Roof or rooftop vegitative base material, and a method for using the structure body or rain water r

PROBLEM TO BE SOLVED : To obtain a vegetational base capable of preventing earth and sand from sliding with rainwater, reduce the weight load on a building, simply carrying out laying and performing the greening execution of a roof or the rooftop at a low cost and to provide a method for greening and a system for reutilizing the rainwater using the vegetational base. SOLUTION : This vegetational base for greening the roof or the rooftop is a thermoplastic resin fibrous structure having >=90% porosity, 5-100 kg/m3 apparent density and 20-100 mm thickness. The method for greening the roof or rooftop comprises carrying out the greening with a vegetational structure prepared by including a mixed soil of peat with a lightweight soil in (4/6) to (1/9) volume ratio in the vegetational base and rearing plants. The system for reutilizing the rainwater is capable of purifying the rainwater with the vegetational structure.