The objective of this study was to determine the moisture content gradient over the thickness of glued laminated timber structures, manufactured as five-layer structures from two different species. Beech (Fagus sylvatica L.) or oak (Quercus robur L.) were used for the faces, and fir (Abies alba Mill.) or lime (Tilia cordata L.) were used for the core layers. Thus, four types of mixed glulam structures resulted. The layers were glued with a polyurethane adhesive for outdoor uses, and then cold-pressed. Samples were also prepared from each individual species as control specimens. All samples were exposed to two types of climate conditions, having cyclic and constant parameters. The first climate test involved cyclic variation in temperature and relative air humidity: 30 °C/40%/12 h, alternating with 10 °C/80%/12 h. The second climate test involved constant exposure conditions, but with higher humidity over a longer period: 15 °C/90%/3 weeks. The moisture content gradient between the layers of the structure was determined and correlated to the delamination effect, assessed by visual analysis. Based on the findings in this work, the lowest values of the moisture gradient were determined for the oak–fir mixed structures, resulting in the total absence of delamination and cracks between the outer oak layers and the inner fir layers.