The invention provides a base plate, and a fastener equipped with such a base plate, whose weight is minimized and whose stiffness is optimised on the one hand and which is suitable for being supported on an elastic intermediate layer on the other hand. To this end, a base plate according to the invention for fastening a rail to a fixed bottom support has, the base plate being made of a plastics material and a stiffening structure which is formed by ribs and by depressions present between the ribs being formed or molded in the underside of the base plate, which underside is associated with the fixed bottom support, depressions of the stiffening structure being closed off, in accordance with the invention, by a filling material.