This paper introduces an in-situ device for measuring the water absorption coefficient of masonry façades. The “Water-Absorption-Measurement” instrument (WAM) allows non-destructive determination of the time-based function of water penetration. In a dense stack box a masonry façade area of 0.40 × 0.51 m is watered by nozzles. The gravimetric measuring system reaches a repeatable accuracy up to ± 8.3 × 10^–4 kg/(m² × √s). Especially on fair-faced masonry, it is possible to measure the water penetration on an average area with bricks and mortar. This report contains detailed information about functionality of the developed measuring concept. Measuring results with laboratory—and a masonry façade are demonstrated. Customary instruments like the RILEM tube are compared to the developed measuring instrument.