Gaps Underneath Fire Doors

We often get asked if gaps are allowed underneath fire doors and if so, how big they can be. We researched on your behalf and compiled this blog to explain the current recommendations.

The Basics

What happens in case of a fire? There is positive pressure in a room on fire (until the fire is vented) but at the base of the fire the pressure must be negative to draw in the air for the fire to receive oxygen. The smoke and the fire will try to push through the side and top gaps around a fire door while (usually) drawing air from underneath the door.

The Development of Fire Door Testing

The old test of fire doors to BS 476 Part 8 (for fire resistance) was carried out simply in a furnace by increasing the temperature to that found in rooms on fire. The door was checked to ascertain when the fire would breach the door, usually at the gaps at the head and sides of the door. This was changed in the eighties to BS 476 Part 22 which was similar, however, the furnace was now pressurised. Old BS 476 Part 8 fire doors failed this new test (usually after around 20 mins for a 30 minute FD30 fire door), so intumescent strips, also called fire door seals, were introduced to ensure fire doors passed the test. However, cold smoke can still seep through the gaps around the door and cause death from smoke inhalation. In some applications  therefore, fire doors with cold smoke protection around the sides and top are required (recognisable by the ending ‘s‘, e.g. FD30s). This is usually achieved by installing fire door seals with integrated cold smoke brushes.

Fire doors tested in accordance with the cold smoke test requirement BS 476 Part 31 ensure that in the early stages of a fire, the cold smoke does not percolate into escape routes and make them impassable. At first, the standard was only concerned with the gaps at the head and the sides of the door, as the air passing through the threshold gap to feed the fire keeps the smoke in the affected room. However, now the standards have changed, and for fire doors requiring cold smoke control, the threshold gap has to be considered as well.

This is quite confusing, as fire fighters were always taught that at floor level there is a layer of an inch thickness of fresh air no matter how severe the fire is and if you get caught, get your nose on the floor to breathe. Consequently, the threshold gap was never considered important and BS 8214:1990 reflected this by stating a threshold gap of 6/8mm was acceptable.

Nevertheless, in BS 8214 this changed and it states that under-door (threshold) gaps should be in accordance with the manufacturer’s installation instructions for the particular doorset design. When fitted, smoke seals should give an even contact with the floor but should not exhibit significant increased frictional forces that could interfere with the closing action of the door (see BS 5588-11). BS 5588-11 is now withdrawn and BS 9999 is the current standard. BS 9999 recommends for fire doors with cold smoke control that the threshold gap should be either less than 3mm in height or should be fitted with a threshold seal.

When speaking with manufacturers, however, a common statement is that a fire doors gap of up to 8 mm underneath the door is permissible. This is referring to normal FD30 applications, as the fire door manufacturer is not involved in upgrading fire doors to cold seal protected standard (FD30s).


Ask your fire door manufacturer for their recommended threshold gap. If this information is not available it seems reasonable to permit a gap of up to 8mm for general fire doors (eg FD30). If you have a requirement for cold smoke control (eg FD30s) your threshold gap should be 3mm or less, or you should fit a threshold smoke seal (most commonly in the form a drop down seal).

This advice applies for most applications, however, special requirements must be considered in a your fire risk assessment.

Leave a Reply

Your email address will not be published. Required fields are marked *