This paper deals with the behaviour of CLT composite T-beams composed of a Cross Laminated Timber (CLT) acting as panel attached to a Glulam (GL) girder. The paper investigates the effect of the configuration of the CLT panel and GL beam on the effective flange width of the CLT composite T-beams. When the CLT composite T-beams are subjected to positive bending, a part of the CLT panel acts as the flange for the GL girder to resist compression. From shear lag assumption, the compressive stress in the flange varies as the distance from the GL girder web changes. When the spacing between the GL girders increases, the stress in the flange right above the web increases with a higher rate than the rate in the extremities. Despite the large number of studies about design of steel-concrete composite structures, there has not been comprehensive research on timber structures. In the presented study, a finite element (FE) model which is experimentally verified is used to analyse the CLT composite T-beams and numerically obtained effective flange width sizes are presented by tables, bar charts and normal stress distribution figures. Based upon a detailed parametric study, it is concluded that the layer arrangement of the CLT panels and its material properties have a significant influence on the effective flange width of the CLT composite T-beams. Any changes that increase the ratio of the transverse layer’s depth to the longitudinal layer’s depth result in an increase of the effective flange width.