The current paper presents the results of an experimental verification of the methodology to evaluate lasting moisture impact on ribbed plates. Conditions of building envelope for ribbed plywood plate with polystyrene insulation were simulated. Internal moisture due the surface exposure to different temperatures and ambient air humidity was accumulated. Within ten months the total mass of the plate increased by 9.7%. After that the plate was dried until its initial mass was reached. Within these three months the plate was loaded in 4 point bending to evaluate its deflection depending on accumulated moisture. After the experiment it was found that the methodology is applicable to assess the effects of lasting moisture impact.