Fire Door Gap Tolerance Between Double Leaves Matters More Than Most People Realise

When people think about fire doors, they often focus on the obvious parts: the door closer, the latch, the frame, the signage. But one of the most important details is much smaller than that. On a pair of fire doors, the gap between the two leaves is a critical part of performance.

That is why fire door gap tolerance matters. Under AS1905.1 Clause 5.5.2, the average gap measured between the two doors must not exceed 3mm. It sounds minor, but in fire safety, a few millimetres can be the difference between a doorset performing as tested and a weak point opening up at the centre of the doorway. The National Construction Code also makes clear that required fire doors must comply with AS 1905.1, and that this standard in turn relies on testing to AS 1530.4.

A double-leaf fire doorset does not work because each leaf is “good enough” on its own. It works because the full assembly has been designed, fitted and tested to operate together. The meeting point between the two leaves is part of that assembly. If the gap at that junction is too wide, the door pair may no longer perform the way the tested system was intended to perform under fire conditions.

In practical terms, the centre gap is where fire, hot gases and smoke will try to exploit any weakness. A pair of leaves that look close enough to the eye can still be outside tolerance. If the average gap exceeds 3mm, the overlap, alignment and resistance expected from the tested doorset can be compromised. That is why this is not just a workmanship issue. It is a compliance issue and, more importantly, a life safety issue.

This becomes even more important in buildings where fire doors protect exits, corridors, stair connections and compartment walls. These doors are there to slow the spread of fire long enough for people to evacuate and for emergency response to begin. If the centre meeting gap is excessive, the doorset may allow fire effects to attack that line earlier than intended. The problem is often hidden in plain sight because the doors may still appear to close and latch. But “closing” is not the same as complying.

One of the most common misconceptions is that if a double fire door latches, the gap must be acceptable. That is not true. Latching is only one part of the picture. A compliant fire doorset depends on correct clearances, correct hardware, correct frame conditions and correct sealing arrangements working together. This is the same reason details like fire door stops and smoke seals matter so much in the overall performance of the assembly.

So what causes the gap between two fire door leaves to become non-compliant?

In many cases, it comes down to wear, movement or poor adjustment. Hinges can loosen. Leaves can drop slightly over time. Frames can move. Latching hardware can become misaligned. Sequence selectors on paired doors can stop working properly. Even repeated heavy use can gradually affect how the two leaves meet at the centre. On new installations, the issue may simply be that the doors were never set correctly in the first place.

Another common issue is carrying out adjustments without understanding that the fire door is a tested system. On ordinary doors, a trade might trim, plane, pack or alter components as part of normal maintenance. Fire doors are different. The assembly has to remain consistent with its tested and approved condition. Uncontrolled alterations can solve one visible problem while creating a more serious compliance issue elsewhere. That is why any proposed rectification should be approached carefully and, where needed, checked against the door manufacturer’s evidence, the installed hardware, and the relevant test basis under AS 1905.1:2015.

The right response is not to ignore the gap and hope the rest of the door will make up for it. The right response is to identify why the leaves are not meeting within tolerance. Sometimes the answer is adjustment of hinges or hardware. Sometimes it is realignment of the leaves. Sometimes the issue is connected to failed hardware, damaged meeting edges, or a broader frame problem. In more serious cases, the doorset may need more substantial rectification to restore compliant operation.

It is also worth remembering that fire doors do not exist in isolation from smoke control. Even where the centre gap issue is being considered from a fire resistance perspective, excessive clearances can also contribute to poor smoke containment outcomes depending on the door type and sealing arrangement. This is one reason regular inspection matters. A door pair that has drifted out of tolerance may still be used every day without complaint, right up until the day performance actually matters.

For owners, managers and committees, this is exactly the kind of defect that should not be written off as minor. A few millimetres may seem trivial in general building maintenance, but in fire safety systems, tolerances are there for a reason. They reflect how the product was meant to be installed and how it was expected to behave when tested. The NCC fire door provisions reinforce that required fire doors must comply with AS 1905.1, which is why these dimensional requirements matter in the first place.

The simplest way to think about it is this: a pair of fire doors is only as reliable as its weakest point. On double doors, that weak point is often the meeting stile. If the average gap between the two leaves exceeds 3mm, the doorset is no longer sitting within the tolerance required by AS1905.1 Clause 5.5.2. And once that happens, confidence in the assembly should drop with it.

A well-performing fire door should never rely on guesswork. It should be correctly fitted, correctly maintained and consistent with the system that was tested. That is what protects people, supports compliance, and gives building stakeholders the peace of mind they should expect from essential safety systems.