Fire Door Letterplates and Letterboxes: The Rules

Can you fit a letterbox to a fire door?

A letterplate — the metal plate — and its slot are cut clean through the door leaf, so a letterbox is an aperture through the door, in the same family as a vision panel or glazed light. Every aperture is a potential weakness in a barrier whose whole job is to hold back fire and smoke for a rated period. On a fire door that leaves one acceptable option: an intumescent, fire-rated letterplate that is part of, or has been proven compatible with, the specific doorset's tested and certified specification. A standard household letterbox is not acceptable, and neither is cutting a new aperture into a certified door on site unless the door's certification expressly allows it.

The BWF Fire Door Alliance is explicit that a fire door "is tested as a complete assembly or doorset in a test furnace and can only work correctly if installed using the same compatible components as when it was tested." It classes letterplates and viewers as non-essential hardware — items that "may also be fitted – but they must be correctly installed, in accordance with the door manufacturer's instructions." Non-essential does not mean fire-neutral: a letterplate puts a hole in a door that was never tested with one unless the manufacturer's evidence says otherwise.

How does an intumescent fire letterplate work?

A fire-rated letterplate is built around an intumescent lining fitted inside the aperture and around the plate. Intumescent materials stay inert at normal temperatures but expand rapidly when heated in a fire, in the same way as the intumescent strips housed in the door edge. As a fire develops, the lining swells to fill the slot and the gap around the letterplate, closing the opening against flame and hot gases so the door can go on resisting fire for its rated period. A standard letterbox has no such lining: its open metal throat is a direct path for flame and hot smoke straight through the leaf.

Intumescent only activates once it gets hot, so on its own it does nothing about cold smoke in the early minutes of a fire — which is when smoke, not flame, does most of the harm to escape routes. Many flat entrance doors are specified as smoke-control doors (the Sa suffix in Approved Document B, giving ratings such as FD30S). On those doors the letterplate must also restrict the passage of ambient-temperature smoke, typically through an integral brush or flap seal — and that smoke-sealing letterplate has to be part of the configuration the doorset was tested and certified to as a smoke door.

Why does cutting a letterbox into a certified fire door void it?

A fire door earns its rating as a complete, tested configuration — leaf, core, frame, seals, glazing and hardware in one specific combination. Cutting a slot through the leaf both removes material from, and adds an opening to, that tested configuration. England's Approved Document B warns in Appendix B that "small differences in detail" between what was tested and what is installed "might significantly affect the performance" — an unauthorised aperture is not a small difference. The door in the wall is no longer the door that passed the furnace test, and its certification no longer describes it.

That is why a fire letterplate can only go into a fire door on the manufacturer's terms. The door's certificate or field-of-application evidence has to permit a letterplate, in the position and size range it sets out; some door cores need the aperture to be lined before the plate is fitted; and where a certification scheme allows apertures to be cut on site, it is only within the scheme's rules and using an approved letterplate. Retro-fitting a letterbox into a certified door you are not otherwise allowed to trim or cut is exactly the kind of on-site alteration that turns a certified doorset into an unproven one.

What certification and evidence should a fire letterplate have?

Because a letterplate is non-essential hardware fitted within a tested doorset, the evidence that matters is the door's evidence, not a letterplate sold in isolation. A fire letterplate should be one the doorset manufacturer's certification or field-of-application report permits — tested as part of, or assessed as compatible with, that doorset. The BWF Fire Door Alliance puts compatibility at the centre: the door "can only work correctly if installed using the same compatible components as when it was tested," and non-essential hardware "must be correctly installed, in accordance with the door manufacturer's instructions."

The underlying fire test is the doorset's own. BS EN 1634-1 (classified under BS EN 13501-2) is the current European method, and historically BS 476-22 — a route Approved Document B removes in England with effect from 2 September 2029, so new evidence and assessments should be built on the EN route. Warringtonfire makes the same practical point as the BWF: fire door performance is proven by testing a complete assembly, and substituting components the door was never tested with breaks the chain of evidence.

On a certified doorset, the certification label or plug ties the door back to its test and assessment paperwork. A responsible person or installer should be able to trace a fitted letterplate to that evidence — confirming the door was permitted to carry one, and that the plate fitted is the type the evidence covers. The BWF's label guidance is what makes that link possible. If the trail does not exist, treat the letterplate as unproven until it does.

What does the Regulation 10 flat entrance door check include?

In England, the Fire Safety (England) Regulations 2022 place duties on the responsible person for buildings containing two or more sets of domestic premises and above 11 metres in height. Under regulation 10, they must "use best endeavours to undertake checks of fire doors at the entrances of individual domestic premises... at least every 12 months," and check "any fire doors in communal areas... at least every 3 months." The government's fire door guidance stresses these checks "should be simple and basic. You should not need to engage a specialist to carry these out" — they are visual and do not involve tools.

The flat entrance door check specifically covers the door's apertures. Among other things, the checker should confirm that:

• Letterboxes are firmly closed and not jammed open — and that any letterbox is fitted correctly and undamaged.
• There is no damage to the door's glazing, and no unauthorised alterations to glazing apertures or air transfer grilles.
• The door has not been replaced with a non-fire door, and no alterations have been made that could affect its fire-resisting qualities (for example a hole where a lock has been removed).
• Intumescent strips and smoke seals are fitted and intact, gaps around the door look consistent, and the self-closing device shuts the door fully onto the latch.

The guidance is specific about newly added letterboxes: "Where a letterbox has been fitted to a door that did not previously have one, the resident will need to confirm that the new letterbox is suitable for use in fire-resisting doors and has been fitted by a specialist contractor." In other words, an added letterbox is not assumed compliant — it has to be shown to be.

What should you do if a flat entrance door has a non-compliant letterbox?

A flat entrance fire door is often the resident's own front door, but it protects the shared escape route, so a suspect letterbox is a building-safety issue, not just a private one. If a check finds a standard letterbox on a fire door, a letterbox that is jammed open or damaged, or one added to a door that previously had none, treat it as a potential defect and follow it up — do not modify the door on the spot.

1. Record it as a finding in the regulation 10 check, noting the flat, the door and exactly what was seen.
2. Ask the resident to confirm the history — if a letterbox has been added, whether it is one "suitable for use in fire-resisting doors" and was "fitted by a specialist contractor," together with any paperwork.
3. Check it against the door's evidence where the doorset is certified: does the certification permit a letterplate, and is the fitted plate within that scope?
4. Escalate to the [responsible person](/compliance/who-is-the-responsible-person), who decides on a competent assessment and any remediation, informed by the building's fire risk assessment.
5. Remediate through a competent contractor — replacing the non-compliant letterplate, or the doorset, so the aperture is once again part of a certified, tested configuration. Do not cut, enlarge or "make good" the aperture as a DIY fix.