Systems of ventilated facades recently gain the increasing popularity. First of all it is explained by multifunctionality of these systems. Ventilated facades not only give the buildings expressive architectural shape, but also provide protection of the building against external influences and effectively cope with thermal protection. The main existing techniques for calculation of these critical structures are based on normative requirements. The hydraulic method of calculation of ventilated façade systems with the help of extremal problems is presented. Extreme conditions divide the actual and virtual movements of viscous air in vertical ventilation gaps. Their application leads to the weak formulations of limit problems convenient for an assessment of hydraulic elements of motion. The method was based on an extremal problem of Cross, the decision was provided by means of Krokko's equations.