Words: Andy Duck

Discover the appeal and practicality of planting on high – a green roof is a visually striking, environmentally friendly solution that helps promote biodiversity and keeps our urban spaces verdant

The construction of human dwellings has come a long way since Emperor Charlemagne decreed in the ninth century that ‘every house roof shall contain plantings of house leeks’. Yet his commandment, issued because the plant was believed to ward off evil, has struck a chord among today’s self-builders and architects, albeit for very different reasons. Concern about the loss of wildlife habitats, overheating cities, rainwater conservation and aesthetic considerations are all contributing to a rise in the number of green roofs across Charlemagne’s former European empire — and now in Britain, too.

The house leek (Sempervivum tectorum, aka sedum) is one of the few plants able to fill what architect Jon Broome describes as that ‘very specific ecological niche’ presented by roofs — alternately bone dry and sopping wet conditions, and next-to- no soil. Many specialist companies produce sedum matting, a convenient but costly ready-made covering that can be slapped down over a waterproof membrane, in much the same way as you’d turf your garden.

image: Andrew Montgomery

But it’s not just sedums that are reappearing on our rooftops. Centuries after sod roofs became obsolete, self-builders are DIYing green roofs, architects and clients are choosing them for extensions and redevelopments and a growing number of people are retrofitting old or leaking roofs with a brand new verdant covering. From thin-soiled sedum pitches to sky gardens with soil deep enough to grow trees, green roofs are again taking root.

I do spend a lot of time on the roof. It was a labour of love, and is a great place to think.

David Matzdorf knew his narrow parcel of north London, part of the garden of a large end-of-terrace, would require a very special self-build house. ‘The roof is green to satisfy three requirements: to lower the house’s green imprint; to meet the planning requirements; and because I like gardening.’ Which is just as well, because his roof — a glorious sweep of emerald accessed by a vertiginous ladder – took a while to get right. To please the planners, it had to descend from west to east, because the house could only be one storey high at its eastern end. Architect Jon Broome proposed the curved monopitch and a ‘loose lay’ waterproof membrane to provide a base for the soil. To increase the variety of plants the roof would support, David insisted on an irrigation system. It pipes tap water into the flatter two-storey section of the roof, from where it percolates down into a runoff drainage channel at the low eastern end. But frequent heavy rainfall in the year after completion waterlogged the soil and turned it anaerobic. David painstakingly restocked his roof with fresh soil and 400 new plants, including sea thrift, rosemary, thyme, agaves and aloes. Does he treat it like a garden?

The basics

Types of roof
Green roofs fall into roughly two categories: intensive and extensive. Intensive roofs feature deep soil and a wide variety of plants. Extensive roofs are low-maintenance, a thin layer of soil supporting a limited range of plants, usually succulents. Intensive roofs are expensive to install and maintain. Extensive roofs are much more common for domestic properties. Either type may require planning permission, but many councils look favourably on green roof applications.

image: Jefferson Smith

Structure
The building must be strong enough to support the weight of soil and plants, plus a capacity for snow loading and precipitation. Creating a support for an intensive roof can be prohibitively expensive. Whatever kind of roof you have it will contain the following: a roof deck, waterproof membrane, insulation layer and growing medium. Optional extras include a root barrier, water- collection system, filtration layer and hard landscaping (plants and lawns are soft landscape).

Growing medium
‘It’s just luck,’ says Royal Institute of Chartered Surveyors’ Dave Richards about getting the growing medium correct on a green roof. Tree shade, microclimate, plant uptake and runoff rates will all affect suitability. Jon Broome has successfully experimented with mixing soil with chalk or sand on the roof of his home. Nigel Bishton tried soil on his rural farmhouse extension but it didn’t retain enough moisture. On flatter roofs, he now uses LECA – Light Expanded Clay Aggregate, a horticultural growing medium made from loam clay – mixed with compost. On pitched roofs, he replaced the original turf with lightweight Grodan rockwool medium. After David Matzdorf’s original soil mixture became waterlogged he put in more conventional plant ‘plugs’ with a mix of compost and grit to allow drainage. ‘Soil is not a sensible choice,’ says Bishton. But in Richards’ experience, ‘clay soil holds the moisture better,’ as long as the soil is well drained.

Planting
As well as sedum, there are many other species that can survive on extensive roofs: with a little care, grasses, alpines, coastal plants and hardy Mediterranean plants can all flourish. David Matzdorf has found that sea thrift has taken well on his roof, but it fights a constant battle against grasses and mosses. As the RICS roof garden proves, there is no limit to what you can grow on an intensive roof. Dave Richards’ picks of his 120 plants include a rare Chilean guava, a blue sausage tree, dwarf banana and tea. ‘Delicates are sometimes easier to grow on a roof, because of the unique microclimate,’ he says. The bottom line is: if you are a keen gardener and intend to have regular access to the roof, experiment. If not, choose low maintenance plants such as sedums and grasses.

An eco-architect’s perspective
Green roofs have had a bad press in Britain in the past, says Jon Broome, architect of David Matzdorf’s turf- topped home, ‘largely because of the failure of flat roofs in the UK,’ which put a question mark over practicality. But on the continent, instead of being fully bonded to the structure of a building, the waterproof membrane on flat roofs has traditionally been laid over loosely, like a tablecloth. This ‘loose lay’ approach stopped any movements in the building being transmitted to the membrane, which could cause it to fail and leak. A loose lay needs to be ballasted: ‘That started us thinking, “this could be done with soil and turf instead of shingle,”’ says Broome. He chose PVC for the membrane of his own green roof – ‘because it is inexpensive and versatile’. Broome suggests using single-ply EPDM or TPO for the membrane, which are less harmful to the environment than PVC.

‘There are a number of ecological benefits to a green roof,’ he says. ‘It minimises surface water runoff and creates wildlife habitats. In London, heavy rainfall causes sewage to wash into the Thames – so any measure to mitigate runoff is important. And with the risk of flooding, controlling runoff in rural areas is also very important.’ Green roofs also help combat the urban heat island effect, keeping buildings cool and reducing the need for air-conditioning. And, as a bonus, an English Nature report found that the green roof on Broome’s south London home supported a wide range of insects and birds.