Guide · updated 9 October 2026
Fire door gaps explained
Short answer: Gaps at the head and edges of a fire door are typically expected to be 2–4 mm, with 3 mm as a practical target, unless the door's certification says otherwise. Under-door gaps depend on the door, and smoke control doors need a tighter threshold or a drop seal.
- Head and edges: typically 2–4 mm
- Door evidence takes precedence
- Smoke doors: tight threshold or drop seal
- Most common reason doors fail
What gap is acceptable around a fire door?
The gap between a fire door leaf and its frame is typically expected to be between 2 and 4 mm at the head and both edges, with about 3 mm as a practical target. The door's own certification or test evidence always takes precedence if it gives a different tolerance.
How big can the gap under a fire door be?
The threshold gap depends on the door. Doors without smoke control often allow a larger gap, in some guidance up to around 8 to 10 mm. Smoke control doors, marked with an S such as FD30S, usually need a gap of 3 mm or less or a tested drop-down seal. Check the door's evidence.
Why do gaps matter so much?
Fire and smoke travel through gaps. Intumescent strips are designed to seal a particular gap size; if the gap is larger, the strip may not close it in time. Excessive gaps between leaf and frame were the most common fault in Fire Door Inspection Scheme data published in 2022, covering doors inspected in 2021.
Why do gaps open up over time?
- Hinges wear and the leaf drops, opening the head gap on the closing side.
- Frames shrink, move or were poorly fixed.
- Leaves warp, especially near heat or damp.
- Doors are planed down after new flooring is laid.
How are gaps measured?
With feeler gauges or a gap gauge at several points along each edge with the door closed. One reading per edge is not enough, because doors are often tight at one end and wide at the other.
How are excessive gaps fixed?
The cause is fixed first: hinges re-packed or replaced, frames re-fixed, or hardwood lippings fitted where the door's evidence allows. Oversized seals used to hide a poor fit are not a reliable repair. If the door cannot be brought within tolerance, replacement is the answer.
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Related questions
What gap is allowed around a fire door?
Most guidance and manufacturers' evidence expect gaps of about 2 to 4 mm at the head and edges, with 3 mm as a practical target. The door's own certification or test evidence takes precedence if it states a different figure.
How big can the gap under a fire door be?
It depends on the door's evidence. Doors without smoke control often allow a larger threshold gap, sometimes up to around 8 to 10 mm, while smoke control doors usually need 3 mm or less or a tested drop seal.
Can intumescent strips be painted?
Brush and fin smoke seals should never be painted, because paint stops them working. Some intumescent strips tolerate a light coat, but the safest rule is to mask all seals before decorating and replace any that have been painted.
What is the difference between an intumescent strip and a smoke seal?
An intumescent strip expands in heat to seal the gap during a fire. A smoke seal, often a brush or blade, blocks cold smoke at lower temperatures. Smoke control doors, marked with an S such as FD30S, need both.
Sources checked 9 October 2026: GOV.UK Fire Safety (England) Regulations 2022 fact sheets; BSI and FPA announcements on BS 8214:2026; FDIS published inspection data. This guide is general information, not legal advice.
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