Mortice locks and wooden door locks, explained simply

A mortice lock is a lock that sits inside a pocket cut into the edge of a door, rather than bolted onto the face of it. The pocket is the "mortice", which is where the name comes from. On a timber front or back door in the UK it is the lock the key turns to throw a solid bolt across into the frame. The two types you will meet are a mortice deadlock (key only, no handle) and a mortice sashlock (a deadbolt plus a sprung latch worked by a pair of lever handles). The insurance-grade version on a final-exit door is a 5-lever lock made to British Standard BS3621, which carries the BSI Kitemark stamped into the faceplate. If your door is uPVC or composite it does not have a mortice lock at all, it has a multi-point mechanism and a euro cylinder, which is a completely different thing.

A mortice deadlock has one job: it throws a single deadbolt across into the frame, operated by a key from both sides, and that is it. No handle, no latch, no spring. You see it on a door that already has a separate nightlatch or handle to hold it shut, with the deadlock added underneath as the secure key-locked bolt. A mortice sashlock does two jobs in one case: it has the same key-operated deadbolt plus a sprung latch (the angled tongue that clicks the door shut), and it takes a pair of lever handles on a spindle to pull that latch back. So a deadlock locks, a sashlock both latches and locks. Which one you need depends on what else is on the door. If you already have a working nightlatch up top, a deadlock underneath is usually right. If the lock has to be the only thing holding and locking the door, you want a sashlock. Both come in BS3621 5-lever versions, and I carry both on the van.

On a wooden final-exit door, usually yes. Most UK home insurers name British Standard BS3621 in the policy wording as the minimum lock on a timber front or back door, and a 3-lever mortice lock cannot be Kitemarked to BS3621, so it will not meet the requirement. A BS3621 lock has at least 5 levers, a bolt that throws at least 20mm, and hardened plates to resist drilling, and it carries the BSI Kitemark plus "BS3621" stamped into the faceplate. The thing to keep in mind is that BS3621 is a timber-door standard. If your door is uPVC or composite, the insurer is really asking for a TS007 3-star cylinder instead, because a mortice lock cannot be fitted to that door. Check your own policy schedule, or send me a photo of the lock and the door on WhatsApp and I will tell you exactly where you stand before you spend anything.

Mortice locks come in two common case sizes. A 2.5 inch case (about 65mm) is the smaller one and suits a narrower door stile, with the key turn sitting around 45mm in from the edge of the door (that distance is the backset). A 3 inch case (about 76mm) is the bigger one, with the backset around 57mm, and it puts the key turn a bit further from the edge. The safest way to get a like-for-like swap right is to take the old lock out and measure the case, or just send me a photo of the faceplate with a tape measure next to it. I carry both sizes in deadlock and sashlock form, in 5-lever BS3621, so on a normal swap I have the right one on the van and there is no second visit. If a door has never had a mortice lock and the pocket has to be cut fresh, that is a bigger job and I will quote it once I have seen the door.

A nightlatch (people often call it a Yale, after the brand) is a rim lock. It bolts onto the inside face of the door rather than sitting inside the edge, and the key cylinder pokes through to the outside. It latches automatically when you pull the door shut, which is why a door can lock itself behind you. A mortice lock is the opposite layout: it lives inside a pocket cut into the door edge and throws a solid key-operated bolt into the frame. On a lot of older timber doors you get both, a nightlatch up top for everyday convenience and a mortice deadlock lower down as the secure lock. For insurance, a standard nightlatch on its own is usually not enough on a final-exit door, but there is a Kitemarked BS3621 nightlatch that does meet the standard, or you pair a non-rated nightlatch with a BS3621 mortice. If you are not sure what you have got, a quick photo on WhatsApp and I will tell you whether it is doing the job your insurer wants.

It depends on what has gone wrong. If the key is stiff, the bolt is sluggish, or the lock just feels notchy, that is often a clean and a re-lube, or freeing off a dropped door that is binding the bolt against the keep, and there is no new lock needed. If a lever has worn or snapped inside the case, or the key has sheared, or someone has tried to force it, then the lock body is usually past saving and a replacement is the honest answer. The other common reason to replace rather than repair is that the old lock is a 2 or 3-lever and your insurer now wants BS3621, in which case repairing it would not get you compliant anyway. I will always tell you on the doorstep which way it falls. If a clean and adjust fixes it, that is what you pay for. I am not going to sell you a new lock to fix a lock that just needed freeing off.

A BS3621 5-lever mortice deadlock or sashlock is from £120 fitted for a like-for-like replacement, which covers my labour plus a Kitemarked lock (Union, ERA or Chubb, depending on the door). That price assumes the door is already morticed for a lock of the same case size, which nearly every timber door that already has a mortice lock is. If the pocket has to be cut fresh into a door that has never had one, that is more work and I will quote it separately once I have seen the door. There is no call-out fee and no VAT added on top, and I photograph the Kitemark and BS3621 stamp once the lock is in so you can email the evidence straight to your insurer.

Three things cause nearly every stuck or stiff mortice lock. The most common is that the door has dropped a few millimetres on its hinges, so the bolt is no longer lining up with the keep on the frame and is binding as you try to throw it. The second is that the lock is dry and gummed up inside, which makes the key feel notchy and hard to turn. The third, the serious one, is a sheared key or a lever that has worn or snapped inside the case. If the key still goes in but will not turn, try lifting the door slightly by the handle as you turn, which takes the weight off a dropped bolt; if that frees it, the fix is usually adjusting the keep and easing the hinges, not a new lock. The same doorstep checks apply to any sticking lock, and I have written them up on my key won't turn guide. If a key has actually sheared off in the lock, do not force the stub, because that can jam it deeper. Send me a photo on WhatsApp with your postcode and I will tell you whether it is a free clean-and-adjust job or a replacement before I set off.

On a standard timber door, no, you stay with a lever mortice lock. A mortice lock and a euro cylinder are two different lock formats: the mortice lock is a self-contained case with its own levers cut into the door edge, while a euro cylinder is a separate barrel that drops into a euro-profile lock case. You can only run a euro cylinder on a timber door if the door and the lock case are converted to a euro-profile case, which means cutting a different pocket and is unusual on timber and rarely worth it. Where euro cylinders are normal is on uPVC and composite doors, which use a multi-point mechanism built around a euro cylinder as standard, and that is covered on my uPVC door lock replacement guide. If your insurer has asked for an upgrade and you are not sure which format your door takes, send me a photo and I will tell you whether you want a BS3621 mortice lock or a TS007 anti-snap cylinder.