Putting down roots on city rooftops

Another Life: The almost voluptuous relish with which our country towns now garland themselves in flowers for the summer, and nurture new trees and shrubs in every spare corner, speaks for a wish to live with natural beauty that goes beyond mere tourist attraction.

In cities, too, the national facelift is finding room for plant-life, if only at the level of the street and window-box. But it's now time to look up (or down, for those sitting at desks in tall office blocks) to the promise of the green and living roof.

The all-round benefits of vegetated roofs, especially for bigger buildings, are catching on at a striking rate in cities throughout Europe, Japan and North America. In Germany, pioneer of the technology, financial incentives in almost half of the cities have led to the installation of more than 30 million square metres of green roofs since 2000.

A visit to Germany inspired Mayor Richard M. Daley of Chicago to create green roofs in his windy city, notably on City Hall. The Ford Motor Company has greened a vast new four-hectare roof at its Dearborn Truck Plant - the largest living roof in the world. In London, green roofs are part of a showpiece block of eco-friendly flats at Tower Hamlets and of buildings regenerating a derelict area beside the River Thames. A new Galway hotel gives guests a view of a flat aluminium roof brocaded with a mat of growing rock-plants, rather than a flat expanse of gravel or bitumen.

Elaborate and intensive roof-gardens are nothing new. Those with deep soil and trees may have a wet weight of a tonne per sq m, even without people, and demand costly reinforcement. Modern mixes of lightweight aggregates, sand and compost can reduce their weight substantially.

But the greening of roofs in what's called the "extensive" style has come with development of reliable, waterproof, root-resisting membranes, and porous, layered blankets into which plants can be rooted or pre-sown. They can be used on flat or steeply pitched roofs and need a minimum of maintenance.

According to research for English Nature (www.english-nature.org.uk), a geotextile blanket sown with sedum and mosses has a saturated weight of only 30 kilos per sq m, and a coir fibre fleece with fescue grasses in the mix weighs 42 kilos. In the UK, the typical price for the supply and installation of an extensive roof system, including the plants, is £98 (€147) per sq m for a 600 sq m roof. This compares well with conventional roofing, and as the roof landscaping industry grows, prices should fall even lower.

The sedum family are stonecrops - low-growing succulents that are flowering now on Ireland's stone walls, rocks and sand dunes. Perennial and hardy, they are naturally drought-resistant, absorbing moisture from the air. Starry flowers in yellow, white or pink, raised from red-tinged leaves, create a Persian carpet in high summer, and their nectar brings bees and butterflies. Bats and beetles, sparrows and wagtails are other beneficiaries of living roofs, especially those that incorporate a few inches of "engineered" soil.

Even the urban economics are persuasive. By protecting the underlying structure from ultra-violet radiation and the "thermal shock" of switches in temperature, a living roof can be expected to last at least twice as long as a conventional one. A city with planted roofs stays far cooler in summer than one with vast stretches of heat-absorbing tarred, slated and paved surfaces., Where new building fills up an open "brownfield" site, its living roof could compensate for the absence of leafy habitat. The buildings themselves may well have lower heating and cooling costs (Chicago City Hall saves over $5,000 a year). Living roofs collect and filter the rain of pollutants, and dramatically slow the run-off of stormwater into sewers. They also shut out noise, in the manner of thatched houses.

And they sweeten the soul. In Switzerland, a hospital at the heart of Basle has spread a 4,000 sq m flat roof with a mix of soil, gravel and peat to plant a garden filled with flowers and birds for the patients overlooking it: so many Irish hospitals, old or new, offer only a prospect of puddled asphalt and ventilation ducts. Thousands of city office workers have little better to inspire them. Green roofs can make new spaces for sociable lunchtime picnics high in the sky, if not for the smoker's meditative exile.

Ecological architecture is making slow headway in Ireland. Even partnerships promoting solar panels, recycled timber and sheep's wool insulation seem shy of living-roof technology.

At Pennsylvania State University, its chief proponent is a professor of ornamental horticulture whose grandfather "was born in a sod-roofed house in Ireland". It looks like yet another time for reinventing the wheel.