Baroque chapel of St. Jan Nepomucene, built in the 18th century, is part of the Renaissance Sarny castle complex in the village of Ścinawka Górna in Poland. It is an ellipse-shaped building, covered with a brick dome decorated with numerous illusionistic paintings made on lime plaster. Above the dome, a roof with a wooden rafter-collar beam structure with purlins was made. Numerous cracks were identified in the area of the walls, especially above the window openings. Moreover, numerous cracks on the intrados of the dome are visible in the interior, mainly in the meridian direction, extending into the lower support ring. One of the causes of the damage were unfavorable soil and water conditions, including clay layers and uncontrolled embankments. Due to the damage to the dome and walls in the form of numerous cracks, it was necessary to carry out a structural analysis to assess the state of danger together with the proposal of strengthening using the FRCM system. A system of strengthening the walls and dome was proposed, both in the latitudinal and meridional directions.