Method for forming an overlap by welding sheets, comprising the steps of: - placing a main sheet and a additional sheet overlaid on one another, connected to one another by means of a contact area; the contact area selected from the group consisting of: a main sheet contact surface, a additional sheet contact surface, a contact sheet and a combination thereof; the contact area being opposite main sheet and additional sheet outer surfaces (4,7); - heating the contact area (3,6) to a melting temperature for melting same, applying pressure between the outer surfaces (4,7); and - cooling; the contact area (3,6) being made of a thermoplastic material and the main sheet outer surface having a main sheet critical temperature; where: the main sheet outer surface is heated to a temperature comprised within a temperature range that is higher than ambient temperature and less than or equal to the main sheet critical temperature; the method being characterized in that the additional sheet outer surface is heated to a temperature that is higher than the main sheet critical temperature and suitable for melting the contact area (3,6) in a heating time. - placing a main sheet and a additional sheet overlaid on one another, connected to one another by means of a contact area; the contact area selected from the group consisting of: a main sheet contact surface, a additional sheet contact surface, a contact sheet and a combination thereof; the contact area being opposite main sheet and additional sheet outer surfaces (4,7); - heating the contact area (3,6) to a melting temperature for melting same, applying pressure between the outer surfaces (4,7); and - cooling; the contact area (3,6) being made of a thermoplastic material and the main sheet outer surface having a main sheet critical temperature; where: the main sheet outer surface is heated to a temperature comprised within a temperature range that is higher than ambient temperature and less than or equal to the main sheet critical temperature; the method being characterized in that the additional sheet outer surface is heated to a temperature that is higher than the main sheet critical temperature and suitable for melting the contact area (3,6) in a heating time. the contact area (3,6) being made of a thermoplastic material and the main sheet outer surface having a main sheet critical temperature; where: the main sheet outer surface is heated to a temperature comprised within a temperature range that is higher than ambient temperature and less than or equal to the main sheet critical temperature; the method being characterized in that the additional sheet outer surface is heated to a temperature that is higher than the main sheet critical temperature and suitable for melting the contact area (3,6) in a heating time. the main sheet outer surface is heated to a temperature comprised within a temperature range that is higher than ambient temperature and less than or equal to the main sheet critical temperature; the method being characterized in that the additional sheet outer surface is heated to a temperature that is higher than the main sheet critical temperature and suitable for melting the contact area (3,6) in a heating time.