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 to the residents. It is still debated whether the green roof will cause fire or it will reduce the chance of one. Some argue that, because the vegetation is about 95% water, the green roof actually reduce the chance of a fire. On the other hand, some argue that during autumn and winder when the leaves are dried up, fire hazard will increase then. A recent study has found, through mathematical model, that when the vegetation itself caught fire, the heat will penetrate down rather slowly, because the thermal conductivity of soil is low, and finally did damage the roof itself. Thus the key to whether the fire caught by the vegetation will damage the roof or not depends on the thickness of the soil. The study also found that by installing a gypsum layer beneath the soil layer, the possibility of damaging the roof can be greatly reduced.
