Fire Door Gaps — What Is Acceptable Under BS 8214?

The numbers

Under BS 8214:2016 (Timber-based fire door assemblies — Code of practice), acceptable gaps for fire-rated doors are:

• Head and jambs — 2-4 mm
• Meeting stiles (double doors) — 2-4 mm
• Threshold (door to floor) — 3 mm (or up to 8 mm if no fire-resisting floor finish)

These are the gaps measured between the door leaf and the frame, not between the door and the floor below the frame.

Why the gap matters

Intumescent seals work by expanding under heat to fill the gap and prevent fire and smoke passage. They are typically rated for gaps of 1-5 mm. Beyond 5 mm, even the best intumescent strip will struggle to bridge the gap.

A 1mm gap that should be 3mm will bind, prevent self-closing and ultimately get propped open. A 6mm gap that should be 3mm provides a smoke and fire path.

How to measure

Use a calibrated gap gauge or feeler gauge — not a credit card. Measure:

• Top edge at three points (left, centre, right)
• Hanging jamb at three points (top, middle, bottom)
• Closing jamb at three points
• Threshold at three points

Any single reading outside the tolerance is a defect that must be remedied.

Common defects

The most frequent BS 8214 defects on our inspections:

• Bottom gap >10 mm — usually after carpet replacement; threshold drops below door level
• Top corner gap >5 mm — door has dropped on its hinges
• Closing stile gap >5 mm — frame has racked or door has warped
• Meeting stile gap <2 mm — paint build-up; doors will bind
• Inconsistent gaps — door is hung incorrectly or frame is out of square

What to do about excessive gaps

Options, in order of preference:

1. Re-hang the door — often a hinge or frame adjustment is enough 2. Replace the door leaf — if warping is the cause 3. Replace the door set — for systemic failures 4. Add additional intumescent products — only as an interim measure, must be on a tested system

Under no circumstances should you screw additional timber to a fire door to reduce the gap — this voids the fire rating.

Documentation

Defensible fire door inspection records under Regulation 10 should include:

• Gap measurements at each location
• Photographic evidence
• Defect priority rating
• Remediation plan with target dates
• Closure evidence on completion

MetroFire delivers FDIS-qualified fire door inspections to BS 8214 across London and Essex. Book a survey.