Moisture is of great importance due to a number of degradation processes. The moisture either accelerates or reduces the degradation velocity depending on the degradation mechanism and material. The degradation of building materials is thus dependent on the time during when the material is exposed to moisture. A model of how the time of wetness on a surface will vary, due to different climate conditions including impact of rain and solar radiation, is developed and presented. Due to the results of some test runs of the developed model, not surprising, both wind and the colour of the surface will have an impact on the time of wetness on a non-hygroscopic surface. However, the model is only tested during some specific climatic conditions and has to be tested in several different climatic conditions with real climatic data of rain and wind.