Country Gardener

Widely regarded as the authority on gardening in the south west

The green roof

Elizabeth McCorquodale takes a look up high and enjoys the wonders of green roofs with all their noise reduction, pollution filtering benefits and shows how you might like to create one

Green roofs attracted quite a lot of attention a few years ago but now they have settled comfortably into the landscape, appearing without fuss on all sorts of buildings from grand avant-garde homes to factories and public buildings (my local council sports a rather lovely example) to modest garden sheds and garages.

I am the fond custodian of an accidental green roof which sprung up, of its own accord, on the top of one of my sheds. It established itself hardly without my noticing after I re-roofed the old shed with a piece of corrugated plastic and then weighed down the plastic with a few shovelfuls of gravel borrowed from my garden path. Over just a few weeks seeds carried by the wind landed on the roof and germinated in the gullies and crevices, and the more the plants grew, the more windblown dust and soil particles got caught among the roots.

 green roofs.jpgThis accumulated substrate is now deep enough to support an entire plant and invertebrate community with no input from me at all, which just goes to show that although green roofs can seem a little exotic, even a little contrived, they are in fact just another patch of garden, albeit a little higher than the rest.

Sedum is, of course, the most popular of all the green roof plants, but from turf roofs –primped and preened or wildly natural – to entire formal gardens, just about any plant and planting style can be incorporated into, or onto, a roofing scheme. It is the depth of the substrate - the growing medium – and the strength and angle of the structure which will determine what you can grow.

Sedum roofs require about 70 to 100 mm of substrate. For grass and wildflower roofs you need a little more, but you can get away with as little as 150 to 200 mm.

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 weigh 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 at all.

sedum roof.JPGThe optimum angle of a green roof is determined by its ability to shed rainwater efficiently and its ability not to shed its plant community. Too flat and the water won’t drain without help, too steep and the water won’t have time to be absorbed and may even result in the plants being washed off the roof.

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 and for this it is a good idea to consult with a specialist green roofing company about the best way to go about it.

Whatever the angle of the roof the substrate needs to be lightweight, it needs to be able to hold water and be able to drain efficiently. This is where coir really comes into its own. Unlike peat, coir will readily re-wet if it dries out, it is lightweight even when it is sodden and it drains well. Use equal parts coir, fine gravel and well-rotted garden compost to make your substrate. The aim is to achieve a stable growing medium that isn’t excessively nutritious in order to provide a home for your chosen plants without encouraging a whole lot of windblown opportunists. Your roof needs to be made up of a stable population of fairly tough, hardy plants that can cope with the harsh environment of fluctuating temperatures and water levels rather than a bunch of fly-by-night tenderlings that won’t last the distance.

The profile of a green roof is simple. It begins with the roof of the building, which is then topped with a moisture-and root-proof membrane such as heavy-duty pond liner. This is topped with the substrate (the growing medium) into which the plants are planted.

As long as the building and roof structure is strong and sound the addition of a membrane and planting will not damage the building. In order for the roof to drain well, the area all around the edge should be designed not to support plant growth in order to allow water to be shed easily and so that plants don’t encroach into gullies or drainpipes. A 15cm border of substrate-free space around the edge of the roof should suffice.

There are three categories of green roof; the intensive, the extensive and the biodiverse. The intensive roof is really a roof garden, with a deep layer of growing medium which may support trees, shrubs and underplanting, as well as other garden feature such as paths and seating, and with its structural design requirements this sort of roof is an architectural rather than a horticultural endeavour.

somerset green roof.jpgMore within our scope is the extensive green roof, and this is what most of us think of when we think of green roofs; a selection of sedums, some grasses and tough wildflowers, or perhaps monocultures of any of these. Extensive systems are found on the roofs of small sheds and in the greening of huge factory buildings and, depending on the species of plants, this type of roof can fulfil just about all the promises of green roofs including reducing the impact of buildings on the built environment, both visually and environmentally.

A biodiverse roof goes one step further and is designed, built and planted especially to support the local ecosystem. The design goes from the membrane up, with the correct substrate and planting complimented by essential ‘extras’ such as small piles of stones or rotting logs (or small branches, on a more modest scale), patches of bare, unplanted ground and small bowls to catch and hold water for the benefit of birds and invertebrates.

Of course there is no strict delineation between any of these designs, and you can do as much or as little as the fancy takes you, incorporating all sorts of creative and environmental goals within the design of your roof. Monocultures are perfectly acceptable and provide a neat and even appearance, but variety is undoubtedly better for supporting wildlife. Sedums only offer nectar and pollen for a scant six weeks or so each summer while a good selection of wildflowers and grasses can support the local invertebrate population all year round.

The best plants for green roofs are those that can cope with shallow substrate and drought conditions 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.

An ordinary green roof requires minimal maintenance once it is established. In the first summer it may be necessary to water your roof during dry periods and you must always be on the lookout for any unwelcome or damaging invaders such as buddleia, but from then on your plant community will gradually settle in and find its own happy equilibrium.

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.

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