Environmental Advances With Plant Surfaces

For the town. Plant surfaces are an important factor in the environment of towns. Green plants bind and break down gases such as nitrogen oxide, carbon dioxide and carbon monoxide and produce oxygen. A combined leaf surface of 150 m2 produces the oxygen needed for one person. A150 m2 roof that has a 100 m2 leaf surface per square metre thus provides for the equivalent of 100 people. Awild, overgrown grass roof produces about 20 times as much oxygen as a well manicured lawn.

Planted surfaces bind dust. A city street without trees has approximately 10 times more circulating dust in the air. Planted surfaces also reduce vertical air movement. Over a conventional roof, vertical air currents of up to 0.5 m/s can be caused by solar heating of the roof material. On metal roofs the temperature can be as high as 100 °C.This air movement can pick up dirt and form clouds of dust over towns. A turf roof will normally reach no more than 30 °C, almost totally eliminating the rising air movement (Figure 15.10).

Planted roofs also function as retention basins, in order to delay the release of storm water that overloads many urban drainage systems. When rainwater passes through a green roof, particulates can also be filtered out of the water.This can reduce pollution downstream.

For the house. Planted surfaces can often provide good thermal insulation since pockets of still air are formed between the plants giving the same effect as a fluffy fur coat. Plants also reduce the effects of wind infiltration into the underlayer. For a large part of the year, a planted surface acts as a solar panel: turf roofs have a particularly high absorption coefficient.

15.11

The roof garden of a large department store from early 20th century in South Kensington, London.

15.11

The roof garden of a large department store from early 20th century in South Kensington, London.

The plants also develop their own warmth during the cold part of the year and thus prevent freezing. During the summer, dew will form on the roof in the morning. For every litre that condenses, an amount of warmth the equivalent of 0.65 kWh is emitted.

The damp earth in theturf roof hasalargecapacitytostorewarmth.Thiscangivethe building a stable, warm indoor climate during the winter and a cooler indoor climate in the summer. Walls covered in plants are also cooled by their shade during the summer.

A turf roof gives a sound reduction effect of 46 dbAwith a 20-cm thickness of the earth layer and about 40 dBA with a 12-cm thickness.This kind of roof is therefore particularly suitable along highways and nearairports.

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