The LWF Blog

Passive fire protection for Building Professionals – Part 13

April 16, 2020 8:18 am

In LWFs blog series on passive fire protection for building professionals and contractors and those with an interest in structural fire protection issues, we aim to give an overview of the important role passive protection plays in the overall fire safety provision of a building. In part 12, LWF looked at the locks used on fire doors and their importance in the overall integrity of the door. In part 13, we will continue looking at the components used in fire doors, beginning with letterboxes.

As with other components installed in fire doors, letterboxes used in a fire door should have been subjected to full testing along with the door into which it is fitted. A fire-rated letterbox which has been inappropriately installed into a fire door can lead to a door which is unable to demonstrate adequate performance in case of a fire. Specialist advice should be sought in order to ensure a fire door is optimised for use.

One of the elements installed into fire doors which is not always obvious to the casual observer is that of intumescent strips and cold smoke seals. While not all fire doors are fitted with cold smoke seals, all tested fire doors will have an intumescent strip at least.

An intumescent strip is fitted around the top three edges of the door, the sides and top, and if exposed to fire they expand to fill the gap between the door and frame. Most seals expand sufficiently to fill a gap of up to 4 mm and a correctly fitted door might be expected to have a gap of between 2 and 4 mm between door and frame.

If the gap between fire door and frame is in excess of 4 mm then specialist advice should be sought to address the issue.

The purpose of the intumescent strip and/or smoke seals is to prevent the egress of smoke and fire gases around the edge of the fire door. They also prevent oxygen from entering the area where the fire has ignited and so will avoid intensifying combustion around the edges and corner of the door.

The cold smoke seal is designed as a flexible element to sit alongside or on top of the intumescent strip. In a fire situation, there will be a point of exposure to the heat at which the cold smoke seal degrades and melts away, allowing the intumescent strip to expand.

In part 14 of this series, LWF will continue discussing the use of intumescent strips in fire doors and cover self-closing devices on fire doors. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact LWF on freephone 0800 410 1130.

Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 25 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.


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