Skip to main content

What is a Drop Seal?

Drop seals plug the common problematic threshold gap between the floor and the door. By plugging this gap, you optimise the door performance and ensure consistent protection against noise, fire/smoke, intruders, energy loss, pests and dust.

Drop Seals Explained

How does the partnership between a door set and drop seal work?

In order for a door to open and close there has to be a threshold gap between the door and the floor. On average, this gap is between 3mm – 15mm. If left unaddressed, this threshold gap will have a significant negative impact on the door set performance. In particular, the fire/smoke, acoustic, security and energy protection of the room the door set serves will be reduced.

A drop seal’s primary function is to plug the threshold gap whilst ensuring the door set maintains uninterrupted functionality.

The role of a drop seal

All door sets should meet performance criteria in accordance with Building Regulations and relevant codes. However, this performance can be compromised because of the threshold gap required for the door to operate.

The role of a drop seal is simple: to plug this problematic threshold gap. If applied correctly, it creates a fully sealed solution, enabling the door set to provide optimum acoustic, fire/smoke, sustainability and security performance.

Door Panel

Generally either Timber, Metal, PVC. All doors have a fire and acoustic specification.

Door Frame

Horizontal and vertical framing (jamb) provide the structural support & door stop for the door panel.

Door Hardware

Provides access and security (if applicable) for the door panel.

Threshold Gap

All doors have a gap of up to 15mm in order to maintain functionality of the door.

Actuator Button

Located on the hinge side of the door panel.

PDB 411M

Mortised into the door panel.

Sealing Bar

Recessed in the PDB 411M casing until the actuator button is compressed.

Threshold Gap

3mm – 15mm gap needed to ensure door functionality.

How do drop seals work?

The PDB drop seal is a market leader in this. 

When closing the door:

  • The actuator button is compressed against the door frame
  • This automatically engages the PDB arm controlled mechanism which lowers the sealing bar in a controlled, evenly distributed, downward motion
  • The sealing bar lowers until the threshold gap between the door and floor is fully closed – this creates a unified seal between the door panel and the floor

When opening the door:

  • The actuator button is decompressed/released from the door frame.
  • The arm controlled mechanism instantly and smoothly retracts the sealing bar back into the drop seal casing. There is no carpet drag.

The PDB drop seals operate on any surface, even or uneven.

Why do drop seals form an integral part of a building?

Fire / Smoke

Doors play an incredibly important role in the protection of buildings and their occupiers against the dangers posed by fire and smoke damage. In the UK, Building Regulations set out specific requirements which have to be met to protect against smoke ingress and fire penetration.

Small gaps exist around all touching edges of the door to ensure it can function properly. If left unattended, these gaps reduce the ability of the door to resist fire and smoke. In this situation, the PDB drop seal plays an important role as it secures the threshold gap and provide protection at the base of the door.

Sound Control

Unwanted sound transfer can directly impact occupants’ enjoyment and wellbeing within a building. Adequate sound mitigation measures are needed to reduce the noise transfer between rooms. UK Building Regulations set out specific door acoustic requirements depending on the building classification.

The small gaps that exist around a door to enable it to function impact the acoustic performance of the door. Drop seals play an integral role in minimising sound transfer between rooms whilst ensuring the door functionality is not impacted. By selecting the PDB drop seal, the door panel can perform at its specified acoustic rating to ensure occupants enjoyment of the building is not impacted.


Threshold gaps present a security risk on all door sets.

Not until the arrival of the PDB drop seal were security benefits associated with drop seals. As a standard feature, the PDB drop seal secures the threshold gap at the bottom of the door, preventing unauthorised access. In addition, due to the air-tight seal formed by the PDB drop seal when activated, attack from noxious materials can be limited, and even potentially a gas attack.

Energy / heat loss

The importance of conserving energy cannot be overstated.

The PDB drop seal helps support this essential task by ensuring the door set provides reliable and sustainable control of draughts and energy loss. This helps in reducing the energy consumption and carbon footprint of the building.


Threshold plates tend to be obstructive and make access difficult. If the correct drop seal is selected, it removes the need for a threshold plate at the bottom of the door.

With the PDB drop seal, no threshold plate is required. The PDB drop seal complies with all UK, EU, US and international standards for access.


The threshold gap offers an opportunity for pests and vermin to enter the building and/or room. If left unaddressed, this can cause disruption to occupants and a significant health and safety issue.

The PDB drop seal offers a simple solution. When the door is closed, it seals off the threshold gap and effectively locks out any pests. The PDB provides complete protection against insects and vermin.

PDB Drop Seal Sectors:

  • Commercial Buildings
  • Residential Buildings
  • Hotels
  • PRS
  • Student Accommodation
  • Hospitals & Medical Practices
  • Educational Buildings
Close Menu

Think you know door seals?
Think again!

The PDB automatic door bottom seal. Innovative, durable and globally patented. A truly all in one, sealed in place solution for doors openings.

Market tested & recognised as providing superior acoustic, sustainability, flood prevention & security performance of all door openings. Applicable across all door applications; any size; any sector.