applied to wherever it will fit. Most of times, actually, the decision to build Green roofs is based on local climate and policy. It is because other than its aesthetics, green roofs are used often for their ability to conserve energy, such as increasing insulation of the building roof, retaining and infiltrating rainwater, and potentially reducing urban heat island effect when it was installed to a certain scale. In Germany, for example, partly because of EU's regulation, 17% of the new roof construction are green roofs. In Washington DC, green roofs are utilized as an alternative storm-water retention technique. Benefits Reducing building energy consumption by increasing the roof insulation: In total energy consumption reduction, green roof would have the best performance relative to a bare roof in a colder climate, which require nighttime heating. The reduction in heating load of the building increase as the soil depth of the green roof increase, though a increased soil depth would mean heavier roof. On the other hand, if a building is cooling-dominated, leaf area index is more important. In peak energy consumption reduction, green roof also has a notable effect, and the leaf area index and soil depth are both positively related to its performance. Rainwater retention and evapotranspiration: a 3-4 inch of soil can retain about 1 inch of rainwater. That is about 75% of precipitation in most areas in United States. By retaining the rainwater in soil, the water would not become runoff, instead they would result in evapotranspiration. Controversies Water runoff quality: When green roof is not able to hold the amount of the precipitation, the excessive rainwater will become runoff. In a field experiment where contaminated water are dripped into a green roof section to mimic rainfall in the green roof, the exfiltrate water was studied and analyzed. It was found that since the average level of suspended solid, nitrogen, and phosphorus concentrations in Green roof water outflow is significantly higher than those in conventional roof outflow, extensive green roofs will become a source of nutrient contamination in urban water environment. Fire Hazard: Green roofs can be more easily ignited than conventional roofs; it is a concern that when the green roof caught fire, the high temperature would damage the roof structure itself. Not only the idea of damaging the roof is contradictory to energy conservation and sustainability, the fire and the roof damage could cause safety issue