This paper evaluates the influence on the positive DC corona onset voltage in conductor-to-plane gaps of a grid of metallic wires placed underneath and parallel to the stressed conductor. A systematic experimental study investigates the dependency of the onset voltage on the gap geometry and the particulars of grounded and negatively stressed grids. These particulars include the wire-to-wire spacing, number, lateral distribution and height of grid wires. It was found that the number and distribution of grid wires are the major factors that influence the corona onset voltage together with the radius of the stressed conductor. In a theoretical approach, calculated values for the onset voltage of positive corona as influenced by the grid particulars are obtained using a method based on the criterion of onset streamers self-sustained growth. This calls for calculating the electric field in the vicinity of the stressed conductor in the presence of the grid. A comparison is made between the calculated onset voltages and those measured experimentally.