Fitting a lock or latch into a fire door is one of those jobs where the quality of the finished result comes down to how well you mark it out before you start cutting.

You can have all the right tools in the world, but if you get the position wrong, cut the mortice too big, drill in the wrong place or don’t get the faceplate sitting flat, you can create problems that follow you all the way through to the finished door.

This is the method I use.

It is based on doing the same things in the same order every time.

Repetition, repetition, repetition.

That is how you stop making mistakes.

A lock or latch is also not just about getting it into the door. It has to work properly with the spindle, handles, cylinder, keep, frame and the rest of the doorset.

And because this is a fire door, there is another level to consider.

The particular lock or latch, its dimensions, the permitted machining, fixings and any intumescent protection need to be compatible with the door’s supporting fire-performance evidence and the manufacturer’s installation instructions. Fire doors are assessed as assemblies, rather than as a door leaf working independently.

Important: This guide covers the general fitting of conventional mechanical fire-door locks and latches. It is not intended to cover every type of specialist locking system.

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Fire Door Ironmongery

This Part 10 is also the main starting point for our Fire Door Ironmongery branch.

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Scope of This Guide

This guide deals with the general installation of conventional mechanical locks and latches commonly fitted to standard fire doors.

There are other locking systems that are much more technical.

These can include:

• Electronic locks
• Electric strikes
• Access-control systems
• Card readers
• Electromagnetic locking systems
• Specialist escape hardware
• Electrically controlled locking systems
• Other electronically controlled door hardware

Those systems can involve electrical installation, access-control requirements, specialist hardware and additional fire-safety considerations.

They also need to have the appropriate supporting evidence for the particular fire-door assembly. BS 8214:2026 places greater emphasis on supporting evidence and covers fire-resisting and smoke-control doors and their installation across different door types and materials.

So don’t read this article and think that because you can fit a normal mortice lock, you can automatically fit every type of electronic or access-control system into a fire door.

A standard mechanical lock is one job. A specialist access-controlled fire-door system is another.

For those systems, follow the hardware manufacturer’s instructions and the fire-door manufacturer’s supporting evidence.

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Choosing the Right Lock or Latch

One thing I always think about before buying the lock or latch is how I’m actually going to cut the mortice.

If I’m going to cut it by hand using chisels, I prefer a square-ended lock or latch.

If I’m going to machine the mortice, a rounded-end lock or latch can make more sense because the cutter naturally leaves a rounded end.

You can machine a rounded mortice for a square-ended lock and then square the corners afterwards with a chisel.

That’s easy enough.

But going the other way is more awkward.

If you’ve cut a rounded-end mortice by hand and then decide you need a square-ended lock, you’ve got to start removing more timber from the ends and carefully square everything up.

So think about the lock and the way you’re going to cut it before you buy it.

It saves time.

Always check first that the lock or latch you’re choosing is actually permitted for the particular fire door. The easiest lock to cut is no good if it isn’t compatible with the door’s certification and installation instructions. BWF guidance specifically stresses that locks and latches need to be compatible with the particular fire-door evidence.

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Setting the Lock Height

The height of the lock can vary depending on the building, the specification and the existing doors.

As a normal working datum, I usually work at 1 metre from the bottom of the door, measured to the centre of the spindle.

That isn’t a universal fire-door requirement. It’s the working position I commonly use.

Sometimes the architectural drawings or specification will give you a different height.

And when you’re working in an existing building, I always look at what is already there.

If every other door in a building has the locks at a particular height, there may be a good reason for it.

So check the existing doors and any drawings or specification before deciding where your new lock is going.

The important thing is that you establish one consistent datum and then use it throughout the job.

I mark the lipping face first.

From there I establish the spindle centre and then mark the rest of the lock from that point.

I use a combination square because it gives me a straight and repeatable reference.

Marking the Lock and Backset

Once I’ve established the spindle height, I look at the actual lock I’m fitting.

The backset is the distance from the edge of the door to the centre of the lock spindle/cylinder arrangement.

Common sizes you will come across include 57 mm and 76 mm, but don’t assume those are the only sizes.

Measure the actual lock.

I mark the backset using my combination square.

Then I mark the lock body and the faceplate.

For a mortice lock, I will also look at where the cylinder holes need to be and mark those positions from the lock itself.

Don’t assume every lock is identical.

Measure the lock you’re actually holding.

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Marking the Faceplate

Now I look at the faceplate or fore-end of the lock.

I measure the width of the faceplate — 25 mm is a common example — and set my marking gauge accordingly.

For a 44 mm door, I commonly use around 9.5 mm from each face.

For a 54 mm door, around 14.5 mm.

These are working measurements I use to centralise the lock in the door, not universal fire-door dimensions.

I set the gauge, make a light mark and then check it.

If I’m not happy with it, I adjust it.

I don’t start making loads of heavy marks around the lock.

I usually make trial marks towards the bottom of the door where they will disappear once everything is finished.

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The Marking Gauge Trick

There is another little trick with the marking gauge that makes things much easier.

You don’t want the gauge running past where you actually want it to stop.

So I’ll make two small pinpricks at the bottom of the lock area.

I then take the marking gauge to the top and run it down the side.

When it reaches the bottom, the gauge drops into those two little pinpricks and stops.

It’s a simple joinery technique, but it gives you a very controlled mark.

And that’s what we’re looking for throughout this job:

Control.

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The Apple-Skin Principle

When I’m marking a lock with a marking gauge, I don’t want to bury the gauge pin into the timber.

I want to break the skin of the timber.

Think of it like taking the skin off an apple.

If you push the pointed gauge pin deeply into the timber, the hole gets wider as it goes in.

You don’t need that.

Make several light passes until you’ve got a fine, clean line.

That gives you a boundary without unnecessarily damaging the lipping.

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Don’t Chisel the Lipping to Reinforce the Lines

This is an important one.

With a normal hinge recess, you can establish the perimeter by carefully reinforcing the line.

I don’t do that with the lock faceplate on the lipping.

The lipping may only be around 9 mm or 12 mm thick depending on the door construction.

If you start hammering a chisel into the sides of the lipping, you can encourage the timber to split along the grain.

That’s exactly what I don’t want.

The marking gauge and knife line have already created the boundary.

Leave it alone.

Now remove the timber towards that line in a controlled way.

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Mark the Centre of the Lock Body

There is an important difference between the shallow faceplate and the deeper lock body.

The faceplate is only the shallow part.

The actual lock body goes much deeper into the door.

For example:

• 44 mm door = 22 mm centre
• 54 mm door = 27 mm centre

Those are simply half the door thickness and are useful working centre marks.

This gives me the centreline for the deeper lock body.

Don’t start by drilling the shallow faceplate recess.

I want to get the deep lock body established first.

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Drilling the Lock Mortice

Now we’re into the deeper part of the job.

The choice of drill bit matters.

I don’t like aggressive bits with a large driver screw or centre point that wants to bury itself into the timber.

Those can encourage splitting, particularly in the lipping.

In my experience, speed bits are very good for this type of timber work because they cut into the timber rather than aggressively pulling themselves through it.

I mark the deepest areas I need to remove.

Then I drill.

I normally drill towards the top and bottom of the deepest section, with the top drill angled slightly upwards and the bottom drill slightly downwards.

That helps remove the excess timber.

But you need to control your depth.

Don’t just drill as deep as the drill will go.

The permitted mortice dimensions and depth are controlled by the particular door and its supporting evidence.

BWF guidance specifically says the lock/latch recess needs to be the correct size and that the manufacturer’s requirements and fire-door evidence need to be followed.

You can put a piece of tape around the drill as a visual depth guide if necessary.

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Don’t Drill the Full Width Straight Away

If the lock body is 25 mm wide, I don’t automatically put a 25 mm drill straight through the timber.

I’ll often use around a 22 mm drill and work from the centre.

That leaves me some timber either side.

If the drill wanders slightly, I haven’t immediately broken through my finished boundary.

Then I work progressively towards the final dimensions.

Again, it’s the same principle:

Path of least resistance.

Let the timber come away towards the void you’ve already created.

Don’t try to make the perfect finished mortice in one operation.

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Chiselling the Faceplate Recess

Once the deep mortice is established, I can work on the shallow faceplate recess.

The perimeter has already been marked.

Now I start removing the timber.

I use controlled 5–10 mm increments rather than trying to take a huge piece out in one go.

Light hammer blows.

Controlled chisel.

Remove the waste.

Check the lock.

Then continue.

I use different chisels for different parts of the job depending on what I’m doing.

The important thing is that you don’t rush it.

You are creating a shallow, accurate recess for the faceplate to sit in.

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Keep the Faceplate Recess Flat

This is one of the things I really want to emphasise.

The faceplate recess needs to be flat.

It isn’t enough for the faceplate to simply be deep enough.

If the recess is deeper on one side than the other, or the bottom isn’t flat, the faceplate can sit at an angle when you screw it down.

And once that faceplate is angled, the lock body behind it is also being held at an angle.

Now you’ve potentially created a problem further down the line.

Think about the spindle.

You want a straight spindle passing through the lock and the handles sitting straight and level.

If the lock body is sitting slightly angled, you’re trying to fit straight hardware into something that isn’t straight.

You can then find yourself trying to straighten the levers to make them look right.

You might adjust the spindle.

You might move one handle.

You might start wondering whether the lock itself is faulty.

But the problem could have started with the faceplate recess.

You’ve created a binding action because you’re trying to make the rest of the hardware compensate for an angle that shouldn’t be there.

The handle may not return properly.

The latch may not retract as freely as it should.

The door can start snagging or sticking.

And you can end up chasing the problem somewhere else.

The same principle applies to the keep.

If the keep is sitting at an angle, the latch isn’t entering a properly aligned opening.

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Drilling the Spindle Hole

Once the lock body is prepared, I drill the spindle hole from the positions I’ve already marked.

The exact drilling arrangement depends on the lock and hardware being used.

The important thing is to make sure the spindle is properly aligned with the lock.

Don’t start enlarging holes unnecessarily just because something doesn’t line up.

If something isn’t lining up, stop and find out why.

It may be the marking.

It may be the lock.

It may be the faceplate.

It may be the door.

Find the actual problem.

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Drilling the Euro Cylinder Hole

With a Euro cylinder, I mark the cylinder position from the lock.

I normally use a 19 mm drill for the cylinder opening.

I drill one hole and then another immediately below it so that the two overlap.

That creates the elongated opening required for the cylinder.

I then use a drill-mounted rasp to shape the sides and clean up the opening.

This is my preferred method.

It gives me control over the final shape without attacking the door with a chisel and trying to create the elongated hole by hand.

Again, the exact hole size and permitted machining depend on the particular hardware and door evidence.

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Clearing the Mortice

Before I trial-fit the lock, I make sure the mortice is clean.

Once the spindle hole has been drilled through, it also gives me an opening into the lock mortice.

I can use that to help clear the waste and debris out of the deeper recess.

Don’t leave bits of timber sitting behind the lock.

You want the lock body to sit properly in the mortice.

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Fitting the Lock Gasket or Intumescent Protection

Some fire-door locks and latches have a gasket or require specific intumescent protection.

Don’t assume one arrangement fits every lock.

Where the particular lock has a supplied gasket, make sure it is correctly seated before the lock is inserted.

Where intumescent protection is required, it needs to be the correct material and arrangement specified by the door’s installation instructions and supporting fire-performance evidence.

BWF guidance specifically states that the correct intumescent material or mastic should be used where required by the installation instructions and fire certificate data.

The important thing is:

Don’t improvise the fire protection around the lock.

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Fitting the Lock

Now I put the lock into the mortice.

It should go in neatly.

It shouldn’t need to be hammered into an undersized hole.

If it doesn’t fit, find out where the timber is stopping it.

Don’t just make the whole mortice bigger.

Remember what we are trying to achieve.

A correctly sized recess.

Not a huge hole with a lock floating around inside it.

Once the lock is in, check:

• Spindle hole alignment
• Cylinder hole alignment
• Faceplate position
• Lock body fit
• Faceplate flatness
• Correct gasket/intumescent arrangement where required

Then screw it in using the correct fixings.

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Understanding the Spindle

The spindle is another area where I’ve seen hardware fitted incorrectly.

There are different spindle arrangements, but the principle is the same.

Some have a split section.

Others have an off-centre hole.

The grub screw in the lever needs to engage with the correct part of the spindle.

With a split spindle, the grub screw engages with the recess and helps grip the lever.

With an off-centre solid spindle, the hole is positioned towards one edge and the grub screw engages with that hole.

If you put the spindle in the wrong way around, the grub screw may not grip properly.

You can then end up with a handle that works loose or doesn’t secure properly.

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Fitting the Levers

Now I fit the levers.

I want the spindle centralised and the two levers sitting properly opposite each other.

I normally fit one lever first.

Tighten the grub screw enough to hold it.

Then fit the other lever.

I make sure the two handles are correctly positioned before finally tightening everything.

On this type of lever furniture there can be three fixing or retention points.

Don’t ignore the third one.

I’ve seen people fit handles using only the short screws and leave the other fixing points.

It might look fine when you’ve finished.

But every time the handle is operated, that movement is being transferred into the fixing.

Over time, things can loosen.

And the last thing you want is somebody pulling a lever and having it come off in their hand.

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Through-Fixing the Levers

Some lever furniture has fixing holes positioned directly behind the lever.

That can make them awkward to access without scratching the finished hardware.

Some designs also allow a degree of adjustment.

I mark the fixing holes, remove the lever where necessary and drill the holes through the door using the appropriate drill size for the hardware.

Then I refit the levers.

Through-fixing bars can have break-off points for different door thicknesses, such as 44 mm, 54 mm or other sizes depending on the hardware.

Make sure you use the correct setting for the door you’re actually working on.

Don’t overtighten the through-fixings.

You can actually start drawing the two sides of the door furniture together.

That can affect the way the lever operates.

You want everything secure without distorting the furniture or the door.

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Escutcheons

If the lever has an escutcheon around it, fit that carefully.

Some escutcheons have a small groove or removable section.

I normally put that at the bottom so that it is hidden and can still be removed later if required.

Different escutcheons use different screw arrangements.

Take your time.

If you’ve got slotted screws, I like to see the slots lined up properly.

That’s one of those little details.

It doesn’t make the door more fire resistant.

But if you’re fitting a professional job, do a proper job.

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Starting the Screws

This is where the apple-skin principle comes back.

If you’re fitting screws into finished timber or finished hardware, don’t let the screw point wander across the surface.

I will often make a tiny starter hole — sometimes only around 1 mm through the surface skin — so the screw has somewhere to start.

Then let the screw do the work.

This is particularly useful with slotted screws because you don’t want the screw skating across the surface and damaging the finish.

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Fitting the Euro Cylinder

The Euro cylinder needs to be fitted so that everything lines up properly.

I usually put the cylinder through from one side and fit the escutcheon and screw arrangement.

Then I remove it and repeat the process from the other side where the design requires it.

That keeps the two sides aligned.

With a thumbturn cylinder, make sure the thumbturn and cylinder are correctly positioned before tightening everything.

When inserting the cylinder, you’ll reach a point where the central black cam is in the correct position to pass through the lock.

Once it is correctly positioned, fit the cylinder retaining screw.

Depending on the design, that screw may be visible or hidden behind the lever furniture.

Don’t force the cylinder.

If it isn’t sliding in correctly, find out why.

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Fine Adjustment of the Escutcheon

Sometimes the escutcheons are only slightly out of alignment.

Don’t immediately start attacking the hole with a drill.

If it is only a very small amount, a rubber-ended hammer or rubber mallet can sometimes be used to make a tiny correction.

The same applies to some lever furniture.

You’re looking for millimetres, not another hole.

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Now Fit the Keep

Once the lock or latch is fitted, we come to the keep.

This is another place where I try to avoid unnecessary measuring.

The easiest way I’ve found is to let the door tell you where the keep needs to go.

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Marking the Position of the Keep

Once the latch is fitted, I virtually close the door.

Then I close it fully.

With the door closed, I mark the top and bottom of the actual latch directly onto the frame.

That’s my exact height.

I now know where the latch will actually land.

I don’t need to get a tape measure out and try to calculate it.

Why introduce another opportunity to make a mistake?

The latch is physically there.

The door is physically there.

Use them.

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Finding the Depth of the Keep

Now I do use the tape measure.

I measure from the back side of the door where it goes into the rebate across to the flat face of the latch.

That gives me the distance from the door rebate to the face of the latch.

Then I measure that same distance from the rebate in the frame.

For example, if the measurement is 35 mm, I measure 35 mm out from the rebate of the frame and mark that point.

Now I know where the edge of the keep needs to sit.

This is important because when the door closes, I want the latch to enter the keep properly.

I don’t want the door rattling around.

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Centralising the Keep

Now I put the keep onto the frame.

I centralise it around the latch.

I leave a little clearance at the top and bottom rather than fitting it so tightly that the latch is almost touching the keep.

I’ve seen people fit keeps too tightly.

Then, when it doesn’t work, they are moving the keep up and down trying to make it work.

It’s much easier to get it centralised properly in the first place.

Make sure it is straight.

Make sure it is in line with the latch.

Then mark around it.

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Marking the Keep

There are a couple of ways to mark the keep.

You can temporarily screw it into position and then run a sharp knife around the perimeter.

Or, if someone is helping you, one person can hold the keep firmly while the other marks around it.

Again, I don’t want a huge deep mark.

I’m simply breaking the skin of the timber.

Just like with the lock:

Light, clean and controlled.

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Marking the Latch Opening

I then use a pencil to mark the inside rectangular part of the keep where the latch actually enters.

If it is a single latch, I mark the single opening.

If it is a double-latch arrangement, I mark both openings.

These areas need to be deeper than the shallow faceplate recess.

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Drilling the Keep Opening

Once the internal openings are marked, I drill them out.

If it is a single latch, I normally drill one straight hole.

I don’t use the angled drilling technique I use on the deeper mortice lock.

But there is another detail that is easily missed.

As the latch operates, the barrel rotates.

Parts of the latch can project slightly above and below the main opening as it operates.

So I also remove a small amount of timber above and below the main latch opening to give those moving parts clearance.

If you don’t do this properly, the latch can catch on the keep.

That’s particularly important when you’re dealing with a tight-fitting keep.

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Chiselling the Keep Recess

Once I’ve drilled the deeper latch opening, I go back to the perimeter.

I use the chisel very lightly around the knife line.

Then I use the same technique as I used for the lock:

5–10 mm increments.

Don’t try to remove the entire recess in one go.

Work your way through it.

Reset the depth gradually.

Keep trying the keep in the recess.

Remove the waste.

Check it again.

The aim is to create a clean, smooth recess with the faceplate sitting properly in the frame.

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Keep the Keep Flat

The keep needs to sit flat, just like the lock faceplate.

If the recess is deeper on one side, the keep can sit at an angle when you screw it down.

That changes the angle at which the latch enters.

You can then end up with the latch catching against the side of the keep.

So don’t just look at whether the keep is recessed.

Look at whether it is flat.

The faceplate should have proper support around it.

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Hardwood Frames – Pre-Drill the Screws

Once I’m happy with the keep, I screw it into position.

And again:

Hardwood frames — always pre-drill your screw holes.

Hardwood can be very unforgiving.

If you try to drive the screw straight into a hard frame without an appropriate pilot hole, you can snap the screw.

And once you’ve snapped a screw in there, you’re in trouble.

BWF guidance also recommends pilot holes of the correct size and depth for the fixings to avoid splitting and says not to over-tighten screws.

So pre-drill.

Use the correct fixing.

Use the correct pilot-hole size for the actual screw and timber.

And don’t over-tighten.

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Checking the Keep

Once the keep is screwed in, close the door.

Now check how much the door rattles.

There is one thing to remember here.

If the door is unpainted, you may have approximately 1 mm of clearance that will subsequently be taken up by the paint finish.

So an unpainted door can sometimes have a little movement at this stage.

But generally, when the door is shut, I don’t want it rattling around in the keep.

It should close positively.

It should latch properly.

And it should feel solid.

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Fine Adjustment of Modern Keeps

Modern, better-quality keeps can sometimes give you a little adjustment.

Some have a small angled section or nib that can be moved slightly.

That allows you to make a fine correction to the amount of play between the latch and the keep.

Sometimes you’re only correcting a millimetre or so.

But that little adjustment can make a big difference.

You don’t need to move the whole keep around if the position itself is correct.

Use the adjustment provided by the keep where appropriate.

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Common Lock and Latch Problems

There are a few mistakes I see repeatedly when locks and latches are fitted.

The lock is too high or too low

The original marking-out was wrong.

Go back to the datum.

Don’t start modifying the hardware to compensate for a bad starting position.

The mortice is too big

Too much timber has been removed.

This is one reason I prefer to work progressively.

You can always remove a little more.

You can’t put the timber back.

The faceplate isn’t flat

The recess hasn’t been cut accurately.

This can angle the lock body and create problems with the spindle and handles.

The handle is binding

Don’t automatically blame the handle.

Check:

• Lock alignment
• Spindle alignment
• Faceplate flatness
• Through-fixing tension
• Handle alignment
• Lock operation

The latch catches the keep

Check:

• Keep height
• Keep depth
• Keep flatness
• Latch position
• Clearance around the latch
• Whether the keep has been fitted square

The door rattles when closed

Check the keep position and adjustment.

Don’t automatically move the whole keep.

The screw has snapped

Especially in hardwood, the first thing I’d ask is:

Did you pre-drill it?

The lock won’t sit properly

Don’t just make the hole bigger.

Find where the lock is actually binding.

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Don’t Forget the Fire-Door Evidence

This is probably the most important part of the whole article.

A lock or latch may physically fit into a fire door and still not be an acceptable installation for that particular door.

The fire door is an assembly.

The leaf, frame, seals, hinges, closer, locks/latches and other relevant hardware all form part of the system. BWF states that essential hardware needs to be compatible with the particular fire-door certificate and supporting documentation.

BS 8214:2026 is the current British Standard for fire-resisting and smoke-control doors and specifically covers installation, ironmongery, holes and fits. It supersedes BS 8214:2016.

So before you start cutting:

Check the door manufacturer’s installation instructions and supporting fire-performance evidence.

Check:

• Lock or latch type
• Permitted size
• Permitted position
• Permitted mortice depth
• Intumescent protection
• Gaskets where specified
• Fixing requirements
• Compatible furniture
• Cylinder arrangements where applicable
• Any restrictions on additional hardware

Don’t assume that because another fire door has a particular lock fitted, yours can have exactly the same thing.

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Final Lock and Latch Check

Before I walk away from the door, I want to check everything.

Lock

• Lock sits correctly in the mortice.
• Faceplate is flat.
• Faceplate is not sitting proud unnecessarily.
• Screws are secure.
• Correct gasket/intumescent arrangement is fitted where required.

Spindle and Handles

• Spindle is correctly positioned.
• Grub screws are engaging correctly.
• Levers are straight.
• Through-fixings are secure.
• Furniture isn’t being pulled together too tightly.
• Handle operates smoothly.

Cylinder

• Cylinder is correctly positioned.
• Cam operates correctly.
• Retaining screw is fitted.
• Escutcheons are aligned.
• Cylinder operates smoothly from both sides.

Keep

• Keep is correctly positioned.
• Keep is centralised around the latch.
• Keep is flat.
• Screws are secure.
• Hardwood pilot holes have been used where appropriate.
• Latch enters cleanly.
• Door doesn’t rattle excessively.

Door Operation

Close the door.

Operate the handle.

Release it.

Close it again.

The latch should retract properly and then engage correctly when the door closes.

You want:

Clunk, click.

Not a door that has to be slammed.

Not a door that rattles.

Not a handle that sticks.

Not a latch that catches.

And not a lock that has been forced into a hole that’s twice the size it needs to be.

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The Main Lesson

Fitting a fire-door lock or latch isn’t really about how quickly you can get the mortice cut.

It’s about how accurately you control the job from the beginning.

Mark it properly.

Use the right lock for the cutting method.

Don’t damage the lippings.

Drill the deep section carefully.

Work towards your lines.

Keep the faceplate flat.

Keep the keep flat.

Pre-drill hardwood.

Align the spindle.

Fit the furniture correctly.

Let the actual latch tell you where the keep goes.

Then close the door and check the finished result.

And above all:

Don’t take more timber out than you need to.

Once the timber is gone, you can’t put it back.

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Fire Door Ironmongery

Part 10 is the main branch for specialist fire-door ironmongery.

Fire Door Installation Series

Part 9 – Installing Fire Door Hinges

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Part 10 – Installing Fire Door Locks & Latches

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