A fire door is not a thicker ordinary door. It is an engineered assembly whose core, edges, seals, glazing and ironmongery were tested together, and the reason it resists fire is mostly invisible from the outside. That is also why identifying one by look or by weight is unreliable.
The fire door leaf
Most certified fire door leaves use a solid core, commonly a particleboard, flaxboard or mineral core chosen for how it behaves in heat, with lippings on the edges and facings on both sides. The core is what resists the fire and the lippings hold the edges together as the leaf moves. Modern cores can be lighter than a heavy old hardwood door with no rating at all, which is why weight tells you nothing.
The rest of the fire door components
The frame, fixed to the wall in a tested way with the perimeter gap correctly sealed. The intumescent and smoke seals in the leaf or the frame. Three hinges as a minimum on most doorsets, of a tested type and fully screwed. The closer. The latch. Any glazing with its beads and intumescent glazing tape. Any air transfer grille or letter plate. Every one of these is part of what was certified.
Identifying a fire door
Look for a label or coloured plug on the top edge or hinge edge. Look for intumescent seals in the leaf or frame edge. Count the hinges and check they are fully screwed. Check for a closer. None of these alone proves a rating, and only the label gives you the evidence. A door with seals and a closer but no identification is a door somebody intended to be a fire door and nobody can prove is one.
Why fire door strips cannot upgrade an ordinary door
Fitting intumescent strips to an ordinary door does not make it a fire door, because the core, the lippings and the facings were never tested. Upgrade systems exist for specific door types, typically for historic doors where replacement is not acceptable, and they are assessed products with their own evidence. Outside those, the answer to an unrated door on a compartment line is replacement.
Other questions people ask
How can I tell if a door is a fire door?
Look for a certification label or coloured plug on the top or hinge edge. Seals, three hinges and a closer are supporting evidence but only the label is proof.
How heavy is a fire door?
Heavier than a hollow core door, but weight is not a test. Modern rated leaves can be lighter than old unrated hardwood doors, so weight should never be used to identify one.
Can I turn an ordinary door into a fire door?
Not by adding strips. Upgrade systems exist for specific cases, typically historic doors, and they are assessed products with their own evidence. Otherwise the door is replaced.
What is the fire door weight of a typical leaf, and why does it matter?
A typical thirty minute leaf is much heavier than an ordinary internal door and a sixty minute leaf heavier again, which is why the hardware and the frame are part of the certification rather than details. Weight decides the number and grade of hinges, whether the frame fixings will hold over years of use, and whether the closer can shut and latch the door from any angle without being wound up so tight that people prop it open. A fire door propped open is the commonest defect on any inspection, and a closer fighting an overweight leaf is one of the reasons why.