Cracking due to corrosion in bed joint reinforcement over openings in masonry facades in the Scandinavian post-war building stock generates increasing retrofitting needs. Removal of the reinforcement can be both costly and labor intensive. The results of retrofitting works are often sub-optimal due to casual inspection practices and lack of knowledge concerning the actual corrosion damage. The objective of the presented research is to increase knowledge about physical factors influencing corrosion of bed joint reinforcement. The research includes a field survey by ocular examination of cracking and decay related to corrosion of reinforcement in joints above openings, and a subsequent laboratory examination of reinforcement extracted from the surveyed buildings. The investigation shows a strong dependence of crack formation on the corrosion depth of the embedded reinforcement. In turn, the corrosion depth is mainly influenced by moisture content and temperature in the façade, two factors related to geographic location, orientation and height above ground. The number of reinforcement bars in the joints has also a large influence on the crack frequency. It is suggested that a practice oriented model able to predict time until cracking, with moisture content and temperature as main factors, would be possible to develop with corrosion models from the field of concrete and the empirical data from the present research as a basis.