1. What are edge strips?
Edge strips are basically strips of timber which frame the outside of the fire door.
Edge strips are predominantly pine or finger jointed timber and should be free of bowing, twists, knot holes and other irregularities all of which should be checked prior to assembly of the fire door, or prior to installation of the fire door.
2. Why are they used?
Edge strips are used predominantly for the following reasons;
- For the prevention of moisture entering the core of the door and
- To allow for site trimming to achieve the stipulated clearances between the door and the frame.
3. What happens to an edge strip if the door is subjected to fire conditions?
As the edge strip is timber, under fire conditions the edge strip will burn.
Although this seems drastic don’t worry. The door will not fall out of the frame as the hinges are fixed into perforated steel plates within the fire door.
You will note that the stop section of a fire rated frame (i.e. the section of frame the door closes on and prevents the door from being swung right through the opening) is bigger than a standard stop section (i.e. 25mm for fire rated frames and 12mm to 15mm for non rated frames). An edge strip is nominally 10mm thick so if it burns away, the core inside the fire door will still overlap the frame and provide a barrier to the spread of a fire for a certain amount of time.
4. Where do edge strips commonly split?
From inspecting thousands of doors over the years, the most predominant area where I have found splitting of the edge strip is above and below the lockset latch (both mortice and cylindrical) and to a lesser extent above and below the hinges.
If you are cutting timber for a fire using an axe, you often find that you start with a little crack in the piece of timber and then with continual force, the crack grows bigger and then follows further along the grain of the timber.
This is true also for timber edge strips. When the lock and hinges are installed, they are usually rebated (referred to as “checking in”) the timber edge strip so that once installed they finish flush in line with the edge strip giving a neat aesthetically pleasing finish.
In doing this, often very small cracks can begin on the corners of the rebated section and over time with the door opening and closing the cracks gradually grow bigger and bigger until a split in the edge strip becomes visible.
Regular maintenance of fire doors can go along way to maximise the life of the fire door by ensuring the door does not slam into the frame.
Like hitting a piece of timber with an axe, slamming of a door can cause the same effect. Even though there is no axe (unless of corse the fire brigade come to visit and you forget to give them a key), the door can be subjected to a similar force which over time allows the crack to develop further until it meets the grain of the timber and off it goes.
To minimise the spread of splits in a timber edge strip make sure the adjustment of the closer is correct so that the door does not to slam into its latching position.
5. What effect does a split edge strip have on the ability of a fire door to perform under fire conditions?
As discussed in Point 3, in a fire the edge strip is most likely burnt so in a simple response we can deduce that a split in the edge strip would have little effect on the doors ability to perform under fire conditions because it is one of the first things to be turned into ash.
Having said this however, we need to be mindful of why edge strips are used.
As discussed in Point 2, one of the main reasons for having an edge strip is to prevent moisture from getting into the core of the door.
If a split in the edge strip is large enough to allow moisture into the core of the door, it requires immediate attention.
Please note that any repair to a fire door must be done in accordance with the requirements of the relevant codes and standards and as such it would be highly recommended to speak to your fire door provider prior to undertaking any repairs to your fire doors to ensure that the proposed method of repair will not effect the integrity of the fire door and further that the repair method is approved.
The internal damage to the door core from moisture can affect the doors ability to perform under fire conditions as hollow pockets can form inside the door where the door core deteriorates and/or collapses.
As the outer covering of the door, like the edge strips, is generally timber (ply, MDF, Duracote etc), when it burns, hollow pockets within the door can be exposed allowing a fire to spread through the door. This obviously defeats the purpose of having the fire door there in the first place.
If you do have fire doors showing the early signs of splitting along the edge strips, get onto it early, don’t ignore it.
You will normally find that with preventative maintenance and regular inspection of the doors you can minimise the spread of splitting and maximise the life of your fire doors.
Edge strips are an integral component of a fire door and although they have little function if there is a fire, they are very important in maintaining the integrity of the door by creating the barrier between the internal core of the door and the moisture in the air so that if a fire does occur, the door can perform as it was designed to.
If you are advised by your service provider that your fire door requires replacement due to splits occurring on the edge strips, it may not be as bad as it seems.
We would suggest that prior to replacing any fire doors you speak with the manufacturer (the company or person who constructed the door) or sponsor (the person or company who manufactured the core of the door) of the particular fire door you have installed (the name of the manufacture and sponsor should be found on the compliance tag installed on the hinge side edge strip of the fire door) and check with them to ascertain if a repair method is available.
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