A drum for building and shaping blanks for radial-ply tyres comprises two co-axial bead holders at least one of which has an extension drum which is co-axial therewith, is secured thereto and has the same outside diameter. the extension drum acts in part as a support for a tyre band while the latter is being secured to the bead holders for shaping. The bead holders are movable axially relative one to the other to position the tyre band for shaping and each is provided with radially movable segments to which end portions of the tyre band are secured in airtight manner. During relative axial movement of the bead holders towards each other, a gaseous medium is introduced into the space bounded by the bead holders and the tyre band so that the band is curved outwards to form a tyre blank.