The task of fire pressure differential systems in a building is to prevent smoke from entering protected spaces such as stairwells and areas that should remain smoke-free in the event of fire. The required overpressure must consider leakage within the protected space. This article presents the results of testing to determine the impact of leakage of the protected space on the conditions for maintaining overpressure. The research was conducted in accordance with draft standard N161-prEN12101-6. The pressure distribution as a function of time was obtained for stairwells with different levels of leakage, using a constant flow aeration fan as well as systems with a variable airflow. Practical application: Designers of differential pressure smoke control systems are required to calculate aeration fan airflow volume. The leakage level of protected spaces should be taken into consideration given that European Standard EN12101-6 suggests that gap areas should be minimal. Consequently, modern building design and construction requires estimation of the leakage area in such protected spaces. The objective is to improve safety of buildings and reduce the system reaction time during a fire.