A process for building a tyre comprises: - supplying a fluid flow through a first nozzle (31; 131) of a pneumatic sensor (30; 130) which is provided in the region of a head (23) of an extruder (20), the fluid flow being directed in a flow direction (A; B) which does not interfere with a second nozzle (32; 132) of the pneumatic sensor which is connected to a pressure measuring unit (33), - starting to extrude a continuous elongate element (21) through an opening (24) which is formed in the head of the extruder, - redirecting the fluid flow via the continuous elongate element (21) into the second nozzle of the pneumatic sensor, - detecting a variation in pressure in the second nozzle (32; 132), thereby determining the presence of the continuous elongate element (21) output from the opening, - after a predetermined time from the detection of the variation in pressure, causing a forming drum (2) to rotate about its own axis of rotation (X), - distributing the continuous elongate element over a radially external surface (3) of the forming drum (2).