Fire Door Gaps: How Much is Too Much?

Gap tolerances are one of the most straightforward aspects of fire door compliance, yet they account for a disproportionate number of inspection failures. The principle is simple: the gaps between the door and frame must be large enough for the door to open and close freely, but small enough to be effectively sealed by the intumescent strips and smoke seals when a fire occurs. Get the gaps wrong, and the entire fire door assembly is compromised.

The Tolerances

The standard gap tolerances for fire doors in the UK are set out in BS 8214:2016 and are consistent across most manufacturers' fitting instructions. The gaps at the head (top) and both jambs (sides) should be between 2mm and 4mm when the door is closed. The threshold gap — between the bottom of the door and the floor finish — should not exceed 8mm. Where a drop seal or threshold plate is fitted, some specifications allow up to 10mm at the threshold.

These tolerances are not arbitrary. They are directly linked to the performance of the intumescent strips fitted to the door. Standard 15mm x 4mm intumescent strips are designed to expand and fill a gap of up to 4mm. If the gap exceeds 4mm, the expanded intumescent material may not bridge the gap completely, creating a path for flames and hot gases. If the gap is less than 2mm, the door may bind in the frame, preventing the self-closer from latching it — which is arguably worse, as the door will stand open during a fire.

How to Measure

Measuring fire door gaps requires a gap gauge — a stepped metal tool with markings at 2mm, 3mm, and 4mm. Alternatively, a set of feeler gauges can be used. Close the door fully (the latch should be engaged) and check the gap at multiple points along each edge: top, middle, and bottom of each jamb, and at both ends and the centre of the head. Gaps can vary significantly along a single edge, particularly if the door or frame has warped or if the hinges have worn.

Record the measurements at each point. If any measurement exceeds 4mm at the head or jambs, or 8mm at the threshold, the door fails. If any measurement is below 2mm and the door is binding, that also requires remediation. Pay particular attention to the hinge side — as hinges wear, the door drops slightly, which typically widens the gap at the top on the hinge side and narrows it at the bottom. This is one of the most common patterns of gap failure.

Remediation

Fixing gap issues depends on the cause and severity. Small increases in gap size (up to about 5mm) can sometimes be addressed by replacing the intumescent strips with a wider specification — for example, swapping 15mm x 4mm strips for 20mm x 4mm — but this must be supported by the manufacturer's test evidence. Adjusting or replacing worn hinges can correct a dropping door and restore correct gap dimensions. If the frame has distorted, it may need to be removed and refitted.

For gaps that are significantly outside tolerance, the door or frame may need to be replaced. Attempting to pack out a frame with timber shims or filler is not an acceptable remedy — the frame must sit squarely in the structural opening and be fire stopped correctly around its full perimeter. Similarly, trimming a door to widen the gaps and prevent binding is only acceptable within the manufacturer's stated trim tolerance, typically 3mm per edge. Beyond that, a made-to-measure replacement is the correct solution.