Glazing, Letterboxes and Cat Flaps in Fire Doors: What's Allowed?

Vision panels and glazing

Glazed apertures are fine when they're part of the tested door design: fire-resisting glass (Georgian wired, ceramics like Pyran/Keralite, or laminated intumescent types such as Pyrostop for insulation ratings) in the certified glazing system — beading, intumescent liners and fixings exactly per specification. The recurring failure: a broken vision panel reglazed by a general glazier with toughened or float glass, visually identical, worthless in fire. Glazing stamps in the corner of the pane are what inspectors hunt with a torch; unstamped glass in a fire door goes on the remedial schedule.

Letterboxes, spy holes and air transfer

• Letterplates: only intumescent-lined fire-rated letterplate assemblies (tested to the rating, fitted per method) — standard hardware-store letterboxes through a flat entrance door are a certification void
• Spy holes: fire-rated viewers exist and are the only acceptable type; an ordinary drilled viewer is a 12mm hole through the core
• Air transfer grilles: intumescent grilles that close in fire are available for specific doors (plant rooms etc.) within tested scope — ordinary vent grilles never
• Numerals, knockers, chains: surface-fixed with appropriate screws is generally fine; through-bolting and deep fixings follow the door manufacturer's guidance
• Cat flaps: none are fire-rated. In a fire door, a cat flap means a failed door — the answer in flats is a different cat arrangement, not a modified door

Modifying certified doors: who may cut what

Certification covers a tested configuration — leaf, frame, seals, ironmongery, apertures. Cutting outside it (new glazing where none was tested, oversized letterplate apertures, trimming beyond allowances) ends the certification, whoever does it. Within scope, work should follow the manufacturer's instructions, use rated components, and be done by competent installers who document it — certificated fire door installers can also perform certain licensed modifications under scheme rules. For flat entrance doors especially, the cheap path (modify on site) and the defensible path (doorset specified with the aperture from manufacture) are different decisions — we spec doorsets with letterplates and viewers built in for exactly this reason.