Isopipe design feature to eliminate sag

Disclosed is an isopipe for use in the manufacture of sheet glass by, and more specifically to an isopipe designed to control sag during use, as well as a method for reducing the sag of an isopipe used in a fusion process for molten glass. In one embodiment, the isopipe comprises a cavity that extends at least partially through the refractory body of the isopipe along its longitudinal length. Thecavity has varying cross-sections configured such that, for at least a portion of the length of the isopipe, the load bending moment is greater than or generally equal to the gravity bending moment. In one embodiment, the neutral axis varies along the length of the cavity and has a similar profile to that of the gravity bending moment diagram.