Surface moisture plays an important role in the deterioration of building surfaces. The extent and duration of surface moisture is generally impossible to predict from meteorological data. The limitations of the ISO 9223 standard for estimating the time of wetness (TOW; RH>80%, T>0°C) is evident in climates with sub-zero temperatures, in environments with significant deposition of pollutants and salt, and in situations where the exchange of radiation between building surfaces and the surrounding environment creates large temperature differences. Consequently, direct measurement of TOW is essential, e.g. using the WETCORR method. This method is suitable for measurements of surface moisture and TOW on building materials in general. The actual sensor consists of an inert electrolytic cell with Au/Au-electrodes combined with a Pt-1000 surface temperature-sensing element.