Gap Under Your Fire Door? Here's What It Means for Compliance

TL;DR: A gap under a fire door is not automatically a fault, but it becomes one the moment it exceeds the tolerance stated on that door's certification. A gap under a fire door lets smoke and hot gases pass underneath before the door has done its job, which is why inspectors treat it as a priority defect rather than something to monitor. If you've spotted one, the door should be assessed against its certification and, where the gap is out of tolerance, scheduled for fire door remedial works rather than left as is.

You've noticed daylight, or a draught, running along the bottom of a fire door. That gap under a fire door is one of the most common findings on a fire door inspection, and whether it matters comes down to one thing: how it compares with the tolerance the door was certified to.

What Counts as a Gap Under a Fire Door?

Fire doors are tested and certified as a complete assembly, leaf, frame, seals, and hardware together, with a specific gap tolerance built into that certification. Third-party schemes such as BM TRADA Q-Mark and BWF-CERTIFIRE typically allow only a few millimetres of clearance under the leaf where no threshold seal is fitted, with a wider tolerance permitted where a certified drop-down or brush seal is installed to close that gap when the door shuts.

The point is that there is no single universal figure that applies to every fire door. The correct tolerance is whatever is stated on that specific door's certification or the manufacturer's fitting instructions. A gap under a fire door that looks fine by eye can still be out of tolerance, and a gap that looks generous can be within spec if the right seal is doing its job.

Why Do Fire Doors Develop a Gap Underneath?

A gap under a fire door rarely appears the day it's installed. It tends to develop over time, or gets introduced during work that had nothing to do with the door itself.

Wear on the Door Leaf or Frame

Timber doors move. Repeated use, humidity changes, and general age can cause the leaf to shrink slightly or the frame to settle, widening the gap at the bottom over months or years.

Incorrect Installation or Alterations

Doors that were trimmed on site, rehung after other works, or fitted with the wrong threshold detail from the outset can end up with a bottom gap that was never within tolerance to begin with.

Flooring Changes After the Door Was Fitted

New carpet, tiling, or an altered floor finish laid after the door went in is one of the most common causes. Removing thick carpet for hard flooring, for example, can leave a fire door with far more clearance underneath than its certification allows.

When Does a Gap Under a Fire Door Become a Compliance Failure?

A gap under a fire door becomes a compliance failure when it exceeds the tolerance on that door's certification and there is no adequate seal compensating for it. During a fire door inspection carried out to BS 8214, the inspector measures the gap at several points along the bottom edge and checks it against the certified allowance, not against a generic rule of thumb.

Where the gap is out of tolerance, the finding is usually recorded alongside other checks, such as the condition of intumescent and smoke seals, so it's worth reading that alongside our guide to getting fire door seals right. A gap under a fire door is also one of several issues that our guide to fire door defects that automatically require remedial works flags as something that shouldn't wait for the next scheduled inspection cycle.

What Should You Do If You Find a Gap Under a Fire Door?

The right response depends on what's causing it.

First, don't assume the door is failed based on sight alone. A responsible person or facilities manager can note the observation, but a proper measurement against the door's certification should come from a qualified fire door inspector, following the process set out in our BS 8214 inspection guide.

Second, avoid quick fixes that aren't certified for the door, such as generic draught excluders or non-fire-rated threshold strips. These can mask the gap without actually restoring the door's fire performance, and in some cases can interfere with the door closing correctly.

Third, once the cause is confirmed, whether it's a worn leaf, a flooring change, or an installation issue, the correct remedy should be specified as part of planned fire door remedial works, carried out by a competent contractor working to the door's certification.

Can a Gap Under a Fire Door Be Fixed Without Full Remedial Work?

Sometimes. If the leaf and frame are otherwise sound and the gap is only marginally out of tolerance, fitting or replacing a certified threshold seal can often bring the door back into compliance without touching the leaf or frame at all.

Where the gap is significant, where it's the result of the leaf or frame moving out of alignment, or where it's combined with other issues such as a fire door not closing properly, a seal alone won't resolve it. In those cases the door needs a wider remedial assessment to establish whether adjustment, partial repair, or replacement is the appropriate fix.

What Happens If a Gap Under a Fire Door Is Left Unaddressed?

A fire door's entire job is to hold back smoke and fire for a defined period, long enough for people to escape and fire crews to respond. A gap under a fire door undermines that from the moment smoke first fills the space on the other side, regardless of how well the rest of the door performs.

Leaving it unaddressed also creates a paper trail problem. Under the Fire Safety Act and Building Safety Act duties, responsible persons are expected to act on known defects within a reasonable timeframe. A gap that was identified on an inspection report and left unresolved is difficult to explain if that door is ever tested by a real fire, or reviewed by a regulator.

Frequently Asked Questions

Is any gap under a fire door a problem? Not necessarily. A small gap is normal and accounted for within the door's certification. It only becomes a problem once it exceeds the tolerance stated for that specific door and isn't compensated for by an appropriate seal.

How is a gap under a fire door measured? A qualified inspector measures the clearance between the bottom of the leaf and the floor or threshold at several points along the width of the door, then compares that measurement against the certified tolerance for that door.

Can carpet or flooring changes cause a gap under a fire door? Yes. Removing or replacing flooring after a fire door was fitted is one of the most common causes, since the door's clearance was originally set against the floor level that existed at installation.

Does a threshold seal fix a gap under a fire door? A certified threshold, drop-down, or brush seal can bring a door back into compliance if the leaf and frame are otherwise sound. It won't resolve a gap caused by a warped leaf or a misaligned frame.

Who should assess a gap under a fire door? A qualified fire door inspector working to BS 8214, using the tolerance stated on that door's certification rather than a general assumption.

Is a gap under a fire door something a fire risk assessment will catch? A fire risk assessment may flag fire doors as an area of concern, but it isn't a substitute for a dedicated fire door inspection. Gaps and seal condition are assessed properly at door level, not at building level.

How quickly should a gap under a fire door be fixed? As soon as reasonably practicable once it's confirmed as out of tolerance. It should be scheduled as remedial work rather than left until the next routine inspection.

If a fire door inspection at your building has flagged a gap underneath one or more doors, Protest ES Ltd can assess the finding against the door's certification and carry out the correct remedial work to bring it back into compliance.