Mercman,
Please, don't stand in awe of my qualifications, or anyone else's.
Most of my advice offered to AAM is experience-based.
Besides, any professional is only as good as their current knowledge.
Case in point - I didn't know that insurance companies required locks on some Fire doors in houses [mine doesn't] and I'd be obliged if you could clarify that by return.
We need to increase the AAM knowledge base...
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There may be a degree of confusion here between requiring a lock on a door which ALSO happens to be a fire door, and requiring a lock on a fire door because its a fire door.
Security and fire safety are often at loggerheads when it comes to buildings which are in operation.
So for example someone may want a lock on a garage door, to prevent casual entry to the house by roundabout means.
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That having been said, I didn't define my terms in the response you quoted and relied on the assumption that it was an inner room served by an accommodation door until it was called a fire door by the OP - IOW, people use this door.
Normally bathrooms are classed as areas of low fire risk and so fire doors in houses are seldom specified for bathroom enclosures.
However, if something is a fire door through which people pass, it normally
- cannot have a lock on it
- can only have a removable locking device - removedion times of operation [padlock and chain]
- must be one way secured to prevent access from the outside of the building [press bar lock] or
- be fitted with a tumbler lock.
The one instance of where locks are normally fitted in apartment buildings is to doors or hatches serving service installations or service ducts.
These are intended to be opened only by trained personally to reduce the risk of disruption of services or hazard.
A difficulty can arise for example when there are multiple lettings and fire stairs serve all storeys that need to be secured.
If one of the stairs is compromised by smoke, and you don't find out until you are in it, how can you get out if the fire door lock behind you?
A thorny one and there are several compromises but no ideal solution.
ONQ.
ONQ.
All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.