Recent investigations on friction welded wood-to-wood connections have shown interesting capacities of these adhesive free joints for further development towards constructional elements. This paper addresses challenges and technical requirements for the enhancement of this technology from small specimens to samples of structural scale. Inhomogeneities in bonding quality and joint strength, herein referred to as scale effects, become relevant with an increase in size of the welded interface. It is assumed that the water vapour and smoke that evolves during the welding process by evaporation of moisture within the cell structure is a reason for these inhomogeneities. In order to achieve a better understanding of these effects, the influence of the water vapour on welded spruce (Picea abies) boards was investigated. It is shown that the vapour increases the internal gas pressure within the welding zone. Thus it strongly influences the welding process and the quality of the joint. The results of these investigations lead to technical solutions, permitting a significant attenuation of the negative effects and an improvement in quality and joint resistance.