Submitted by editor on
published on 25/03/2026

UK Fire Cavity Barriers - A Guide

Fire doors and letterbox systems play a crucial role in protecting buildings from the spread of fire. As part of your escape routes, fire doors must remain effective during an emergency, and any openings, such as letterboxes, must not compromise their safety or performance. The right systems can help slow fire spread, protect occupants, and reduce damage to property.

What is Passive Fire Protection in Doors & Letterboxes?

Passive fire protection refers to built-in fire safety measures designed to slow or contain the spread of fire within a building. This includes fire-resistant walls, floors, and doors that help keep fire contained within a specific area, giving people more time to evacuate safely.

Fire doors are a key part of this approach, and any additions, such as letterboxes, must maintain the door’s fire resistance. Fire-rated letterboxes are specifically designed to ensure that openings in doors do not allow flames or smoke to pass through more easily.

How Do Fire Rated Letterboxes Work?

Fire rated letterboxes are designed to protect the integrity of fire doors during a fire. One of the most common solutions is the use of intumescent letterbox systems, which contain materials that expand when exposed to heat. This expansion seals the opening, preventing fire and smoke from spreading through the door.

These systems are typically rated to provide protection for a set period, such as 30 or 60 minutes, helping to slow the spread of fire and support safe evacuation.

Anti-arson letterboxes provide additional protection by preventing fires from being deliberately started through letter openings. They work by containing flammable substances and, in some cases, extinguishing flames using fire-resistant materials or built-in suppression features.

Where Should Fire Doors and Letterboxes be Used?

Fire doors and letterbox systems are used across a wide range of environments, including residential buildings, offices, retail spaces, and public sector properties. They are particularly important in buildings where secure entry points and fire safety must be carefully balanced.

Intumescent (expandable material that, when heated, prevents the travel of fire and smoke) letterboxes are commonly used in commercial environments to support fire containment and maintain door performance during a fire. Anti-arson systems are often installed in higher-risk locations where there is a concern about deliberate fire-setting.

By selecting the right type of fire-rated letterbox system, building owners can improve safety, support compliance with fire regulations, and protect both occupants and property.