What is the best option to do on a smoke seal fire door when the threshold gap is 5mm. The doors in question are hospital corridor doors & the doors are from the 90’s   & the company have gone bust so unable to speak to the manufacturer. External DDS would just get damaged. Any ideas would be much appreciated as the door survey we had done picked up on it.

I would advise that a floor survey be carried out to ensure it is suitably level for a concealed automatic threshold drop-seal to be fitted.