How to Check Intumescent Seals (and Know When They’ve Failed)

15 x 4mm Double Fin (centred) Intumescent Strip

Practical Guide #18

For: Installers, inspectors, caretakers, housing staff

What Do Intumescent Seals Actually Do?

When exposed to heat, intumescent seals expand rapidly — filling the gap between the door and frame. This slows down the spread of smoke, flame, and hot gases, buying critical escape time and protecting the structure.

But seals only work if they:

• Exist

• Are continuous

• Are the right type and size

• Haven’t been damaged, painted, or replaced incorrectly

1. Where to Find Intumescent Seals

Most commonly fitted:

• In the groove of the frame (edge of the door opening)

• Occasionally in the door edge itself (particularly on glazed doors)

They run all the way around the perimeter except the bottom (threshold).

2. What You’re Looking For

Inspection Item	Pass/Fail Clues
Presence	Continuous seal visible on head and both jambs
Size	Usually 10mm x 4mm or 15mm x 4mm for FD30/FD60
Type	Intumescent only or combined smoke/heat seals (with brush or fin)
Fixing	Tightly seated in groove, no peeling or gaps
Damage	No tears, gouges, shrinkage or discolouration
Paint	Should not be painted over – this impairs expansion
Consistency	Same size, type and brand all around (not mix-and-match!)

3. Special Notes

• Glazed fire doors may have intumescent glazing gaskets or liners – these also need checking

• Older buildings may use seals in the door edge instead of frame — still valid if tested

• On non-brush seals, you might see a thin plastic casing around the strip — this is normal

💡 If you're unsure of the material, rub gently with a dry cloth. A powdery or crumbly texture = replace it.

4. When to Replace a Seal

Replace the seal if:

• It’s damaged, missing sections, or partially detached

• It has been painted over

• It’s the wrong type for the door rating

• There are signs of shrinkage (exposed groove or cracked corners)

• The smoke seal element (brush or fin) is torn, distorted or missing

Seals are usually cheap and easy to replace, but must match:

• Size

• Material

• Fire rating

• Door certification (check the door-set's data sheet)

5. Quick Field Test

• Insert a business card or slip of paper between door and frame — there should be light resistance, not a gap

• Push lightly on the brush seal — it should spring back to shape

• Run your finger along the groove — you should feel no steps, gaps, or lumps

  
  

If anything feels inconsistent or loose, log it for remedial action.

Final Word

Intumescent seals are one of the most overlooked elements in fire door safety — yet one of the most critical.

Treat them like a firefighter’s hose: you hope you never need them, but when you do, they must perform without question.