TL;DR
- A fire door is certified as a complete doorset: leaf, frame and ironmongery tested together. A rated leaf hung in an untested frame is not a fire door.
- FD30 holds fire and smoke back for 30 minutes, FD60 for 60. FD30 covers almost everything in an ordinary house.
- The gap around the leaf should never be more than 4mm, and as small as practicable at the floor. That one measurement fails more doors than any other fault.
- Glazing is allowed, but the glass, beads and seals have to be part of the same tested assembly. A glazed fire door is not an ordinary door with fire glass dropped into it.
Short version: the leaf is the cheap part. What makes it a fire door is everything bolted to it, and that is the part most people buy last.
Most homeowners meet fire door regulations halfway through a project, usually when building control asks a question nobody was expecting. The answer is narrower than it sounds. In a normal two storey house you may need none at all. Add a third storey, convert the loft, or put a door between the house and an integral garage, and you need a proper one, in the right place, fitted to a tolerance you can measure with a coin.
What actually makes a door a fire door
Not the leaf on its own. Government guidance on Approved Document B puts it plainly: a fire doorset should be regarded as a complete installed assembly, so the door, the frame and any ironmongery all count when you judge whether it is suitable.
That sentence quietly rules out the most common mistake in home renovation. Somebody buys an FD30 leaf, hangs it in the existing frame on the existing hinges, and assumes the job is done. It is not. The hinges have to be rated. The latch has to hold the leaf shut under pressure. The frame needs a proper stop, and the intumescent strip that swells and seals the gap has to be present and unpainted. Miss one of those and the certificate the leaf came with no longer describes what is on your wall.
The ratings themselves are simple enough. FD30 means the assembly resisted fire for 30 minutes under test. FD60 means an hour. An “S” suffix, as in FD30S, adds a cold smoke seal, which matters more than people expect, since smoke reaches you long before flame does. Testing is done to BS 476 Part 22 or the European equivalent, BS EN 1634-1, and the rating belongs to the tested configuration rather than to the door as an idea.
| Rating | Resists fire for | Where it usually turns up in a house |
|---|---|---|
| FD30 | 30 minutes | Loft conversions, third storey landings, house to garage |
| FD30S | 30 minutes plus smoke seal | Flat entrance doors, doors onto shared corridors |
| FD60 | 60 minutes | Larger buildings, some basements, commercial conversions |
Glass in a fire door, and what it changes
Glazed fire doors are the part people assume is a compromise, and they are not. A vision panel is often the only reason a fire door is bearable in a hallway with no other light, and certified fire resisting glass has been standard for years. What changes is that the glazing becomes part of the tested assembly rather than a detail chosen afterwards.
So the glass, the beads holding it, the gasket and the maximum pane size were all fixed at the point of test. You cannot enlarge the pane, swap the beading for something prettier, or reglaze a damaged panel with ordinary toughened glass and keep the rating. Steel framed designs have become popular here for a practical reason: a slim steel section carries a large certified pane without the chunky timber surround that makes a glazed fire door look like a fire door.
The tested configurations are normally published alongside the product, as they are for the internal fire rated range at https://blacksteeldoors.co.uk/internal/fire-rated/, and that is the page to read before a size gets agreed with anybody. Sizes drive the rating far more than finish does, and finding out afterwards is expensive.
A steel framed glazed fire door is easier to understand once you have seen one hung and closing, so this short manufacturer walkthrough is worth two minutes.
Video: https://youtu.be/rMdkbUKezuo
Where the rules actually bite in a house
Three situations account for nearly every fire door in a private home.
The first is the third storey. Convert a loft in a two storey house and you have created a three storey dwelling, which changes the escape route. The stairway becomes a protected escape route and the doors opening onto it have to hold fire back long enough for everyone above to get past. This is the single most common reason a fire door appears in a renovation, and it applies to the existing floors as well as the new one. Approved Document B volume 1 covers dwellings and is the document your building control officer will be working from.
Then there is the integral garage. A door between a house and an attached garage separates the living space from where the petrol, the paint and the car are, so it gets a fire door and a self closer.
Flats are the third case, and they carry their own duties. Internal doors in flats are treated differently again, which is worth knowing before you fit closers everywhere: the official FAQ on Approved Document B confirms door closers are not required on internal fire doorsets in flats used for lobby protection. Plenty of people fit them anyway and then live with doors that slam.
Cupboards and bathrooms off a protected stairway are the grey area, and the answer depends on the layout. Ask before you order, not after.
The 4mm rule
Government guidance on checking fire doors states that the industry standard is a gap of no more than 4mm between door and frame, and that at the bottom of the door the gap should be as small as practicable. The full checking guidance sits on gov.uk and takes about ten minutes to read.
Four millimetres is roughly two pound coins stacked. It sounds fussy until you understand what the intumescent strip is doing: it expands to many times its thickness and closes the gap, and it can only close a gap it can reach. Trim the leaf to fit a bowed old frame and you have quietly moved the door outside the conditions it was tested in.
The other faults on the official checklist are equally unglamorous. Damaged or painted over intumescent strips. Missing or loose hinge screws. A leaf that does not fully close into the frame from any angle. Cracked or replaced glazing. None of it is difficult, all of it gets missed.
What to ask before you buy
Ask for the test evidence rather than the marketing. Which standard was it tested to, what leaf sizes does that test cover, is the frame included, and what glazing configuration was in the door when it passed. A supplier who cannot answer those four questions in writing is selling you a door, not a doorset.
Then ask who is fitting it. The fitting is where the rating usually gets lost, and a certified door fitted badly is worth less than an honest solid core door, because it invites you to stop worrying about it. If the fire door is part of a bigger project, sequence it properly instead of leaving it to the end. Our renovation project guide covers where a decision like this fits in the running order.
Twenty or thirty minutes is what the whole exercise buys the people upstairs, and it rests on details you can check with a tape measure and five minutes of attention.