A study on the slab effective width of steel-concrete composite decks is presented. By using an analytical beam-model presented by the authors, the shear-lag effect is investigated on continuous twin-girder composite decks under static, geometrical and thermal actions and when internal and external prestressing techniques are adopted. Separately for each action, the elastic and the long-term stress distributions on the slab are reported. The dependence of the slab effective width on the type of the action is put in evidence. A parametric analysis shows that the slab effective width is weakly dependent on time, shear connection stiffness and steel beam inertia while it is strongly influenced by the span length and by the beam spacing. Finally, some correlation formulas between the slab effective width and the span length and between the slab effective width and the steel beam spacing are proposed.