Botany and building are two professions you wouldn't usually draw close associations between

The UK construction industry is warming to the benefits of covering buildings in vegetation. Tim Wood of Permanite Engineered Roofing Systems discusses the issues involved in bringing horticulture to the roofing industry.

THE UK has traditionally lagged behind other European countries when it comes to installing green or living roofs, but growing recognition of their many environmental and performance benefits means they are being increasingly specified by developers, architects and contractors alike. The past two years have witnessed a particular surge of interest in living roofs, which has been bolstered by political backing from figures such as Mayor of London Ken Livingstone.

Roofs covered in living plants deliver almost instant benefits for occupiers and the environment. As well as providing an important habitat for wildlife, the plants improve air quality by attracting particles to the damp surfaces of the vegetation and soil, as well as absorbing air-borne pollutants through their leaves. During photosynthesis the plants improve air quality by converting carbon dioxide to oxygen.

Getting to the root of the roof

The life of the roof and particularly the waterproofing membrane is dramatically increased due to an extra layer of protection being provided to the roof fabric. Storm-water run-off, which has become an increasing problem due to the prevalence of impermeable building materials in modern construction, is controlled, reducing the general flood risk and ensuring that water does not collect on the roof. What’s more, both the acoustic and thermal performance of the roof is vastly improved as the vegetation provides extra soundproofing and insulation.

The development of green roofs has been driven by advances in flat roofing waterproofing technology from the 1970s onwards. Modified bituminous membranes provide an ideal base for living roofs as they require less maintenance and are capable of supporting increased weights. As a result, green roof systems now provide a fully functioning built-up roof, consisting of membranes to seal the building onto which a growing medium is subsequently placed. There are now a number of waterproofing options which can be used with green roofs, including built-up bituminous  membranes, polymermodified asphalt systems, polymeric single-ply systems and hot melt systems.

Using green roofs extensively

There are two main types of green roof; extensive and intensive. Extensive green roofs are self-supporting floral communities which exist as a wild space on the roof, providing a haven for birds and wildlife. They have a shallow soil depth which can be installed quickly and offer low maintenance, excellent aesthetic qualities and long-term performance. Sedum plants are favoured for these applications. These plants are herbaceous perennials with succulent leaves which are easy to grow and which give excellent ground cover. Sedum grown in lightweight soils are generally used with extensive green roofs as they provide protection from the elements and are particularly resilient to harsh conditions.

Extensive green roofs have a typical substrate depth of 25-50mm, and can therefore be installed on a wide range of buildings, from schools and houses to offices and even sheds. Intensive green roofs provide accessible garden space, which can include bushes, lawns, ornate plants, benches and even water features, leading to stunning visual effects. They are much heavier than extensive roofs as they require a greater depth of soil, often over 600mm,. Consequently they require a robust structure that is capable of supporting their weight.

Green roofing evolution

Most extensive green roofs in the UK use pre-grown sedum mats rolled out over the roof membranes. However, a new type of extensive roof is now being developed. Designed specifically to replace the natural habitat of local wildlife and recreate the environment lost to building development, they use a substrate of recycled local  aggregate from building or industrial waste and can be tailored to blend in with the surrounding location. The recycled aggregate is either simply left to colonise naturally or seeded with an annual wildflower mix or a local seed source. Such roofs tend to drain particularly well and are low nutrient in character, offering increased ease of maintenance as well as greater biodiversity.

New systems are also being developed to overcome some of the structural barriers that can prevent the installation of green roofs.

Typically, minimum roof falls of 1:60 are recommended for green roofs as anything less may require special drainage. However, developers are exploring ways of delivering green roof systems which are suitable for a wider range of buildings. There are possibilities for retro-fit roofs, which would allow a green roof to be added to pre-existing buildings if the existing roof structure is capable of withstanding the additional load that would be imposed.

The slope of the roof is also a problem, and green roofs are being developed to address irrigation and slippage  issues. Extensive green roofs are not confined to flat roofs, but severely sloping living roofs often begin to look threadbare at the apex and overly lush lower down the roof. However, architects and self-builders are showing real interest in the possibility of installing vegetation on sloping roofs.

Green roofs are now becoming popular on a range of buildings, from private houses to public sector buildings. Extensive green roof systems, for example, are a particularly popular option for school buildings as the enhanced sound proofing and thermal protection improves the learning environment within the building and the increased wildlife the vegetation attracts can provide a useful educational tool. Intensive roof gardens are often specified for office blocks as the garden area provides a relaxing haven for employees and building users.

The UK construction industry is warming to living roofs, and the latest developments are bringing green roofs to a wider range of buildings. Green roofs benefit the environment and the performance of a building and, contrary to popular belief, they are easy to install and maintain. One of our greatest assets is just above our heads.