A study was made of radon-safe building in 300 Finnish low-rise residential buildings using data obtainedfrom a questionnaire study. The study also aims at finding the main defects in design andimplementation and how the guidance given on radon-safe building in slab-on-grade houses has beenfollowed. According to the guidance, the prevention of the flow of radon-bearing air from the soil intothe house is recommended to be carried out through installation of aluminized bitumen felt and use ofelastic sealants.