What is a threshold gap?
The threshold gap is the clearance between the bottom of a fire door leaf and the floor. It has to be small enough that the door performs as tested, and it is one of the most commonly failed checks.
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<img src="https://fireoptix.co.uk/glossary/threshold-gap/diagram.svg" alt="threshold gap — diagram from FireOptix" width="880" height="620" loading="lazy">
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<a href="https://fireoptix.co.uk/glossary/threshold-gap">What is a threshold gap?</a> — diagram by
<a href="https://fireoptix.co.uk">FireOptix</a>
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</figure>Every gap around a fire door is part of the tested assembly, and the one at the bottom is the one that changes most. Floor coverings get replaced, screed gets added, and carpet gets laid over carpet — each raising or lowering the leaf relative to the floor.
Where smoke control is required at the threshold, a seal or a drop-down device closes it. Where it is not, the gap is simply limited, and the limit comes from the door's own certification rather than from a universal figure.
It is easy to measure and easy to evidence, which is why inspectors go to it early. A door failing on threshold gap usually indicates nobody has checked the doors since the last time the floor was touched.
In practice
A door with a measured gap at the threshold and no note of the unit.
The inspector wrote a number. Nobody recorded whether it was millimetres, and nobody recorded what the door was certificated to.
A measurement is only useful if it can be read against the right limit later, which requires knowing both the figure and the standard for that door.
What the gap is permitted to be is in the certification, not on a checklist.
What goes wrong in practice
- New floor covering laid under a fire door, changing the gap with nobody re-checking it.
- A gap made worse by trimming the leaf so it stops binding.
- Drop-down seals fitted but seized, so they no longer deploy.