Taiwan government specifies that the average roof thermal transmittance must be less than 1.0(w/(m2·k)) for the design of all residential buildings in order to implement the policy of saving energy. However, self-disciplined architects practice the design of aesthetic roof to blend in with green landscape so that they urgently expect the academia to provide roof-greening technical information to support their idea of designing green roofs for residential buildings. In this research, a single family housing unit is used for investigating the possibility of applying extensive roof greening to achieve building sustainable development. The experiment tasks focused on the soil denudation caused by rainwater washing and replenishing the soil carbon by irrigating the soil with gray water. Using tap water to irrigate the green roof for 12, 16 and 14 weeks causes nitrogen, phosphate and potassium, respectively, to be reduced to less than the original levels, respectively. Applying gray water to irrigate the green roof soil will raise the soil fertility by improving nitrogen and phosphate but not obvious for potassium.
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