where conditions could be very heterogeneous: different surface finish, low vegetation cover in first years, lack of maintenance, etc. In this article, different aspects about this topic are discussed. Moreover, the results of monitoring a real extensive green roof during the first year after plantation in order to study the thermal behaviour are presented. In these conditions of low plant cover (10 %), the main conclusions refer to the substrate, which although it preserves cool near the base roof, it heated and cooled very quickly. This fact confirms that the substrate limited thermal mass effect, due basically to its composition and small thickness. The material composition and the thickness of the substrate should be studied in depth because it can affect the thermal performance of the roof.