Fire Door Gaps Too Big? Tolerances, Fixes and What They Cost
By the DC Fire & Security engineering team — installing and maintaining fire and security systems since 2010. Updated July 2026.
Quick answer
Fire door gaps should sit around 3mm (2–4mm is the working band) at the sides and top, and no more than 8mm under the leaf — closer to 3mm where smoke control matters. Anything bigger gets recorded as a defect at inspection. The fix is usually cheap: £250–£350 a door covers new seals, hinge adjustment or a drop seal. A new doorset (£1,350–£1,500) is the last resort, not the default.
The tolerances, in one place
These are the numbers inspectors carry in their heads. They come from test evidence — a fire door is only proven to hold back fire in the configuration it was tested in, and gap size is part of that configuration.
| Where | Allowed gap | What a fail looks like |
|---|---|---|
| Sides and top (leaf to frame) | 2–4mm, aim for 3mm | A visible wedge of daylight, or a £1 coin (about 3mm) sliding through with room to spare |
| Under the door (no smoke control) | Up to 8mm | New flooring fitted and the threshold gap now swallows your fingers |
| Under the door (smoke control, FD30S/FD60S) | About 3mm, or a fitted drop seal | Light showing under a flat entrance door |
| Meeting stiles (double doors) | 2–4mm, even along the join | Leaves that clash, or a gap that widens toward the top |
Why inspectors fail doors on gaps
Intumescent seals only work if the gap is small enough for the expanding strip to bridge it. At 3mm they close the gap early in a fire. At 6 or 7mm they may never seal it at all, and smoke — which kills long before flames arrive — streams through from the first minute.
This used to be caught once a year at best. Since the Fire Safety (England) Regulations 2022, residential buildings over 11 metres need quarterly checks on communal fire doors and annual checks on flat entrance doors, and gaps are the most common defect those checks record. Workplaces pick it up through the fire risk assessment. Either way, an oversized gap now gets written down, and someone is expected to deal with it.
What fixing an oversized gap costs
Prices below are our standard retail rates, +VAT. Most gap defects land in the first two rows.
| Fix | When it's the answer | Price (+VAT) |
|---|---|---|
| Smoke seal replacement | Perished, painted-over or missing brush/blade seals | £250 |
| Light remedial visit + pass certificate | Seals plus hinge packing, keeps, closer tweak — one visit, re-checked and certificated | £350 |
| Drop-down smoke seal | Threshold gaps up to about 14mm after flooring changes — beats planing a certified door | £250–£350 fitted |
| Carpenter day rate (re-hang, lipping, multiple doors) | Volume remedials across a block — a day covers several doors | £625/day |
| Full door refurbishment | Damaged leaf, several defects on one door | £950 |
| New FD30S doorset, supplied and fitted | Door beyond economic repair or unprovable certification | £1,350–£1,500 |
Never fill a fire door gap with foam or filler
Decorator's foam and general filler have no fire rating and no test evidence behind them. Packing a gap with them doesn't fix the defect — it hides it, and it tells the next inspector the door has been interfered with. The same goes for planing a door to cure binding: certified doors have permitted adjustment limits, and going past them voids the certification. If a gap can't be closed with seals or hinge work, the answer is lipping or a new leaf, sized and recorded by someone who knows the limits for that door type.
We see foam most often on flat entrance doors after a flooring or decorating job. It costs more to undo than the original fix would have cost to do properly.
Who this lands on
We inspect and fix across Luton, Bedfordshire, Hertfordshire and North London — one visit, priced per fix, with photos and a certificate at the end. Fire door inspections and remedials are separate line items, so the report stands on its own.
- Blocks of flats over 11m: the responsible person has to run quarterly communal and annual flat-door checks — oversized gaps are the defect those checks find most
- HMO landlords: door condition comes up at licensing inspections, and councils do measure
- Schools, offices, care settings: the fire risk assessment flags gaps and expects them closed off with evidence
- Anyone who's just had flooring fitted: new carpet or LVT routinely eats the threshold margin
Frequently Asked Questions
Is 5mm really a fail?
What gap should a fire door have under it after new flooring?
Do all fire doors need smoke seals as well as intumescent strips?
How often should gaps be re-checked?
Can I fix fire door gaps myself?
Does an oversized gap mean I need a new door?
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