What Is a Mortice Deadlock?
A mortice deadlock is a mortice lock stripped to its essential function: a deadbolt, a lever mechanism, and a key. Nothing else. No spring latch, no handle follower, no spindle. The bolt stays locked until the correct key retracts it, and it stays unlocked until the key throws it again. That is why it is called a dead lock — the bolt is dead in its position, held there by mechanical resistance rather than a spring.
This simplicity is the point. A deadlock has no internal springs or latches that can be manipulated. There is no handle to force, no latch tongue to card or shim. The only way to move the bolt is with the correct key. That makes the deadlock the preferred security lock for timber doors across the UK.
Where Mortice Deadlocks Are Used
The most common placement is on front doors, fitted below a night latch or Yale lock. The night latch handles everyday opening and closing. The deadlock provides the security deadbolt that you throw when you leave the house or go to bed. This two-lock configuration is the standard that most UK home insurers expect.
Deadlocks are also fitted as standalone locks on back doors, side doors, garage connecting doors, and any timber door where a key-only deadbolt is sufficient and a handle is not needed. For doors that need both a handle and a deadbolt in one unit, a mortice sashlock is the alternative. Learn more on our locksmith advice page.
Mortice Deadlock vs Mortice Sashlock
Both are mortice locks fitted inside timber doors. The difference is whether the lock includes a latch and handle alongside the deadbolt.
Mortice Deadlock
Contains only a deadbolt. No latch, no handle, no springs. Operated by key from both sides. The bolt stays in its position — locked or unlocked — until the key moves it. Nothing can be manipulated from outside without the correct key.
The compact case is smaller than a sashlock because there is no latch mechanism or follower to house. This makes deadlocks slightly easier to fit in narrow door stiles and period doors with less available timber.
Best for:
Front doors (below a night latch), back doors, side doors, any external timber door needing BS3621 where a separate handle is not required.
Mortice Sashlock
Combines a deadbolt and a spring latch in one case. The latch is operated by a pair of handles; the deadbolt is thrown by a key. One lock provides both everyday door operation and security locking.
The latch spring means the door clicks shut when closed and can be opened by pushing the handle without unlocking. This adds convenience but also adds a component (the spring latch) that a sashlock has and a deadlock does not — one more thing that could be exploited by a determined attacker.
Best for:
Back doors, side doors, internal doors — any door where you need both a handle for everyday use and a deadbolt for security in one fitting.
The Night Latch + Deadlock Combination
Most UK front doors use two locks working together. A night latch (or deadlocking night latch) sits at roughly handle height. A mortice deadlock sits about a third of the way up from the bottom. Between them, they provide both convenience and security.
How They Work Together
Night latch handles everyday use
You pull the door shut, the latch clicks, the door is closed. You open it from inside with the knob, from outside with the key. Quick, easy, no fumbling with a deadbolt every time you walk in and out.
Deadlock provides the real security
When you leave the house or go to bed, you throw the deadbolt. The bolt extends into the keep in the door frame and stays there. A night latch alone can be shimmed or forced — the deadlock is what makes the door properly secure.
Insurance requires the deadlock
Your insurer does not care about the night latch. They care about the 5-lever BS3621 deadlock. That is the lock they check after a break-in. Without it, the claim may be reduced or refused.
Why Not Just Use a Sashlock Instead?
Some front doors do use a single mortice sashlock instead of the two-lock setup. This gives you a handle and a deadbolt in one unit. It works, but the two-lock arrangement has advantages: the night latch provides an additional locking point higher up the door, the deadlock provides a second locking point lower down, and together they resist twisting and levering forces better than a single lock in the middle. For properties across South London — especially the Victorian and Edwardian terraces with older timber doors — the two-lock setup remains the recommendation. See our about us page for more on how we assess door security.
3-Lever vs 5-Lever Deadlocks
Deadlocks come in both 3-lever and 5-lever versions. The lever count determines security level and insurance eligibility.
3-Lever Deadlock
Three levers, limited key combinations, no anti-pick or anti-drill features. Does not meet BS3621. Cannot be used on any external door if you want your insurance to be valid.
Use for: Internal doors only — storage rooms, cupboards, utility doors where key-only locking is needed without handle operation.
5-Lever Deadlock (BS3621)
Five levers, tens of thousands of key combinations, anti-drill plates, anti-pick lever profiles, hardened anti-saw bolt inserts, restricted key blanks. Passes BS3621 testing. Accepted by every UK home insurer.
Use for: All external timber doors — front, back, side. The standard deadlock for insurance compliance on UK entry doors.
Not sure which yours is?
Open the door and check the faceplate on the edge. Look for the Kitemark, "BS3621", and "5 LEVER". If any are missing or worn, send a photo on WhatsApp and we will confirm. Free check, no obligation. Visit our contact page for all options.
How a Mortice Deadlock Works
A deadlock is a purely mechanical device. No electronics, no batteries, no springs that return to a default state. The bolt is moved by the key and stays where the key puts it. Here is the sequence.
Locking the Door
1. Insert the key through the keyhole into the key guide inside the lock case.
2. Turn the key. The bit (the shaped end of the key) lifts each lever to its correct height. On a 5-lever deadlock, five separate levers must each reach the exact position where their gates align.
3. With all gates aligned, the fence clears and the bolt thrower pushes the deadbolt out of the case and into the keep morticed into the door frame.
4. Remove the key. The lever springs push the levers back to their resting positions, misaligning the gates. The fence can no longer move. The bolt is now dead — locked in place with no way to retract it without the correct key.
Why "Dead" Lock?
The word "dead" refers to the bolt's state once the key is removed. A spring latch is alive — it can be pushed back by pressing the handle or by sliding a card between the door and frame. A deadbolt is dead — it does not move without the key. There is no spring tension to exploit, no handle mechanism to force, no latch tongue to shim. The bolt sits in the keep under its own weight and the mechanical lock of the misaligned lever gates. That fundamental difference is why deadlocks are the security standard, not sashlocks.
Why Deadlocks Are the Preferred Security Lock
The absence of moving parts that a burglar can exploit is the deadlock's core advantage. Here is what makes them harder to defeat than any latch-based lock.
No Latch to Card or Shim
A spring latch can be bypassed with a credit card, a strip of plastic, or a specialist shim tool. A deadlock has no latch at all. There is nothing to push back, nothing to slide between the door and frame.
No Handle to Force
A sashlock has a handle linked to the latch via a spindle and follower. That handle is an attack surface — it can be gripped, levered, or broken off. A deadlock has no handle, no spindle, no follower. There is nothing on the outside to grip or force.
No Internal Springs to Exploit
A sashlock has a latch spring that returns the latch to its extended position. That spring is a potential weakness — it can be manipulated. A deadlock has only lever springs, which push the levers down after the key is removed. There is no spring holding the bolt in position; the bolt is held by the lever gates themselves.
Smaller Case, Fewer Weak Points
The compact case houses only the bolt, lever pack, and key mechanism. No follower hole weakening the case wall. No spindle hole through the door. Fewer openings means fewer points where an attacker can insert tools.
Bolt Stays Put Without Power
A deadbolt is not spring-loaded. Cut the power (not that there is any), wait a week, shake the door — the bolt does not retract. It stays in the keep until the correct key turns it back. This is mechanical persistence that no electronic or spring-based lock can match.
BS3621 Anti-Attack Features
A BS3621 deadlock adds hardened anti-drill plates, anti-pick false gates, anti-saw bolt rollers, and restricted key blanks. Combined with the inherent security of the deadlock design, this package is what the UK insurance industry recognises as the minimum standard for external doors.
BS3621 and Home Insurance
A BS3621 certified 5-lever mortice deadlock is what most UK home insurance policies require on every external timber door. The deadlock is the lock the loss adjuster checks first after a break-in. If it is missing, non-compliant, or a 3-lever instead of a 5-lever, the insurer has grounds to reduce or reject the claim.
How to Verify Your Deadlock Is Compliant
Open the door. Look at the faceplate — the metal plate on the door edge. Three markings confirm compliance: the BSI Kitemark (heart-shaped symbol with a tick), the text BS3621 (or BS3621:2007), and 5 LEVER. If any are missing or illegible, the lock may not be compliant. Older locks manufactured before BS3621 existed will lack these markings regardless of quality.
We offer a free security audit across South London. We check every external door, photograph the faceplates, and confirm whether your locks meet your policy requirements. No charge, no obligation. Call 020 8050 2017 to book. See areas we cover.
Common Mortice Deadlock Problems
Bolt Will Not Throw Fully
Cause: Misaligned strike plate or keep. The door or frame has moved due to seasonal timber expansion, settlement, or hinge drop. The bolt hits the frame face instead of entering the keep cleanly.
Fix: Reposition the keep and strike plate, or deepen the keep mortice. If the frame has shifted significantly, the keep may need refitting at the new alignment.
Key Stiff or Grinding
Cause: Worn lever springs, dirt accumulation in the keyway, or a poorly cut duplicate key. In older South London properties, decades of use without servicing causes gradual degradation.
Fix: Rekeying (replacing the lever pack with new levers and keys) or full lock replacement if the case is corroded. Rekeying is available for Chubb, Union, and ERA deadlocks.
Key Snaps Inside the Lock
Cause: Forcing a stiff lock, a weakened old brass key, or a combination of misalignment and excessive force. The broken piece blocks the keyway completely.
Fix: Professional extraction of the broken key, then diagnosis of why it was stiff. From £120. We carry extraction tools on the van for same-day resolution.
Deadlock Not Insurance Compliant
Cause: The lock is a 3-lever deadlock on an external door, a pre-BS3621 model, or a non-certified 5-lever lock where the Kitemark is absent.
Fix: Upgrade to a BS3621 certified 5-lever deadlock. From £180 supplied and fitted. Same-day service available.
Mortice Deadlock Services in South London
New Fitting
Fresh mortice pocket cut into your timber door. Deadlock case, strike plate, keep, and escutcheons fitted. 30–60 minutes.
from £180Replacement
Existing deadlock removed, new BS3621 lock fitted into the same mortice pocket. Matched backset and case size. 20 minutes.
from £180Repair / Rekey
Stiff mechanisms serviced, broken keys extracted, worn levers rekeyed. Chubb and Union rekeying changes keys without replacing the case.
WhatsApp for quoteNight Latch + Deadlock
Full front door lock set: night latch above, BS3621 deadlock below. Both supplied and fitted in one visit.
from £430How We Work
Call or WhatsApp
Tell us which door needs a deadlock. We give you a fixed price before setting off. Send a photo on WhatsApp to confirm the lock specification.
Same-Day Arrival
We aim for 30 minutes across South London. On arrival we inspect the door, confirm the work, and agree the price before starting.
Fit, Test, Handover
Deadlock fitted, tested from both sides with all keys, strike plate aligned, Kitemark photographed. You pay the quoted price. Card or cash accepted.
Why Choose Locksmith South London
DBS Checked
Enhanced DBS check completed and verifiable.
5.0 Google Rating
Perfect score from verified South London customers.
30-Minute Response
Based across South London. Same-day as standard.
No Call-Out Fee
You pay for the work, not for us turning up.
Fully Insured
Public liability insurance on every job.
90-Day Guarantee
Workmanship guarantee on every lock we fit.
Mortice Deadlock Costs South London
All prices are fixed and include labour. No VAT. No call-out fee. Card or cash accepted.
What Our Customers Say
"The old deadlock on our front door had been getting stiffer for months. Lloyd came out, diagnosed worn lever springs, and fitted a new Union 2134 BS3621 deadlock in under half an hour. Key turns smoothly now and we have the Kitemark photo for our insurer. Exactly what we needed."
Tom F.
Carshalton
"We needed a night latch and deadlock fitted on a new timber front door after a renovation. Lloyd fitted both locks in the same visit, aligned everything properly, and the door locks and latches perfectly. He also checked the back door and flagged that it had a 3-lever that needed upgrading. Honest, thorough, good price."
Helen W.
Coulsdon
"Key snapped in the Chubb deadlock on a Friday night. Called and they were here within the hour. Extracted the broken piece, rekeyed the existing Chubb case with a new lever set and keys, and it works perfectly again. Saved us the cost of a full replacement. Very impressed with the speed and skill."
Steve M.
Thornton Heath
Mortice Deadlock Fitting Across South London
We carry BS3621 certified deadlocks on the van and cover every South London postcode.
Frequently Asked Questions
What is a mortice deadlock?
What is the difference between a deadlock and a sashlock?
How much does a mortice deadlock cost to fit?
Do I need a deadlock on my front door?
Can a mortice deadlock be fitted to any door?
What is the difference between a 3-lever and 5-lever deadlock?
How do I check if my deadlock is BS3621?
Which brands of deadlock do you fit?
Can a deadlock be repaired instead of replaced?
Do you offer emergency deadlock services?
Need a Mortice Deadlock Fitted?
Call for a fixed-price quote or send a photo of your door on WhatsApp. We carry BS3621 certified deadlocks on the van and fit same day across South London.
No call-out fee · Fixed prices · Card or cash accepted
24/7 emergency locksmith serving South London since 2015 — trusted by homeowners, landlords and businesses across Sutton, Mitcham, Wallington, Cheam, Purley and surrounding areas.