Fire Door Gap Tolerances: The 3mm Rule Explained

Why a few millimetres decide everything

In fire, the intumescent strip expands many times its size to seal the door-to-frame gap — but it's tested to bridge a defined gap. Beyond ~4mm, expansion may not close the opening before fire passes; hot gases accelerate through wider gaps and the 30-minute door fails in minutes. Too tight has its own failure: leaves bind seasonally, closers can't overcome the friction, and the 'fire door' spends its life ajar. The under-door gap matters mainly for smoke: cold smoke kills along corridors long before flame, which is why escape-route doors get smoke seals and tighter thresholds.

How to measure properly

• Use a gap gauge (we use calibrated wedge gauges) or the £1 coin (~3mm) for quick checks
• Measure at multiple points per edge — doors and frames are never uniformly straight
• Check the meeting stile gap on double doors — the most commonly out-of-spec edge
• Under-door: measure to finished floor including any threshold; new carpets are the classic gap-changer
• Record per-door measurements; trend drift between inspections tells you about building movement and hinge wear

How out-of-spec gaps are fixed

Oversized side/top gaps: re-hanging on packed hinges, frame adjustment, or fitting larger-section intumescent seals rated for bigger gaps; beyond repairable limits, leaf or frame replacement. Under-door excess: thresholds, drop-down seals (which self-lower on closing — the standard fix where smoke control meets uneven floors), or re-hanging. Binding doors: easing within strict limits — planing a certified door beyond its permitted adjustment voids it, so easing decisions belong with someone who knows the limits per door type. Every fix should end with re-measurement and a record.