In situ-measurement of the water absorption behaviour of facades – Evaluation of measurement results on three façade constructions. The water absorption behaviour of façade constructions of historical buildings is an important aspect for their durability. To determine the water absorption coefficient of façade construction materials, samples can be taken on site and examined in the laboratory according to the state of the art. With regard to the protection of historical buildings or the influence of mortar joints in fair-faced masonry, this procedure is not appropriate in all cases. In situ measuring instruments work non-destructively and directly at the building, but often provide only limited results. This paper contains parts of the dissertation Stelzmann [1]. It contains approaches for the estimation of various effects that can influence in situ tests on water absorption behaviour. These include the correction of three-dimensional suction in the area of the edge of the wetting surface, varying water temperatures or initial moisture content of the facade materials. After physical derivation, three façade constructions were examined in a comprehensive study with the water absorption measuring instrument developed in [1]. The statistical evaluation of the data gives an indication of the respective proportions of the various effects of in situ measurements.