The bending strength and elastic properties of engineered timber products such as glulam (GL T) and cross laminated timber (CL T) are commonly described via load-bearing models based on the tensile properties parallel to the grain of boards and finger joints. These models assume that strength graded boards remain unsplit in the final product. In practice, however, products can be lengthwise split (e.g. resawn glulam) or may include a significant share of split lamellas, as in CL T beams or slabs. After splitting, the assigned strength class no longer applies. Lengthwise split lamellas also occur in the novel products flex_GLT and flex_CLT. The first part covers the develops a probabilistic board model capable of describing knot distributions and tensile properties not only for full-section but also for lengthwise split boards. Combined with a newly developed stochastic-numerical beam model, the load-bearing behaviour of beams containing such split lamellas can be calculated. The model covers products with uni- or orthogonally oriented lamellas and is validated by tests. System and size effects as well as the influence of split lamellas are analysed. Finally, load bearing models and minimal geometric requirements for the newly developed flex_GLT-CLT products are presented.