Fire Door Drop Seals: When They’re Required and How They Work

What Are Drop-Down Seals?

Drop-down seals (also known as drop seals or automatic threshold seals) are concealed within the bottom edge of a fire door. When the door closes, the mechanism drops to form a tight seal with the floor; when opened, it retracts. They sit inside the door leaf—either mortised or face-fixed—so they don't interfere with the door's movement.

Why You Need Them

1. Stop Smoke Spreading

Smoke often travels under the door faster than fire. A small gap—especially in smoke-rated doors—can be a critical failure point. Drop seals seal that gap, preventing smoke migration.

Regulations:

• Fire-only doors: bottom gap should be 8–10 mm

• Fire-and-smoke doors: bottom gap must be 3 mm—wider gaps require a drop seal.

2. Maintain Fire Door Integrity

The effectiveness of a fire door is only as strong as its weakest point. Ensuring all perimeters, including the bottom gap, are properly sealed keeps the door’s fire and smoke-resistant rating intact.

3. Better Acoustic & Thermal Control

Drop seals also improve sound and energy performance. They're tested for acoustic isolation (e.g., up to 40 dB) and airtightness under standards like BS 476:31.1.

When Are Drop Seals Required?

• If the gap is greater than 3 mm on a smoke-rated door.

• If your construction method or floor height creates excessive gaps.

• Ideal when installing in hotels, schools, offices—anywhere smoke containment and noise reduction are priorities.

Quick check:

• ≤ 3 mm? You may not need a seal—just ensure perimeter and bottom gaps comply.

• > 3 mm but ≤ 10 mm? Use a drop seal.

• > 10 mm? Drop seal alone might not suffice—consider threshold plate + drop seal for sure fit.

Types of Drop Seals

Type	Description	Benefits
Mortised (recessed)	Installed inside a groove cut into the door’s bottom	Sleek, protected installation
Face-fixed / Retro-fit	Surface-mounted onto door bottom	Quick install for existing doors
Certified models	e.g., Sealmaster DropSeal 2712 — tested per BS/EN standards	Durable, smoke/fire rated, tested up to 1 million cycles

Other options:

• Perimeter seals (e.g., Delta seals) cover head and jamb edges to block smoke/sound leakage.(firewise)

• Use alongside drop seals for a full door assembly that’s airtight and compliant.

Installation & Maintenance Tips

Installation

1. Choose quality drop seals (e.g., Sealmaster, FAS45) tested to relevant fire/smoke standards.(coulsycontractsolutions.co.uk, Fire & Acoustics Seals)

2. Fit flush or mortised so the mechanism is protected and unobtrusive.

3. Use a threshold plate where necessary—especially when sealing uneven flooring or combining materials like carpet and vinyl.(Lorient UK)

Maintenance

• Test regularly to ensure the seal fully closes when the door shuts.

• Clean dust/debris—especially in high-traffic or dusty sites.

• Replace worn seals—over time the spring mechanism or seal material may fail.

• Visual checks during fire door inspections should include the gap and operation of the drop seal.(Noberne)

Common Mistakes to Avoid

• Installing a drop seal without first measuring the actual gap—could compromise operation.

• Assuming that perimeter seals alone are enough for smoke control.

• Neglecting the need for threshold plates—especially with mixed-floor surfaces, which can break the seal.

• Skipping maintenance—drop seals are moving parts and need regular checks like any other hardware.

Quick Summary (Executable Steps for MVP)

1. Measure bottom gap of the fire door.

2. If gap > 3 mm on smoke-rated, install drop-down seal.

3. Select right type:

   • If the door is new or has a rebate, mortised is best.

   • For existing doors, face-fixed is efficient.

4. Use a threshold plate where floor surface varies or seal could be compromised.

5. Test seal operation regularly and inspect gaps during fire door checks.