Two variants (the basic and approximate) of the stability analysis method for rectangular plates pivoted on sides and strengthened by the orthogonal frame of ribs have been proposed. A necessity of allowance for the discrete arrangement of ribs both when determining the parameter of critical stresses and when determining wave formation pattern appearing as a result of stability loss has been demonstrated on numerical examples. The approximate procedure is constructed in a assumption that one can disregard the tangent constituents of transfer components under stability analysis of the ribbed plates. This procedure permits determining the parameter of critical stresses with sufficiently high accuracy in the range of ribs parameters.