How to Fit Intumescent Seals, Smoke Seals, and Drop Seals

Practical Guide #6

For: Joiners, door installers, repair teams, fire door inspectors, and maintenance staff.

Why This Matters

No matter how good the door, if the seals are missing, damaged, or incorrectly fitted — the entire fire door system can fail.

Intumescent seals expand in heat to block smoke and fire. Smoke seals stop cold smoke from spreading during the early stages of a fire. Drop seals reduce the under-door gap for better protection, especially on acoustic and smoke-rated doors.

This guide walks you through fitting them right — whether new, retrofit, or replacement.

1. Understanding the Seals

Intumescent Seals

• Usually 10–15mm wide, heat-activated material

• Expand rapidly when exposed to ~180–200°C

• Fitted in a routed groove in either the door or the frame

Smoke Seals

• Typically brush or fin-type

• Often combined with intumescent in a single strip

• Create a tight seal against cold smoke movement

Drop Seals

• Mechanical seal that lowers automatically when door closes

• Usually surface-mounted or rebated into the bottom edge

• Vital when an under-door gap of 3mm is required (e.g., FD30S)

2. Fitting Intumescent Seals

Factory-Fitted or Retrofit?

• Some doors come pre-grooved and fitted — always match manufacturer’s test data

• For retrofit: grooves must be clean, straight, and cut to correct depth (typically 4mm)

Key Installation Tips:

• Check the certification for required size and type

• Do not use glue or mastic unless the product allows it — most are self-adhesive

• Press seals firmly into place, ensuring no gaps or lifted corners

• Ensure the seal runs continuously around the head and both jambs

• Do not cut seals around hinges or locks — use split seals or re-route carefully

If you're painting the door, paint before fitting the seal — not after. Paint over seals reduces effectiveness.

3. Fitting Smoke Seals

• Often combined with intumescent in a twin-seal strip

• Fin or brush must make contact with the opposing surface without excessive drag

• Ensure the seal isn’t crushed or bent during fitting

• Watch for obstruction by hinge screws or warped frames

4. Installing Drop Seals

Used when the under-door gap must be <8mm, or ≤3mm for smoke-rated doors.

Two Types:

1. Rebated Drop Seal:

• Requires cutting a groove into the bottom edge

• Neat and discreet, commonly used in commercial projects

• Activated by button on hinge side that triggers when door closes

2. Surface-Mounted Drop Seal:

• Quicker to install

• Screws onto the door’s surface (usually inside face)

• Slightly more visible but easier to retrofit

Fitting Process:

• Measure and mark carefully — the seal must span the full width of the door

• Align so that the seal drops vertically and contacts the floor evenly

• Avoid crushing on thick carpet or uneven thresholds

• Test the activation — adjust screw tension or height if needed

5. What Can Go Wrong

• Missing seals

• Painted-over intumescent strips

• Gaps or overlaps between seal lengths

• Smoke seals too tight or too loose

• Drop seal doesn’t contact floor fully

• Split seals cut too short or misaligned

• Seals not matching the fire test evidence of the doorset

6. Inspection and Maintenance Notes

• Seals must be replaced if damaged, perished, painted over, or missing

• Regularly inspect for:

  • Compression or wear on brush seals

  • Peeling of adhesive

  • Poor contact from drop seals

• Keep a stock of tested, approved replacement seals on site

• Always replace like-for-like, referring to original manufacturer data where possible

Final Reminder

Seals are a small part of a fire door — but without them, the entire door-set is non-compliant.