This paper reports the development of a new design guideline for the stair slabs, supported on the structural member spanning at right angle to the direction of the flight. The study is based on finite-element analyses of two common types of stairs, dog-legged and open-well stairs. Results of the finite-element investigation have been compared with the experimental findings, ensuring reliability of the predicted response. The stair slabs, usually supported on walls or beams at landing levels, derive significant rigidity from such supporting arrangement. This results in an efficient distribution of moment along the length of the flight. This distinctive feature of stair slab is not recognized by the leading codes of practice. In order to establish the characteristic behavior of stairs of the aforenamed types, a sensitivity study for the geometric parameters of the stair slab has been carried out. Based on the findings of the present study, a new design rationale for stair slab has been proposed. The guidelines presented here enable straightforward estimation of the design forces, which are generally overestimated by 300–400% when determined by the conventional design practices.