The present study aims to investigate the water balance in conventional and green roofs and also to monitor the development of peanut grass (Arachis repens Handro), relating the type of vegetation cover or not with its water retention capacity and, consequently, to obtain the coefficient of runoff for each scenario tested. The scenarios tested were: (1) conventional collection surface with fiber cement tile, (2) green roof structure with substrate and no plant, and (3) planted green roof. The rains incident on the roof were obtained for the city of Itajubá, in Minas Gerais, by the curve of i-d-f with 5min of duration and 5years of return period according to the recommendation of NBR 10844 (ABNT,1989). The volumes of storm water runoff and drained from the bottom of the roofs studied were collected in calibrated graduated containers. It was concluded that: the peanut grass showed satisfactory development and high recovery capacity; the peanut grass took only 1½ month to cover the entire roof; the coefficient of runoff of the green roof for intense rainy season was on average 0.569 and in the dry season it was 0.003. While the conventional roof presented average surface runoff coefficient of 0.995 for the rainy season and 0.901 for the dry season; the component of the green roof that contributed most to the rainwater retention was the substrate. Therefore, attention should be paid to the choice of substrate so that it can adequately grow the plant, retain rainwater and not contaminate the bottom drainage water of the roof that can be used for less noble uses.