The following paper is focused on hygrothermal behaviour of roofs with ventilated air layers subjected to air leakages in their internal decks (the internal partial construction located below the ventilated air layer). Possibilities of CFD modelling of such constructions are discussed in the first part of the paper including the problem of introduction of water vapour diffusion into calculation. A brief comparison of results from CFD analysis and from calculation based on a technical standard is also included. The main part of the paper includes a case study concerning various effects of leakages in three basic types of ventilated roofs. General conclusion discussing the significance of leakages is included in the final part of the paper.