A landlord fits a plain brass letterbox into a brand-new FD30 fire door because it matches the handles. The door still looks rated. It isn't. The moment that hole was cut without an intumescent liner, the door's 30-minute certification was voided, and no fire risk assessor is going to miss it on the next inspection.
Any uncertified aperture cut into a fire door breaks its rating, and a standard letterbox is exactly that. A fire-rated intumescent letter plate seals shut under heat within seconds, restoring the door's protection. For HMOs and communal entrances, this is a five-minute, sub-£25 fix that stops a routine EICR or fire door inspection turning into a C2 finding.
Why the Hole Matters More Than the Door
A fire door is only as good as its weakest opening. The door leaf, frame, intumescent seals around the edges and the self-closer are all tested together as a certified assembly. Cut an uncertified hole through that leaf, for a letterbox, a spyhole, or a cat flap, and you've created a direct path for fire and smoke that the certification never accounted for. The door can still carry an FD30 label on the edge and fail completely at the one point someone actually cut into it.
This is exactly why the products sold as "letterboxes" for fire doors aren't just a brass plate with a flap. A fire-rated intumescent letter plate carries its own liner that expands under heat and seals the aperture shut, tested separately to BS 476-20/22 so the whole door assembly keeps its rating even with a hole through the middle of it.
| Fitting | Standard letterbox | Fire-rated (FD30) letter plate |
|---|---|---|
| Fire rating impact | Voids door certification | Maintains FD30 rating |
| Behaviour under heat | Stays open, flap melts or warps | Intumescent liner expands and seals |
| Standard tested to | None | BS 476-20/22 |
| Typical price | £8-£15 | £19.99 |
This is one of the most commonly missed items on a fire door inspection. A door can pass every other check, correct leaf, correct seals, self-closer working, and still fail because of an uncertified letterbox fitted years earlier during a routine refurb. Under the Approved Document B fire safety guidance, any penetration through a fire-resisting door must maintain the door's overall performance, and a standard domestic letterbox was never tested to do that.
What to Check Before Your Next Inspection
If a property has fire doors on flat entrances, communal corridors or HMO bedrooms, the letterbox on each one is worth a five-minute check before it becomes someone else's finding.
- Confirm every fire door with a postal letterbox has a certified fire-rated plate, not a standard domestic one fitted during an earlier refurb
- Match the plate's fire rating to the door's own rating, an FD30 plate on an FD60 door doesn't carry the door's full protection
- Check the cut-out size before ordering, the 10" and 12" variants use different internal slot dimensions and aren't interchangeable without re-cutting the door
- Keep the product's fire test certificate with the property's fire door log, an assessor will ask for it alongside the door's own certification
For the wider fire door compliance picture, our guide to fire door hold-open devices covers the self-closer side of the same problem, and our Fire Safety Equipment Buying Guide covers passive protection more broadly, including the intumescent fire and smoke seals and intumescent mastic that go around the frame while the letter plate handles the aperture.
Frequently Asked Questions
Auditing Fire Doors Across a Property?
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