How to Check the Fire Door Gap and Why It Matters

Practical Guide #19

For: Installers, inspectors, caretakers, housing officers

Why the Gap Is Critical

The gap between the fire door and the frame must be tight enough to:

• Prevent smoke and flames from passing through

• Allow the intumescent seals to activate correctly

• Let the door close smoothly without binding

Too wide? Smoke and flames break through.

Too tight? The door may not close or latch — and could even warp under pressure.

Either way = non-compliant.

1. The UK Standard Fire Door Gap Tolerances

According to BS 8214 and BS 476:

Position	Tolerance
Sides & top	2mm – 4mm
Bottom (threshold)	Typically up to 10mm (unless a smoke seal is required — then ≤3mm)

Ideal total perimeter gap = 3mm, consistent on both sides and top.

2. How to Measure the Gap

Tools:

• Gap gauge (ideal)

• Feeler gauge or calibrated wedge

• Steel rule (in a pinch)

• Flashlight or torch (for visual assist)

  
  

Steps:

1. Close the door slowly

2. Check top and both sides — is the gap uniform?

3. Insert feeler gauge and test resistance

4. Shine a torch from one side to spot light bleed

5. Measure the bottom gap (check if threshold seal is present — this affects tolerance)

   
   
   
   

3. Common Fail Points

Issue	What It Means
Gap >4mm at top/sides	Smoke and fire compromise risk; seals may not engage
Gap <2mm	Risk of binding, sticking, incomplete closure
Bottom gap >10mm	Unacceptable unless permitted by doorset spec
Inconsistent gap	Poor fitting or hinge issues
Gap varies over time	Door or frame may be warping, swelling, or shifting

Fire door gaps aren’t just a spec — they’re a life-saving detail.

4. Adjusting an Out-of-Spec Door Gap

Depending on the cause, use:

• Hinge packers to adjust alignment

• Plane or sand the door edge (fire door rated tools only!)

• Re-bed or adjust frame if door is severely misaligned

• Replace door if irreparably out of tolerance or not approved for trimming

Important: Fire-rated doors have strict trimming limits — often no more than 3mm per edge, and sometimes none at all.

Check the manufacturer’s datasheet before modifying.

5. Reporting Format for Gap Issues

Example log format:

• Door ID: FD-012

• Gaps:

  • Left: 3mm

  • Right: 6mm ❌

  • Top: 3mm

  • Bottom: 12mm ❌

• Notes: Over-trimmed at bottom, inconsistent gaps right side

• Action: Recommend replacement or professional refitting

• Inspector name & date

Final Word

In a fire, pressure builds fast — and the smallest weakness can become a fatal flaw.

A few millimetres too much… and smoke finds a way through.

Measure. Log. Adjust if permitted. Replace if needed.

Because in fire door compliance, precision isn’t optional — it’s everything.