Fire Door Gaps Too Big? Tolerances, Fixes and What They Cost

What is the maximum gap around a fire door?

4mm is the maximum at the sides, the top and between double doors. Aim for about 3mm, with 2mm as the practical minimum so the door does not bind.

These figures come from industry practice set out in BS 8214, the code of practice for fire doors, and from the government's fire door guidance for blocks of flats. The guidance says plainly that the gap should never be more than 4mm.

The door's own certification wins if it says something different. A door is only proven to the gaps it was fire tested with, so the manufacturer's fitting instructions are the final word.

How big can the gap under a fire door be?

Under the door, about 8mm is the usual maximum inspectors work to, and about 3mm where the door must also hold back smoke.

The government guidance only says the bottom gap should be as small as practicable. In practice most door test evidence allows up to about 8mm on a standard fire door. Smoke-control doors, marked with an S, need the gap closed to around 3mm or sealed with a drop seal, because cold smoke travels under the door long before any heat reaches it.

New flooring is the usual cause of trouble. Lift a carpet and the gap grows overnight. Trim a door to clear new carpet and it can easily pass 8mm once the carpet wears down.

Why does a 5mm gap fail a fire door inspection?

A 5mm gap fails because it is outside the 4mm limit, and at that size the seals may not close the gap in time to stop smoke and flame.

Intumescent strips swell when they get hot and fill the gap between the door and the frame. They are designed to bridge a gap of a few millimetres. Push the gap to 5mm, 6mm or 7mm and the seal may expand without ever meeting the other face. Smoke seals have the same problem. A brush or fin seal that cannot touch the door does nothing.

A large gap is also a sign of something else going on. Loose hinges, a dropped leaf, a warped door or a frame that has moved all show up first as uneven gaps.

How common are oversized fire door gaps?

Very common. In our inspections, 42% of doors measured had a leaf-to-frame gap over 4mm (156 of 371, Sept 2025 to Oct 2026), and 25% had a gap under the door over 8mm.

Gaps or misalignment were the single most common fault we recorded, on 39% of 448 itemised doors. A gap under the door that was too big, or a missing drop seal, came up on 16%. The full figures are on our fire door inspection statistics page.

Most of these doors were fixed rather than replaced. Across three finished programmes, 95% of failed doors were repaired.

How do you fix a fire door gap that is too big?

Most gaps are fixed by adjusting or packing the hinges, refitting the seals or adding a drop seal. Repairs usually cost £150 to £900 a door + VAT, depending on the fault and how many doors are done on one visit.

Never fill a fire door gap with expanding foam, decorator's caulk or wood filler. None of them are fire rated, they hide the fault from the next inspector, and they can stop the door closing. Planing a door to cure a tight spot can also take it outside its certification if too much comes off. See fire door repairs and fire door remedial costs for more detail.

How do you measure a fire door gap?

Close the door and check the gap at at least three points on each side and the top with a gap gauge or a set of feeler gauges.

A cheap plastic gap gauge is enough for routine checks. The government guidance says these checks are visual and do not need a specialist. A full inspection by a qualified inspector goes further and records each gap in millimetres.

• Measure with the door fully closed and latched.
• Check top, middle and bottom on both sides, and three points across the head.
• Measure the bottom gap at both corners and the middle.
• On double doors, check the meeting stiles top to bottom.
• Write the readings down so you can see if a door is moving over time.

Who has to check fire door gaps, and how often?

In blocks of flats over 11 metres, communal fire doors must be checked at least every 3 months and flat entrance doors at least every 12 months, under the Fire Safety (England) Regulations 2022.

In other buildings the Regulatory Reform (Fire Safety) Order 2005 requires the responsible person to keep fire doors maintained in good working order. The fire risk assessment sets how often they are checked. Schools, offices, care homes and HMOs commonly have doors checked every 6 to 12 months, and more often where doors take heavy use. Our guide on how often fire doors need inspecting covers this in detail, and the fire door inspection checklist shows what to look for.

We inspect and repair fire doors across Bedfordshire, Hertfordshire, London, the Thames Valley and the East Midlands.

Sources

• GOV.UK: Fire Safety (England) Regulations 2022, fire door guidance (updated 8 August 2025)
• The Fire Safety (England) Regulations 2022
• Regulatory Reform (Fire Safety) Order 2005
• Approved Document B: Fire safety
• BS 8214 code of practice for fire doors (2026 edition, replacing the 2016 version), FPA report
• DC Fire & Security: fire door inspection statistics

Updated October 2026 by Sebastian Grabowski, DC Fire & Security. We inspect and repair fire doors across Bedfordshire, Hertfordshire, London, the Thames Valley and the East Midlands. Our own figures: fire door inspection statistics 2026.