T-beams are acknowledged as economic and efficient structural members widely used for floor slab construction systems. In many cases, according to practice in some countries, the beams do not present transverse reinforcement, and their shear strength is governing for dimensioning the width of the web. Although experimental investigations have shown that the presence of the compression flange enhances the shear capacity with respect to equivalent rectangular cross sections, most current design codes neglect this phenomenon, which leads to the overdesign of these members. In this paper, the role of the compression flange of slender T-beams with concentrated loads is investigated with reference to its influence on the shape of the critical shear crack and to the associated shear transfer actions (STA) of the beam. The flanges are considered elements that allow the smearing of applied loads over a certain length of the web. This consideration, in combination with the mechanical model of the Critical Shear Crack Theory (CSCT), allows a consistent treatment of the phenomenon and leads to simple design expressions accounting for the role of flanges. The results of the proposed model are compared together with design codes (Model Code 2010, Eurocode 2, and ACI 318-11) and other shear design approaches to a database of 239 beams on T-shaped members. The comparison shows that the role of flanges is finely accounted with the proposal based on the CSCT, leading to consistent agreement and to strength predictions that are more suitable for design purposes than the other investigated design models.