Component 07 of 10 · Ironmongery

Lock & latch

Mortice lock / latch. The positive hold.

A lock secures a door; a latch holds it closed. Some fire doors are tested and specified latched, and there the latch is what the closer closes into: a positive hold that keeps the leaf seated while pressure builds on the fire side. Others are tested and specified unlatched, relying on the closer and the seals, and the installed configuration must match the door's evidence either way. Cutting a mortice takes timber out of the leaf and puts metal in its place, which changes how that part of the door behaves in fire. The lock shown here has no intumescent on it. Many fire doors need it around the case, and whether yours does is set by the evidence rather than by eye.

Key facts

Function: Secures the door, and where a latch is specified holds the closed leaf positively in the rebate
Specification: Mortice latch, lock or sashlock with fire test evidence for this type of door in its tested configuration, latched or unlatched, plus intumescent protection where that evidence calls for it
Materials: Steel case and bolt; intumescent wrap or paste around the mortice where the evidence calls for it
Critical tolerances: Where a latch is specified it must engage the keep fully under closer power alone. Locks, latches, keeps, intumescent protection and operating furniture must be compatible with the evidence
Standards: Conformity marked to the product standard within whose scope the device falls, for example BS EN 12209 for mechanically operated locks and latches, or BS EN 179 for an emergency exit device. Proven within the fire door's test evidence
Role in fire: Where a latch is specified, resists the pressure and distortion that try to push the leaf open in fire
Where it sits: Morticed into the closing edge, engaging a keep in the jamb

Compliance

The mortice is a deliberate hole in a fire barrier. The lock's fire test evidence, including any intumescent protection it was tested with, is what makes that hole acceptable. An unrated replacement is not a like-for-like swap.

Did you know

In a furnace test the pressure of the fire itself pushes on the leaf. On a door tested latched, the small spring bolt is doing real structural work. On a door tested unlatched, the closer, the seals and the hinges were proven to hold it without one.

On the model

Latchbolt: Sprung positive hold, where specified
Lock case: Intumescent where the evidence calls for it
Faceplate: Flush in the leaf edge
Spindle follower: Drives bolt from the handle

What inspectors check

Work through the checklist with the door closed, then in motion. Every line here is a recordable outcome for a Fire Door Inspector.

Confirm the configuration, latched or unlatched, matches the door's evidence
Where a latch is specified, let the closer shut the door on its own; the latch must engage every time
Check every screw is present and tight
Push and pull the closed leaf: excessive rattle means the bolt is not held in the keep
Check the keep alignment, and that the bolt enters cleanly rather than riding the edge
Ask whether the evidence calls for intumescent around the case, and whether it is there
Look at the mortice for oversizing, ragged edges and voids around the case
Check the lock position against the door's evidence, not against habit
Look for tape, packers or anything defeating the latch

Common defects

Screws missing or loose
Intumescent protection absent where the evidence calls for it
Lock not compatible with the leaf it is fitted to
Fitted in a position the evidence does not cover
Configuration, latched or unlatched, not the one the evidence covers
Misaligned with the keep
Not latching when the door closes on its own, where a latch is specified
Excessive rattle in the closed door
Oversized or ragged mortice, leaving voids around the case
Latch taped or held back in daily use

Go deeper

What a mortice costs the door

Timber chars from the exposed face. Cut a hole in it, fill the hole with steel, and that part of the door stops behaving like the rest. This is the whole reason locks on fire doors are a specification question rather than a shopping one.

Material removed: A mortice takes out part of the stile. That is insulation gone, and it also weakens the leaf's resistance to distortion at the moment the fire is trying hardest to twist it out of the frame.
Metal in place of timber: Steel conducts heat where timber resisted it. Where enough timber has been replaced, and only thin sections are left around the case, that part of the door gives way sooner than the rest of it would.
Voids: A mortice cut larger than it needs to be leaves air pockets inside the leaf. Clearance holes for spindles, cylinders and keys should be no bigger than the fitting instructions state, for the same reason.
Through fixings: Anything passing right through the leaf is a path for heat to cross it, and a bigger fixing is a bigger path. Keep them as small as the instructions allow.
Intumescent around the case: Where the fire evidence used intumescent to make up for the timber taken out, that material has to be there. It also fills any irregularities in the lock pocket, especially where the mortice was cut by hand rather than by machine. The lock modelled here does not show it, which is normal for a product shot and is not how the lock should sit in a door that needs it.This is not a judgement to make on site. Whether a given door needs intumescent behind or around the lock is answered by the manufacturer and the evidence, and fitting it where it was not tested is as much a change as leaving it out where it was.

Latched or unlatched

Whether a fire door needs a latch is not a universal rule. It is a question the door's evidence answers.

Tested and specified latched: The latch was part of the tested configuration, and the evidence relies on it: in fire the bolt holds the leaf in the rebate as the leaf tries to bow and the pressure pushes on it. On a door like this the latch must engage under closer power alone, every time.
Tested and specified unlatched: Many fire doors are tested without a latch, which gives the evidence scope to cover both arrangements; closing-device manufacturers test unlatched to prove the closer holds the door on its own. A door specified unlatched relies on the closer, the seals and the hinges to keep it shut, and the evidence proved that they do.
Match the evidence: The installed configuration must match the applicable evidence for that door. Locks, latches, keeps, intumescent protection and operating furniture all have to be compatible with it, and adding or removing a latch is a change to the configuration, not a detail.

Types, and which belong on a fire door

The code of practice groups locking devices into several families. Being in the list does not make a device suitable: some are actively unhelpful on a fire door.

Mortice latches and mortice locks: Morticed into the closing edge. On a timber door the surrounding leaf shields the case from the fire for a while, which is an advantage the surface-fixed types do not have. Sashlocks combine latch and deadbolt in one case.
Rim latches and rim locks: Fixed to the face of the door rather than buried in it, so it needs evidence covering that arrangement. A rim lock may also need a rim cylinder or operating furniture that itself requires a hole through the leaf.
Roller bolt catches: A sprung roller rather than a bolt. It cannot be relied on to hold the door, and it can stop the leaf coming fully into the frame, which defeats the seal line.Not recommended on a fire door that has no self-closing device. Note the distinction: some latches withdrawn by a handle use a roller instead of a bevelled bolt and do hold the door positively. Those are a different thing.
Multi-point locks and bored-in knobsets: Both exist on fire doors and both need evidence for the door type and configuration they are going into. Multi-point locks in particular remove a lot from the stile.
On a steel door: No timber comes out, so voids are not the issue. The problem moves to heat conducted through the door itself, and to making sure the case is rigidly supported with enough metal for the fixing screws to hold.

Evidence, marking and escape

Two separate questions get confused here: whether the lock is a properly made lock, and whether it has been proven on a door like yours.

Conformity marking: Product standards have defined scopes. BS EN 12209 covers mechanically operated locks and latches; multi-point locks and electromechanical locks fall under other designated standards; and a BS EN 179 emergency exit device is only a conforming device as a set with the lever or pad it was tested with. A UKCA or CE mark tells you the device meets the standard within whose scope it falls. Third party certification on top of that is worth having.
Fire evidence is a separate question: A conformity mark says the lock meets its product standard. It does not say the lock has been proven on a fire door. That comes from having been in a fire test, on a door built like yours and fitted the way yours is fitted. Ask for the evidence.
Bolt throw: Where the door's performance depends on the bolt engaging, the throw matters. A shorter throw than the tested one holds less of the leaf in the frame under pressure.
One single action on escape routes: Guidance to the Building Regulations across the UK and Ireland is that a locked door on an escape route opens with one action, without manipulating more than one mechanism. Security that conflicts with that needs a risk assessment, not a second thumbturn.

Fire Door Anatomy is an introduction to fire doors, not the whole subject. Always defer to the fire door’s certification, its test evidence and a competent inspection.