Radon emanation has been studied in different rooms of buildings in Marrakech by using LR-115 and CR-39 solid state nuclear track detectors (SSNTD). A new calibration method has been developed based on calculating the probability for an emitted α-particle to be registered on a SSNTD. The thoron-to-radon ratios as well as thoron and radon activity concentrations in the dwellings studied have been determined by exploiting the measured track densities, due to the α-particles emitted by radon, thoron and their daughters, registered on the SSNTD. The influences of building materials, pollution and painting layers and papers were also investigated.