How to Seal Gaps Around Fire Door Frames (Legally and Effectively)

Practical Guide #11

For: Joiners, installers, maintenance teams, compliance officers

Why Sealing Frame Gaps Matters

Fire doors rely on precise installation. Even a top-quality FD60 door-set won’t perform correctly if it’s surrounded by 10mm gaps and foamed-in like a kitchen unit.

The gap between the fire door frame and the structural opening is critical. If incorrectly sealed, it can:

• Compromise fire and smoke resistance

• Invalidate third-party certification

• Lead to inspection failure or enforcement action

Let’s walk through when, how, and what to seal with — correctly.

1. Understand the Tolerance First

The frame-to-wall gap should ideally be:

Wall Material	Acceptable Gap (per side)	Notes
Masonry / concrete	≤10mm	Common in commercial installs
Timber / stud	≤5mm	Used in timber partitions
Steel stud / MF	≤5mm	Requires fire-rated backing

🔥 If the gap exceeds these tolerances, check the door-set manufacturer’s installation instructions or EXAP documents (Extended Application data) — they might allow special products or packing.

2. Approved Gap-Filling Materials

You can’t just grab a foam gun or slap in pink mastic.Here's what’s typically approved — check certification to be sure:

Material	Usage	Notes
Fire-rated PU foam (e.g., Illbruck FM330, Soudal FR)	Gaps ≤10mm	Must be cut back and sealed over
Mineral wool (e.g., Rockwool, Knauf)	Larger voids or irregular gaps	Must be compressed and sealed
Intumescent mastic (e.g., Nullifire FS703, Everbuild Fire Mate)	Surface sealing over mineral wool or foam	Must be CE marked
Fire-rated acrylic sealants	For acoustic/smoke performance	Often used in schools/hospitals
Timber packers (fire rated or hardwood)	Support and fixing	Avoid plastic or softwood shims

3. How to Seal the Gap (Step-by-Step)

1. Inspect and clean the cavityRemove debris, crumbling mortar, loose foam or plaster.

   
   

2. Pack the gap appropriately

   
   

   • For small, even gaps: use backing rod or mineral wool

   • For larger gaps: fire-rated foam (if approved)

   • Always support the frame mechanically — don’t rely on foam to hold weight

     
     

3. Cut back excess foam flush to the wall (if used)

   
   

4. Seal with mastic or acrylic

   
   

   • Apply a consistent bead around the entire perimeter

   • Use a spatula or finger to tool the joint smooth

   • Label if required (e.g., “Fire Rated Seal” near the frame)

     
     

5. Apply architraves or facings

   Optional, but check the certification — some door-sets require a specific type or depth of facing.

   
   
   
   

4. What Not to Do

• ❌ Don’t use expanding foam from a DIY store unless fire-rated and certified

• ❌ Don’t leave foam unsealed or exposed

• ❌ Don’t over-pack mineral wool — it must remain compressible

• ❌ Don’t cover up massive gaps — build out the wall or consult the fire engineer

• ❌ Don’t assume a “pink label” = compliant — check datasheets and test evidence

5. When to Get Sign-Off

If you're unsure about the seal detail:

• Refer to the manufacturer’s installation instructions

• Contact the certifying body (BM TRADA, IFC, etc.)

• Log a photo and mark it for review

This is especially important for:

• FD60 or FD90 installs

• Steel frame installations

• Retrofitted doors in listed or heritage buildings

Final Word

Getting the frame-to-wall detail right is just as important as hanging the leaf. A fire door is a system — and it’s only as strong as its weakest edge.

Take a few minutes to get this right, and you avoid failed inspections, costly rework, and dangerous non-compliance.