A fire door is not a door. It is a doorset: leaf, frame, intumescent seals, hinges, closer, latch, glazing and any other ironmongery, tested together as an assembly. The certification belongs to the assembly, which is why changing one component can end it, and why the question is never whether the leaf is fire rated but whether the whole thing is still as tested.
The leaf and the frame
The leaf carries the core that provides the integrity. The frame has to be of a compatible construction and fixed to the structure in the way the certification requires, with the correct packing and fixings at the correct centres. A certified leaf in a frame that was fixed with foam and no packers is not a fire door, and this is among the most common installation defects found.
Seals
Intumescent strips expand when heated and close the gap between leaf and frame, which is what keeps the flame out once the timber has shrunk back. Where smoke control is required as well, a brush or fin smoke seal is combined with the intumescent. Seals have to be continuous, in the groove they were designed for, unpainted and undamaged.
Hinges, closer and latch
Hinges have to be of the grade and number the certification requires and fitted with the specified screws into the specified material. The closer has to shut the door fully from any angle, including the last part of the swing, and the latch has to engage so that the door is held closed against the pressure a fire produces. A door that closes but does not latch has failed.
Gaps, glazing and the rest
The gap around the leaf has to sit inside the range the certification states, at head, jambs and threshold. Glazing has to be a tested fire resisting glass in a tested bead and glazing system. Air transfer grilles, letterplates, kick plates, locks and closers all have to be products tested with a door of that type, which is why the manufacturer's installation instructions are a compliance document rather than a suggestion.
Questions people ask about fire door requirements
What are the fire door requirements uk law actually sets?
The law sets outcomes: adequate limitation of internal fire spread when work is done, and maintenance in efficient working order afterwards. The component detail comes from the guidance and from the door's own certification.
Does every internal door need to be a fire door?
No. Fire doors go where the guidance requires a compartment or a protected route to be maintained. Fitting them elsewhere costs money and creates maintenance obligations without adding safety.
Can components be mixed between manufacturers?
Only where the certification covers it. Third party certified doorsets state which hardware may be used, and substituting outside that list puts the burden of proving equivalence on whoever did it.