Since water is inextricably linked with life, its presence in green building constructions is an inevitable aspect in the process of hygrothermal behaviour analysis. Water content in the substrate of green structures is a variable phenomenon throughout a year. During this period it has a strong influence on the building's hygrothermal behaviour. The WUFI simulation tool analyses hygrothermal behaviour of the tested extensive green roof in the climatic conditions of East Slovakia (Kosice). The presented simulation scenarios are aimed at assessing the influence of water in the green roof substrate on the roof membrane temperature and the fluctuation of heat fluxes through the roof during summer period (June 1-August 31). Presence of water and its movement through capillary forces is an essential aspect of hygrothermal behaviour and cannot be neglected in the simulation process. Water in the tested green roof substrate has reduced the average daily heat flow across the roof by 11%, which significantly contributed to lowering its summer overheating. Calculated water content in the substrate was influenced by the selected initial water content condition for up to two-thirds of the simulated period's length. (C) 2018 Elsevier B.V. All rights reserved.