Structural timber is specific, because its quality is set by visual or mechanical grading. Visual grading method by PILODYN 6J device is examined in the paper. Experiments were carried out using 5 spruce boards (Picea abies Karst. L) from Slovakia. Dimensions of boards were 40 x 200 x 2500 mm. Density of wood was set by gravimetric method according to EN 408 (2013). Depth of penetrations was measured using the device PILODYN 6J (hp). Wood structure was described by the rate of growth (RoG) according to DIN 1052 (2004). Three most used visual grading strength classes C30, C24 and C18 according to EN 338 (2016) were specified. Dependencies between measured characteristics were expressed by multistage parallel scale model (penetration depth ~ number of growth rings ~ rate of growth ~ strength class and wood density). It is possible to predict visual strength class of board and indicative density of wood (EN 338) by the proposed model in situ. Methods of model are easy to use, reliable and economically undemanding.