One method used to strengthen brick masonry columns is reinforced concrete (RC) jacket. However, this method is not always effective and does not align with conservation doctrine. The article presents the negative consequences of applying this method to a historic building. In the 14th-century town hall in Zary (in western Poland), strengthening of brick walls and pillars were made in the middle of the last century. Reinforced concrete jackets for the pillars were used in one of the rooms. Currently, there is severe moisture and cracks in the brick pillars and RC jackets cracking. The lack of damp proofing and, at the same time, the use of jackets for the brick pillars have resulted in increased moisture in the masonry elements. The RC jackets impede the evaporation of capillary rising water from the ground from the masonry parts. In addition, there are cracks in the masonry and reinforced concrete elements. The damage and cracking of jackets is caused by the corrosion of the reinforcement bars. The corrosion of the reinforcement is a result of the high moisture content of the pillars. The moistened masonry elements did not have the possibility of natural evaporation of water, by which the process of damage to the jackets was intensified. The use of improper strengthen caused additional damage to the brick pillars.