What is a green roof?

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Find out why a living roof could be a good option for your self build, extension or outbuilding

The term green roof (describes a roofcovered with any kind of growing plants, whether turf, hardy evergreen plants such  as sedum or a full-blown roof garden with trees and flowering shrubs.

There are two types: extensive and intensive. Extensive green roofs have a thin layer of soil and are suitable for sloping roofs or roofs that are for visual effect rather than for walking on; intensive green roofs are flat roof gardens planted on a deep layer of soil. Whereas turf is suitable for intensive green roofs, a mix of hardy plants is appropriate for visual appeal on extensive green roofs.

How is a turf roof constructed?

A turf roof is built up of several layers on a suitably strengthened roof structure: note that it’s important to have a structural survey to assess roof strength, also it is only viable on a slope of up to 35 degrees, as retention is too difficult over that limit. Guttering is still needed around the edge of the roof so that excess water can run off. The layers comprise:

  1. A waterproof and root-resistant liner such as pond liner or other 300 micron damp-proof membrane, with an edging of wooden or aluminium battens to hold it in place on a sloping roof
  2. A moisture-retaining blanket or piece of carpet
  3. A layer of grit, stone or aggregate that aids drainage
  4. A filter (typically fleece) that allows water to pass through but retains the soil
  5. The plant layer and its soil
  6. Some turf roofs may incorporate additional layers of insulation and vapour control

How environmentally friendly is the structure?

Plant and grit layers are green in every sense, the waterproof liner is polyethylene – but this is made from recycled plastic. The filter can be made of sheep’s wool.

What are the advantages of a green roof?

A turf roof is attractive to birds and insects. It is pleasing to the eye, a factor that can endear it to neighbours and planning authorities, especially if you are building a new house that is overlooked. If it’s a flat roof structure, it can be a place to sit out and relax – especially valuable in a city centre. It adds some insulation and also reduces sound transmission by up to 40 per cent. Because a proportion of rainwater is soaked up and held by the plant layer, there is less run-off and so less demand on drains.

What about the disadvantages?

On the minus side, although evergreen sedums, lavender and other plants are chosen for green roofs, because they are hardy and drought-resistant and can cope without water for up to a month, beyond that you need to water, as with a garden. Turf will only be as good as the attention it receives. You may also need to control any unwanted bugs such as greenfly if these get too prolific. The plant-bearing moisture mat must make good overall contact with the liner: if there are any air pockets plants can dry out and die.

What does a living roof cost?

You can have an extensive turf roof fully installed for £50-100 per m², or £130-140 per m² for an intensive green roof. (The wide variation depends on how close you are to the supplier.)

Alternatively, you can build up the layers yourself, and buy the growing layer as a length of planted matting. The cost varies depending on how mature the plants are, and this in turn affects how long it takes to achieve a visually pleasing all-over green coverage without gaps. NatureMat, a 24m² length of mixed plants on a growing medium and geotextile base layer, costs from £25 per m² from Blackdown Horticultural Consultants.

Generally it’s reckoned that plants take between 12-18 months to fully establish, and the more mature (and therefore more expensive) the plants you buy, the sooner you will have good all-over coverage. Marketed as ‘A green roof in a box’, UrbanMAT is a 12m² length of sedum-planted matting that includes drainage and waterproof backing, and that costs £590 from Urban Roof  Gardens. Bear in mind that plant matting is very heavy and needs at least two people to handle the laying.

Who are the major suppliers?

Major suppliers of the complete green roof package are Blackdown Horticultural Consultants and Bauder. These companies and others such as British Field Products and Urban Roof Gardens, will also supply the planted matting layer on its own. Liners are sold by Flag and International Construction Bureau.

Photo: Alex Pratt

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