This thesis covers calculations of the equivalent measuring depth in the joint concrete between hollow-core slabs performed using the calculation and simulation program WUFI Pro. The calculations are based on input data from field measurements performed by Polygon|AK in Uppsala. Input from the company has been combined with a moisture profile from the calculation program Torka S to construct a basic model in WUFI. Simulations have been made based on the basic model with constant water-cement ratio in joint concrete, three varying parameters in terms of relative humidity in spackle, leveling height for spackle and vapor resistance in carpets. In order to get a broader result basis, 18 different typical cases for further analysis have been studied.  The calculation results indicate that the industry standard according to RBK, which states the equivalent measuring depth to 50 % of the hollow-core slabs joint height, does not correspond to the calculation basis for this work. The result of this work indicates that the actual equivalent measuring depth is about 15-18 % of the height of the joint, depending on the selected parameters. The difference between the industry standard and the result of this work can be explained by the fact that in the previous calculations of the equivalent measuring depth assumes a completely dense surface layer and no drying downwards and moisture from glue has not been taken into account. The assumption that the water contained in the cross section does not dry out and only redistributes leads to results that do not correspond to today's real conditions in terms of more diffusion-open carpets, moisture from carpet glue and different moisture conditions in spackle.