same structure you will notice that the runoff will be reduced by as much as 50%  once the plants are established.  This is caused by a combination of evaporation and transpiration and is one of the great advantages of green roofs; it cuts down on the water runoff, and on a large scale can be beneficial to controlling flooding.

Green roofs will not damage new or existing roofing material if it is sound and undamaged. Indeed it can extend the life of many roofing materials as it protects the surface from extremes in temperature and UV rays.

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There are 3 main types of green roof; the intensive, the extensive and the biodiverse roof. The intensive roof is really a roof garden, with a deep layer of growing medium, often in held in raised beds, tall plants and even trees, as well as underplanting and other garden feature such as paths and seating. In intensive green roof requires a lot of maintenance and demands the attention of a structural engineer, at least, as well as building regulations approval. The intensive green roof is a large project which  is beyond the scope of this article, but it is a worthy and worthwhile option to reduce the impact of any new build.

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The extensive green roof is what most of us think of when we think of green roofs - a selection of several species of sedums, probably, some grasses and tough wildflowers, or perhaps monocultures of any of these. Extensive systems are found all over the place, from the roofs of small sheds to the large greening of factory buildings. Depending on the species of plants chosen, these roofs can fulfil just about all the promises of green roofs...from their insulating qualities, both temperature and sound (one sedum roof near Kings Cross in London, is planted with a green roof as a method of sound proofing. This green roof is on a theatre so I am not sure whether it is to keep the sound in or out!) and for reducing the impact of buildings on the built environment, visually and environmentally.

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The difference between extensive roofs and biodiverse roofs is that the biodiverse roofs are planted especially to support the local ecosystems in one way or another and in the way they are built. A biodiverse roof is built using the most eco-friendly materials available locally and with one or more particular species in mind. The design goes from the membrane up, with the correct substrate and planting complimented by essential ‘extras’ such as small piles of rotting logs and stones, patches of bare, unplanted ground and small areas to  catch and hold water in order to support a invertebrate or animal or bird community. There is one biodiverse roof planting in London, for instance, that was designed specifically to support the return of the rare black redstart, one of our rarest breeding birds. Others have been designed to support a particular butterfly or beetle and of course, there are urban wildflower roof gardens specifically designed to provide pollen and nectar for both honey and bumble bees. It can be a very exciting and rewarding project.

 

Extensive and biodiverse green roofs are low maintenance, though not no maintenance, once they are established. In the first summer it may be necessary to water your roof during dry periods and patches may appear where one or another or your plants fail to establish. These can either be replanted with the same species or left so that wind born seeds can take root. If, however,  invasive species such as buddlia try to take hold, hoik them out immediately as they will damage the structure of your roof if allowed to persist.

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Check to ensure that drains and pipes remain clear of plants and substrate and ensure that no plants grow over the edge of the roof (unless the roof has been designed in this manner) or it will allow water ingress as drips curl down and under the eaves. On roofs of over a few meters square, and especially in exposed areas, there should be a upstand surrounding the roof to prevent wind from lifting the planting and substrate off the roof, while still allowing efficient drainage.

Choosing the plants and setting them in place is without doubt the fun part of the exercise. Sedums are evergreen, hardy and drought tolerant...everything you might want from a green roof, but for increasing biodiversity they leave something to be desired. Sedums only really support nectaring and pollinating insects for about 6 weeks in June and July and there are other plants that you can plant to share the roof area, though they lend such an even and regular appearance to your green roof space.

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Monocultures have their place, but variety is generally better for supporting wildlife.

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Last summer I decided to increase the size of my flower beds by taking away some of my lawn. I  had laid this lawn some 8 years ago and I thought it would be easy.  It wasn’t. I neglected to take into account that this lawn had been nursery grown on a nylon mesh. No doubt this makes it easier or the nursery to harvest, roll  and transport, with fewer losses from broken turves, but it made it a real pig to remove. After 8 years the nylon mesh was as tough as the day it came out of the factory and wouldn’t tear. I couldn’t dig through it, though I could cut through it with a serrated bread knife, which was what I had to do in the end. Roots that attempted to grow through had  nylon mesh digging deep into their flesh and all in all it was jolly hard and frustrating work;  and this is the just the problem with mats of roofing plants.  Unless they are furnished with  tiny breadknives and infinite patience, no burrowing beetles, butterfly larva or foraging birds are going to have any luck penetrating nylon mesh so check carefully with the supplier if you are thinking of  buying mats of plants rather than planting plugs or sowing seed. Some grow their mats of plants in trays or fields  but some are grown on huge webs of nylon mesh. Avoid these for the sake of your roof visitors.  

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Most of us will choose a compromise between biodiverse and extensive roofs, using up readily available materials and perhaps choosing a mixture of plants that will be easy to maintain and to source while at the same time thinking of a favourite plant, bird, animal or insect that we would like to encourage. 

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Some of the best plants for green roofs are those that can cope with shallow substrate and drought condition and many of these can be found in the alpine section of nurseries and garden centers.  Some of the best for shallow substrate (70-100mm) are: Allium shoenoprasum, Dianthus deltoids, Muscari, Potentilla verena, Sempervivum species, Sedum species, Thymus serpyllum and Petrorhagia saxifrage.  The deeper your substrate, the broader your choice of plants will be and that, in turn, will broaden the sorts of visitors that you will receive.

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After building and planting your green roof, keep it damp for 5 to 10 weeks, depending on the weather,  while the plants become well established. After this period, as long as there is not a prolonged drought, there should be little or no extra watering needed. Thoroughly inspect the roof, inside and out every six months to carry out regular maintenance and  to check on the health of your high level habitat.

• Contact your local council or wildlife trust to find out about the local Biodiversity Action Plan which can inform you of which plants to grow to best support local wildlife.

• If your substrate is deeper than 100mm your plant choices will be greater and you can choose plants that will happily colonise any dry verge or gravel garden, but be aware that it is an exposed environment, so keep your plants below 30cm tall if you can.  If your green roof has challenging conditions such as it is very steep, exposed to sea breezes or is in deep shade, seek specialist advice from a green roofing company. Most are happy to offer advice in the hope that you will consider using their services in the future.

• Including small piles of logs or stones, bare ground and containers to hold water will increase the number of vertebrate and invertebrate visitors you receive.