The passive evaporative cooling roof in building is an important passive thermal insulation technology with the development of green roof, spraying water roof and storing water roof. For its complexity of heat and moisture transfer process only some sample experiments could be done on some controlling conditions. In this paper on the basis of analyzing the unsteady heat transfer model with the first boundary conditions and by using the integral method based on the concept of thermal boundary layer the critical penetration time, the critical penetration thickness and the temperature attenuation ratio are calculated and analyzed for different materials with different thickness. The research results show that the critical penetration time is gradually attenuated with the increasing of thermal diffusivity. The critical penetration time of being researched materials is lower than one hour when the thickness of materials is 0.1 meter, when the thickness of materials is 0.2 meter the critical penetration time is between 46.83 and 214.55 minutes, when the thickness is 0.3 meter the time is between 105.36 and 482.73 minutes. The sandy soil has the maximal critical penetration time and the critical penetration thickness in all analyzed materials, and the loose sawdust has the minimum critical penetration time and the maximal critical penetration thickness. On the side of temperature attenuation ratio the different material thickness has different attenuation law.