During the reconstruction and restauration of the U-tower of the former Union Brewery in Dortmund inhomogeneities in the load-bearing outer walls were detected and needed to be investigated. The 50 cm thick walls made of clinker bricks featured several locations where concrete was found behind an outer brick lining of merely 7 cm thickness. Since these spots differed from the original clinker wall structure they could negatively affect the structural integrity and statics of the whole building depending on their size and location. However these spots were not visible from neither outside nor inside of the building. Thus the high resolution ground penetrating radar technology was applied to detect and identify location, size and distribution of the relevant areas in the building's facade. Further goals of the investigation were the detection of beams and other metallic objects as well as areas of increased moisture within the walls.