Abstract: The laser alignment system of apparatus for leveling flowable material, in which two radiant energy beam detectors mounted on opposite ends of an elongated leveling member are responsive to a projected radiant energy beam establishing a leveling plane, is modified for operation when one of the detectors is blocked from receiving the projected radiant energy bean by an obstruction such as a support column, equipment or personnel. Whereas each detector signal is normally used to generate a separate adjustment signal for an adjustment device such as an hydraulic cylinder to raise or lower the associated end of the elongated leveling member, when one detector is blocked from receiving the radiant energy beam the detector elevation signal generated by the other, unblocked, detector is used to generate the adjustment signals for both ends of the elongated leveling member.