Sun. Strong solar radiation can cause the planted surface to dry out too rapidly, especially if it is on a relatively steep roof facing south. In drier climates, roofs with an angle more than 20° need to be shaded or supplied with a thicker layer of earth to give a higher waterand heat capacity.
Wind. The impact of wind depends upon the height and shape of the building and local wind conditions; the stronger the wind, the slower the plant growth. Wind also has a cooling effect and can increase the drying rate, even causing physical damage in certain situations. For very exposed areas, plants should surround the building to protect it, with a thicker layer of earth on the roof, mixed with stones to give the roots a better hold.
Rainfall. Even if the earth can, in certain cases, be waterlogged, water is something the planted roof needs in very large quantities. As there is no ground water reserve for them to draw on during a dry period, they are totally dependent upon the storage capacity of the layer of earth on the roof. A short dry period is no problem; after a little rain the plants can quickly recover. Shading can reduce solar penetration and a thicker layer of earth can store more water, especially if it contains more clay than sand. Solutions might include an automatic watering system, especially if vegetables are to be grown. Grey drainage water from the household can be used for extra fertilization.
Pollution. Green roofs have a very positive effect on air pollution, but they can also be damaged by it.This will only occur in situations of extreme pollution where there are strong concentrations of ozone, or dust that settles on the leaves and prevents photosynthesis. If the earth becomes too acid, lime can be added.
Erosion. Green roofs do not receive any nutrition from the natural nutritional cycle, but are constantly losing humus, minerals, salts, etc., as these are washed out. Much can be gained by starting with a rich earth. A little compost can be added occasionally, and autumn leaves should be left lying. The correct mix of plants can also add to the richness of the earth.
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