Green Roofs Homebuilding & Renovating

Green Roofs Homebuilding & Renovating

How ‘Green’ Are They?

Green roofs are, without doubt, one of the more visible signals that you have built a sustainable home. There is a lot of cynicism that they don’t actually serve enough ‘eco purpose’ to be worthwhile, but they do have some genuine benefits:

  • they absorb heat (from the sun)
  • they absorb CO₂
  • they absorb a large proportion of the water that falls on them (up to 70%)
  • they provide a degree of insulation – more cooling the building in summer as opposed to staying warm in winter
  • critically, they replace the ecology that the building stands upon.

The benefits are now gaining increasing recognition — London now has regulations in place requiring buildings of a certain size and function to have a green roof.

The Systems

There are three basic types of green roof: intensive (thick), extensive (thin) and semi-extensive (somewhere in between).

Intensive: Consists of a thick layer of soil (50-200mm+) in which a variety of grasses, herbs, flowers and shrubs can grow. These need to be accessible gardens (for maintenance) and can even be used as a recreation space. They provide a valuable habitat for wildlife but place significant weight on the building and need substantial roof support. They offer good insulation and better water absorption than the thin types, but need a lot of care .

Extensive: Generally a shallow layer (20-100mm) of substrate planted with low-growing, stress-tolerant grasses, mosses and sedum. These lightweight systems require little maintenance. The insulation quality is relatively low and extensive systems always incorporate conventional insulation .

Semi-Extensive: Of slightly greater depth than extensive systems (100-200mm) but constructed on the same principles to allow for a greater diversity of plants. It is also relatively low-maintenance .

Installation

Green roofs need to be installed on roofs pitched at no more than 30°. The planting is just the tip of the iceberg — underneath, a moisture-retention fleece keeps water in the soil rather than letting it pool on the membrane surface; an aqua drain carries away excess water. A good membrane is nonetheless required to prevent leakage, while a vapour control layer manages the passage of internal water vapour through the roof, preventing condensation inside.

Green Roofs Homebuilding & Renovating

What Do They Cost?

The cost will vary with the green roof system and the particular situation. As a guide, prices can vary from £50/m² for a sedum roof to £200/m² for a fully planted intensive roof — the cost of any reinforcement needed to the roof and/or wall structures is additional to this.

Is a Green Roof for Me?

A green roof is, in ecological terms, a great and simple thing to do. It replaces the ecology the new house sits upon, thereby replacing the habitat that a house takes away. Used over a whole roof, a green roof will be the single overriding design influence on the house as well as impacting on the construction. right down to the foundations.

But green roofs do not necessarily need to be applied to the whole roof, or even the house itself. If the roof is accessible, having just a section planted is visually pleasing and a step in the right direction. Alternatively, install a green roof on an existing shed or garage. or install an extensive system on a flat-roofed extension.

The right system used in the right place has no downside. So a better question, perhaps, is which green roof system is best for your project?

How to Choose a System

Extensive roofs (ABOVE LEFT) use low-growing grasses, mosses and sedum. An intensive option (ABOVE MIDDLE) showing the most grass-like option visually. Meadow roofs (ABOVE RIGHT) grow flowers and grasses to promote biodiversity.

Intensive systems and meadow roofs need to be designed in. as they are heavy and the supporting structure needs to be designed to carry the weight. Extensive and semi-extensive roofs are less of a problem. They offer less water retention and insulation, but are easier to install and maintain.

The thicker intensive systems are really for specialist applications — buildings that want to make that big statement about their eco credentials. Any growing medium will attract other species – that is sort of the point – and maintenance is likely to remain an issue whatever system you install, but the thinner extensive systems are generally the right choice for most domestic situations. They still need conventional insulation, and rainwater run-off will still need to be dealt with, but they offer good ecological benefits, are cheaper to install, easier to maintain and have good visual appeal. On a shed or garage they make a great talking point.

The Standards

The German FLL guidelines are followed internationally, due to the nation’s dominance in the market. There are no official UK standards so ensure your installer understands and follows the FLL.

History of Green Roofs

Turf and sod roofs have been common in many cultures for hundreds, if not thousands, of years — the earliest recorded being from the 7th century BC. Yet the idea of a deliberately planted roof perhaps dates to 1950s Germany, where covering wet bitumen with 75mm of sand as a fire protection measure was common practice.

Vegetation would seed itself even in sand and that, in turn, led to the idea of deliberate planting. The technology developed fairly quickly and by the early 1970s cheap, relatively light, more effective systems were on the market. By the 1990s the market was also offering green wall systems.


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