This paper describes mathematical modelling procedure of the rib-reinforced floor/ceiling structures, which are made up of components with irregular shapes and physical parameters. Exact determination of the vibration of a composite structure becomes impossible beyond the low-frequency range due to the uncertainties in the structures. Therefore, the prediction model in the mid- to high-frequency range must include the e ects of the irregularities. Statistical estimates of the solutions will then give appropriate mean and variance of the vibration of the structure for the given severity of the irregularities. We discuss the nature of the randomness in the lightweight constructions and compare the existing methods, such as Statistical Energy Analysis and Finite Element methods.