Assessing the condition of concrete in place is an important part of building safety verification programmes. For the study of existing concrete structures, it is important to know the strength of different elements. The assessment of the compressive strength of structures can be carried out by destructive coring tests in varying quantities and calibrated indirect methods, combining destructive coring with non- or semi-destructive techniques. When planning ultrasonic indirect measurements, it must be taken into account that in inhabited houses the columns are covered with plaster that often cannot be easily removed and restored (even more so if, in addition to being a protective covering, it also has a decorative function): indirect measurements can be performed, but it is necessary to evaluate the impact of the presence of plaster. An analytical model has been developed to predict the surface wave propagation velocities in concrete columns, both with and without a plaster covering. The aim is to support the interpretation of time-of-flight measurements in indirect ultrasonic testing.