Providing adequate strength and stiffness is considered a primary principle for structural design. Roof flexural members have a remarkable role to provide lateral and gravity stiffness. A vital issue in the analysis and stiffness assessment is to apply stiffness modification factor for the mentioned members in RC structures so that their impact in gravity and lateral load bearing could be changed. In the present study, to achieve an appropriate coefficient to decrease the stiffness of reinforced concrete slabs under simultaneous gravity and lateral loading, 20 structures from 2 to 20-stories with various roofs including flat slab, beam-column slab, and one-way and two-way waffle slabs were designed and analyzed, and then an equivalent overall coefficient for slabs stiffness decrease was obtained. The results indicate that the coefficient of 0.25 recommended by the building codes for flat slabs is around 60 percent conservative. In other categories of slabs for which the available building codes have not recommended a determined coefficient, the coefficient of 0.45 to 0.5 is achieved based on the analysis of the current study. تفاصيل المقالة