We discuss the spreading of a viscous drop hanging on the earth-faced (bottom) side of a smooth horizontal plate, which is either uniformly heated or cooled. The model of Ehrhard and Davis (J. Fluid Mech. 229, 365 (1991)) for the spreading of axisymmetric drops is applied to this situation, where gravitational forces point off the plate. The model holds for thin drops under fairly general thermal conditions. Solutions valid under such conditions are inferred. Experiments by Levinson et al. (Rev. Phys. Appl. 23, 1009 (1988)), using isothermal silicone oil drops hanging below a glass plate (completely wetting system), are used to validate the model predictions. It is found that the model is able to describe the observed experimental time behavior reasonably. In particular the effect of gravity is reflected correctly, as the model predicts a spreading toward an equilibrium state, with the final drop radius depending on the Bond number. This behavior is contrary to previous findings for spreading drops on the top surface of the plate.