How to Check Fire Door Gaps (and Why They Matter More Than You Think)

Practical Guide #13

For: Installers, inspectors, site supervisors, and maintenance teams.

Why Fire Door Gaps Are Critical

In a fire, every millimetre matters.Gaps that are too wide let smoke and heat pass through. Gaps that are too tight can stop the door from closing and latching properly.

Fire resistance is only guaranteed when the door closes fully and holds its integrity under pressure — and that starts with correct gap tolerances.

1. Acceptable Fire Door Gap Sizes

Location	Ideal Gap	Tolerance	Notes
Top & sides	2 – 4 mm	±1 mm	Measured when door is closed
Bottom (threshold)	Up to 10 mm	See below	Depends on floor type and rating
Meeting stile (double doors)	2 – 4 mm	With or without rebated edges

🚪 Undercut at bottom: Timber or tiled floors = max 10 mm Carpeted floors = reduce undercut accordingly Doors with drop seals = must fully seal when closed

2. How to Measure Fire Door Gaps Accurately

Tools:

• Gap gauge (specialist plastic or steel gauge cards)

• Feeler gauges (mechanical engineers may use)

• Calipers or ruler (not ideal but passable)

• Visual check = rough, not reliable

Steps:

1. Close the door gently until it latches

2. Insert your gauge at three points on each side (top, middle, bottom)

3. Log or photograph any gaps outside spec

4. Repeat for:

   • Bottom undercut

   • Meeting stiles (if double leaf)

   • Intumescent seal alignment (should remain compressed when door is shut)

   
   
   
   

3. Common Gap Issues (and What They Mean)

Issue	Risk
>5mm at head or sides	Smoke & heat bypass seals
<2mm gaps	Door can bind — may not close/latch
12mm+ bottom gap	Fails smoke control and air pressure tests
Uneven gaps	Poor installation or warped leaf
Rebated meeting stile not aligned	Leaf-to-leaf seal breaks under pressure

4. Fixing Out-of-Spec Gaps

Too wide?

• Rehang the door with corrected hinges or packers

• Fit intumescent edge seals with brush/smoke inserts

• Add a drop-down seal to bottom edge

• Insert intumescent or acoustic strips into frame rebates

Too tight?

• Plane door edges within certification limits (often 3 mm max trim per edge)

• Check for frame swelling, warping or misplaced stops

• Adjust or remove excess paint build-up

⚠️ Warning: If trimming, use certified intumescent paint to recoat edges. Don’t trim more than allowed by the door-set certification.

5. Record It All

Gap measurement is now a common compliance check for:

• Fire risk assessments

• FRAEW reports (external wall systems)

• Insurance inspections

• Regulatory enforcement visits

Create a basic form or use a digital tool (like FireCheck360 Lite) to log:

• Door ID / Location

• Gap sizes (top/side/bottom)

• Pass/fail status

• Date checked

• Inspector initials/signature

Final Word

Gap checking is low effort, high impact.

A 6mm side gap may not seem like much — until smoke starts billowing through it in a corridor packed with sleeping residents.

Check them. Record them. Fix them. It's a quick win that could save lives.