Hanging a Fire Door to the Gap Tolerances

Nine readings, and two of them fail

Forty-one flat entrance doors on a nine-storey block, all replaced two years earlier, all with the plug in the top edge and the paperwork in the file. The quarterly check took a gap gauge round each leaf at nine points — three down each stile, three across the head — and thirty-nine doors came back with at least one reading over. Not one of them was hung badly by any ordinary standard. The margins were even to the eye, every leaf closed sweetly, and the decorator had left them looking better than the day they went in. They failed because on a fire doorset an even margin is not the same thing as a correct margin, and because the number the gauge is being compared against is not a number anybody on the fitting crew had ever read.

That is the whole difference between this work and hanging a door. Everywhere else in joinery the leaf is the adjustable part: you scribe it, plane it, ease the lock stile, hang it twice and let the architrave hide the rest. Here the leaf is the one thing you may not touch beyond a stated allowance measured in single millimetres, because it was tested as part of an assembly and every shaving takes it further from the thing that was tested. So the adjustment has to happen somewhere else, and the only place left is the opening. You build the hole to suit the doorset instead of easing the doorset into the hole.

Everything awkward on site follows from that inversion. The structural opening acquires a maximum as well as a minimum. The void behind the frame is a specified fill rather than whatever foam is on the van. The seals have to be continuous through the hinge positions. And the door has to latch every time, against a closer somebody will later want turned down because a resident finds it heavy. None of it is difficult. All of it is measured afterwards by a person holding a gauge, which is not true of any other door in the building.

What the gauge is actually reading

There are four gaps and they are not one specification. Head and both stiles are commonly quoted as a 3 mm nominal with a millimetre either way, and that pairing is deliberate on both sides: too tight and the leaf binds as timber takes up moisture, or as the intumescent begins to swell and has nowhere to go except against a leaf that then cannot shut; too slack and there is more gap than the seal was tested to bridge, so the fire finds a slot before the seal has closed it. The gap under the leaf behaves differently again, and it varies far more between schedules than the other three, because a doorset with a smoke classification has to close that gap with a drop seal or a brush while a plain fire door without one is generally allowed more.

The operative number is not in a code. BS 8214, Timber-based fire door assemblies — Code of practice, is the document that governs how a timber fire doorset is installed in the UK, and NFPA 80, Standard for Fire Doors and Other Opening Protectives, does the equivalent job in North America, where it states clearances at the top and vertical edges for wood and for steel leaves separately and caps the clearance under the bottom of the door. Read the edition your jurisdiction has actually adopted rather than a figure somebody remembers. Underneath both of them sits the thing that really decides it: the certification schedule and field of application for the specific doorset in front of you, issued by whoever tested it to BS EN 1634-1 or BS 476-22, or to NFPA 252 and UL 10C. Where the schedule and a remembered rule of thumb disagree, the schedule wins and the rule of thumb is what gets you a remedial.

Get that document onto the site before the first frame is fixed, not after the first inspection. It is a page or two, it names the gaps, the seal sizes and positions, the hinge grade, the permitted trim and the permitted ironmongery, and every argument that happens on this job later is settled by it. Crews that have it pinned inside the site cabin door hit the tolerances; crews working from what worked last time do not.

Seals that stop at the hinge

An intumescent seal works by being continuous. It swells to many times its own volume and closes the gap it sits beside, and a run that is continuous for nine tenths of its length has left a slot at the tenth — which is where the smoke goes, because smoke is looking for exactly that. So the failures worth hunting are all failures of continuity rather than of specification, and they cluster in four places.

The corners are the first. Where a head run meets a stile run the two either butt tight or are mitred, depending on what the schedule shows, and a five-millimetre shortfall at the top of each stile is two holes in the same door. The hinge positions are the second, and they are the ones that catch experienced fitters: on many frames the seal groove runs straight through the hinge recess, and a hinge blade let in over it either interrupts the run or has to be paired with an intumescent hinge pad behind the blade, which is a listed component and not a washer somebody improvises. The third is decoration. A seal painted over four times has a skin on it, and a seal that has been painted, filled and sanded with the architrave has lost the fins on a brush type entirely. The fourth is substitution — a smoke seal swapped for a plain intumescent because the merchant was out, on a doorset whose classification depends on the smoke half.

Smoke is the quieter half of the specification and the half that gets dropped. Ambient-temperature smoke leakage is tested separately under BS EN 1634-3 and classified under BS EN 13501-2, and a doorset carrying that classification has a brush or a fin or a blade seal doing work that the intumescent does not do at all — intumescent is inert until it gets hot, and by the time it has activated the smoke has been moving for several minutes. This is why the threshold gap is a specification question rather than a joinery one: a leaf hung to a generous undercut for return air is a leaf with a smoke path along its whole width, and the answer is a drop seal or a sealed threshold rather than a leaf trimmed to a gap somebody guessed.

Set the seals after the frame is fixed and before the architrave goes on, and gauge the four gaps with the leaf hung and the seals in — not before, because a seal proud of its groove by a millimetre changes every reading you took. Where the schedule puts the seals in the leaf edge rather than the frame, that is where they go; the two are not interchangeable and the tested arrangement is the one on the certificate.

1. Check the seal size, type and position against the schedule before opening the first pack, including which of the two components carries them.
2. Run each length full, butted or mitred at the corners exactly as the schedule shows, with no short pieces made up from offcuts.
3. At every hinge, confirm whether the run continues behind the blade or whether an intumescent hinge pad is specified — and fit the pad if it is.
4. Keep paint, filler and caulk off the seals entirely, and mask them before the decorator arrives rather than cleaning them afterwards.
5. Hang the leaf, gauge all four gaps with the seals in place, and photograph the readings while the perimeter joint is still open.

Closing is not the test — latching is

A fire door that closes but does not latch has failed. Not marginally: the leaf is held against the stop by nothing but a spring, and the pressure differential across a door in a fire is more than enough to push it open again. Every certified assembly is tested with the latch engaged, so the latch is a structural part of the thing, and the single commonest defect found on inspection is a door that swings shut, touches the keep and stops a few millimetres short of throwing.

The cause is nearly always the last few degrees of travel. A closer to BS EN 1154 is classed by power size, the schedule names a minimum, and somebody has wound it below that minimum because a resident complained the door was heavy — which reduces the latching action precisely where the door is fighting the latch spring, the smoke seal and whatever air the corridor is moving. Latch and lock cases carry their own route in BS EN 12209, panic and emergency hardware in BS EN 1125 and BS EN 179, and hinges in BS EN 1935; on a certified doorset all of it is named on the schedule and none of it is a like-for-like substitution made at the merchant's counter. Free-swing and hold-open devices to BS EN 1155 are a legitimate answer to the heavy-door complaint where the doorset permits them, because they release on the alarm and let the closer do its job at full power — but they are a specified item with a wiring implication, not an adjustment.

Test it the way it will be tested. Open the leaf to ninety degrees, let go, and watch it through the last twenty degrees without helping it. Do it again from about thirty degrees, which is the position a door in real use is most often released from and the one that finds a weak closer. Then check that the bolt has actually thrown rather than resting on the lip of the keep — a bolt that has to be nudged home with a finger has not latched. Where the keep is a fraction out, the fix is the keep or the frame, because the leaf is not available to you: this is the point where the habits from ordinary door hanging have to be consciously put down.

The paperwork is half the installation

An installed fire doorset is a claim, and the claim needs evidence attached to it or it is worth nothing at the first inspection. Photograph the perimeter joint packed and sealed before the architrave goes on. Photograph the plug or label in the top edge with the door number visible in the same frame. Record the gap readings at the nine points, the closer make and power setting, the hinge and latch models actually fitted, and any deviation from the schedule with the reason for it. Third-party installation schemes exist precisely because this record is the difficult part — the BWF Fire Door Alliance and the BM TRADA Q-Mark installation schemes both require it and both give the building owner something an inspector can read.

The ongoing regime is somebody else's duty and it is worth knowing anyway, because it tells you what your work will be measured against for the next thirty years. In England the Fire Safety (England) Regulations 2022 place duties on the responsible person for higher blocks — checks of fire doors in the common parts on a quarterly cycle and best endeavours to check flat entrance doors annually — sitting under the Regulatory Reform (Fire Safety) Order 2005. NFPA 80 requires fire door assemblies to be inspected and tested not less than annually with a written record of the result. Both regimes measure the same things you have just built: the gaps, the seals, the self-closing action and whether the door latches.

So hand over a schedule written for the person holding the gauge in three years: which doorsets are on the job, what each one's certificate says its gaps should be, and where those documents live. A door that passes on the day with no record behind it gets argued about.

Where this page stops

Steel doorsets are a different discipline with a different specification route — ANSI/SDI A250.8 in North America, and the European product standard BS EN 16034 covering the fire and smoke characteristics of doorsets alongside their base product standard — and while the gap logic is the same, the tolerances, the frame fixing and the permitted field modifications are not. Glazed apertures are not a site operation on any leaf: the aperture, the beads and the glass are a tested system, and specifying that system has a page of its own on this site. Upgrading an existing panelled door with intumescent products is a question about test evidence for a specific construction rather than a fitting technique, and it belongs with the guidance on which openings need a fire door in the first place.

The claim here is deliberately narrow: build the opening to the doorset, fill the perimeter with what was tested, keep the seals continuous through the hinges and corners, and prove the thing latches from thirty degrees rather than from ninety. Clear width, the height chain, scribing a leaf and marking a keep are ordinary joinery covered elsewhere on this site — necessary here, and nowhere near sufficient.