Additional horizontal sealing in mineral building materials often produce insufficient or varying results despite the application of the injection agent being carried out according to the manufacturers' specifications. There is no current scientific explanation for this effect, which prompted this research. The main idea was to solve the question of whether the increasing filling of the capillary pores of mineral building materials with water (and therefore the degree of moisture penetration) can be connected to the varying spread of the injection agent. After developing a test procedure, different degrees of moisture penetration were applied to samples of bricks and mortar cubes and hydrophobic injection agents were applied to the test specimens. The results showed that the degree of spreading of the injection agent was inversely proportional to the degree of moisture penetration. Consequently there is a dependence of the above mentioned parameters (the existing degree of moisture penetration and the spreading of the injection agent) and a model was developed of the spread of injection agents dependent on the existing degree of moisture penetration in building materials.