A toroidal forming drum and a process for budding tyres. The toroidal forming drum is expanded within a shaped carcass sleeve for supporting the carcass sleeve against an abutment surface externally provided by the forming drum. An elementary semi--finished product is applied around the shaped carcass sleeve by pressing the elementary semi-finished product toward the abutment surface. The abutment surface has circumferential rows of solid portions alternated with hollow portions. The solid portions, arranged along axially opposite circumferential edges of the abutment surface, have a transverse size between 10% and 60% of a transverse size presented by the solid portions arranged in proximity to an axial centreline plane of the abutment surface.