Hi can you help please. I work in the care industry as a property manager and wish to confirm the protocols for a subsequent fit of spy glass in a client's bedroom door. So long as these are fire rated can these be self fit or do they require professional fit and certification. I would have thought the latter. Can you also confirm the max base gap allowed under a fire door.....Can it be up to 10mm. regards Kevin

There is a correct answer and a practical answer.

The correct answer is, that as you will have to drill all the way through the door, which will create a weakness, this will affect the fire resistance, which is impossible to predict to what extent, so the whole assembly should be subject to test.

On the other hand you have conflicting hazards of personal assault and fire safety perhaps it would be reasonable to balance these risks by using a fire resistance viewer, drill with care to ensure the exact fit without any gaps, and use intumescent paste around the viewer to pack the hole.

I suggest any decent joiner should be able to do this provided the RP applies a good level of instruction and supervision.

There are fire rated door viewers available at safelincs

If the fire door require cold smoke seals then the maximum threshold gap is 3mm +/- 1mm if this cannot be achieved then threshold smoke seal should be fitted, check out https://www.safelincs.co.uk/blog/2013/08/09/gaps-underneath-fire-doors/