<p>The bending behaviour of steel–timber composite beams is not only determined by the bending deformation, but also by the shear deformations of the shear connectors and the transverse layers of cross-laminated timber. This deformation behaviour was investigated in bending tests that validate three different numerical models. The three model types differ in whether spatial or planar numerical elements are used and how the shear connection is modelled. In the simple models with plane finite elements, a plane strain state prevailed, as known from steel–concrete composite beams. Due to the low modulus of elasticity of timber at right angles to the fibre and the low rolling shear modulus of timber, a spatial strain state arises with numerical 3D-solid elements. This spatial strain state significantly changes the composite effect and differs from previous knowledge. Whether a spatial or planar strain state in the cross-laminated timber better reflects reality in a composite beam strongly depends on the structural configuration of the shear connection. Based on the numerical simulations, this article sets out clear demarcations and explains the mechanical basis for this. Finally, it is formulated which findings of the numerical analysis must be included in a simplified analytical calculation to correctly describe the strain state of the steel–timber composite beam.</p>