The carrying capacity of simply supported rectangular plates under uniaxial, in-plane compressive loading is investigated. The ultimate load is determined as the load corresponding to the intersection of an elastic loading line and a rigid–plastic unloading line. An attempt is made to formulate a plastic roof mechanism for the rectangular plate; the square buckle pattern mechanism for long plates is obtained as a special case. The effective width method is re-examined and is shown to give good agreement with experimental evidence. The recommendations of CSA S136-1974 are briefly reviewed in the light of the results obtained.