The purpose of this paper is to study the effectiveness of a green roof by simple mathematical model. The developed model simulates a green roof during the summer. The results explain and support the claims in the literature such that effectiveness of a green roof depends on the climate condition, and on the insulation condition of the roof. If insolation can not be reduced more than 60% by the shading effects of a green roof, more active measures than green roof seem necessary. The analysis tools like presented in the paper is necessary especially to consider climate effect, to design, and also to test green roof.