What you need to know before you start

A patio door lock replacement is a straightforward job you can do yourself in about 30 minutes with basic tools. The lock sits on the inside of the door frame, held by two or four screws, and slides out once you remove them. You do not need to take the door off its track or call a professional unless the lock housing itself is cracked or the door frame is damaged.

The first step is identifying what type of lock you have. Most sliding patio doors use one of three styles: a mortise lock (the most common, with a rectangular metal box recessed into the frame), a surface-mounted lock (a small metal box bolted to the inside face of the frame), or a keyed handle lock (where the lock mechanism is built into the handle itself). Knowing which one you have tells you what replacement part to buy and how to remove it.

Key Takeaways

  • Most patio door locks are held by two or four screws on the inside of the frame and can be removed without tools beyond a screwdriver.
  • Measure the backset (the distance from the edge of the door to the center of the lock hole) before buying a replacement, or bring the old lock to the hardware store to match it.
  • Mortise locks sit recessed in the frame; surface-mounted locks bolt to the face; keyed handle locks are part of the handle itself.
  • After installing the new lock, test it from both inside and outside before closing the door fully, because adjustment is harder once the door is sealed.

Identifying your lock type and measuring for a replacement

Look at the inside of your patio door frame where the lock sits. If you see a rectangular metal box recessed flush into the frame edge, you have a mortise lock — the most common type. If the lock is a small metal box bolted to the flat inside surface of the frame (not recessed), it is a surface-mounted lock. If the lock is part of the handle itself and you turn a key in the handle to lock it, you have a keyed handle lock.

Once you know the type, you need the backset measurement. This is the distance from the edge of the door frame to the center of the lock hole. Measure it with a ruler or tape measure — it is usually 2 3/8 inches or 2 3/4 inches for standard patio doors. Write this number down. If you are unsure, remove the old lock (see the next section) and bring it to a hardware store; staff can match it to a replacement or measure it for you. Do not guess, because a lock with the wrong backset will not line up with the existing hole.

Removing the old mortise lock

A mortise lock is the easiest to replace. Open the patio door and look at the inside edge of the frame where the lock sits. You will see two or four screws holding the metal lock box in place. Use a screwdriver (usually Phillips head, sometimes flathead) to remove all of them. Keep the screws in a small container so you do not lose them — you may reuse them for the new lock.

Once the screws are out, pull the lock box straight toward you. It should slide out without resistance. If it does not, check that you have removed all screws and that nothing is blocking it. Do not force it. If the lock is stuck, spray a small amount of penetrating oil (like WD-40) around the edges, wait five minutes, and try again.

After the lock is out, look inside the frame cavity. You may see a metal strike plate (the part the lock bolt hits when you close the door) still attached to the opposite frame. Leave it in place unless it is damaged or the new lock requires a different strike plate. Most replacement locks use the existing strike plate.

Removing a surface-mounted lock

A surface-mounted lock sits on the flat inside face of the door frame, not recessed into it. You will see a small metal box with bolts or screws running through it into the frame. Use a screwdriver or wrench (depending on whether it has screws or bolts) to remove all fasteners. There are usually two to four of them.

Once the fasteners are out, pull the lock box away from the frame. Check the frame surface for any damage, cracks, or splintering wood. If the frame is damaged, you may need to fill small holes with wood filler or have a professional assess whether the frame is safe to use. If the frame is sound, you are ready to install the new lock.

Removing a keyed handle lock

If your lock is built into the handle, you will need to replace the entire handle assembly, not just a lock box. Look for screws on the inside of the handle or on the frame behind it. Remove those screws and slide the handle out. Some keyed handles also have a small set screw that holds the key cylinder in place — check for it before you pull the handle free.

Once the handle is out, compare it to the replacement you bought to make sure the backset and hole spacing match. If you are unsure whether a replacement handle will fit, take the old one to the hardware store and ask staff to confirm the fit before you buy.

Installing the new lock

For a mortise lock, slide the new lock box into the cavity where the old one was. The bolt should face into the door frame (toward the outside of the house). Align the screw holes and insert the screws by hand first to make sure they thread correctly, then tighten them with a screwdriver. Do not over-tighten — snug is enough. The lock should not move when you wiggle it.

For a surface-mounted lock, position the new lock box on the frame face, align the bolt toward the outside, and insert the fasteners. Again, hand-thread first, then tighten. For a keyed handle lock, slide the new handle into place, thread the screws by hand, and tighten them. If there is a set screw for the key cylinder, tighten that last.

After installation, test the lock from both sides before you close the door. Turn the key or knob from the outside to make sure the bolt extends and retracts smoothly. Then test from the inside. The bolt should move freely and the door should lock and unlock without binding. If the bolt sticks or does not move all the way, loosen the screws slightly and adjust the lock box position a fraction of an inch until it works smoothly, then re-tighten.

Adjusting the strike plate if the door does not close smoothly

After you install the new lock, close the door gently and see if the bolt slides into the strike plate (the metal slot on the opposite frame) without resistance. If the door closes smoothly and the bolt catches, you are done. If the door feels tight or the bolt does not line up with the strike plate opening, the strike plate may need adjustment.

Loosen the strike plate screws slightly (usually two screws) and slide the plate up, down, or sideways until the bolt aligns with the opening. Close the door again to test. Once it closes smoothly, tighten the strike plate screws. If the bolt still does not catch or the door binds, the frame itself may be slightly warped — this is when a professional should inspect it, because forcing the door closed can damage the new lock.

When to call a professional

Call a professional if the door frame is cracked, splintered, or visibly warped. A damaged frame will not hold a lock securely and may not be safe to use. Also call if the bolt does not align with the strike plate even after you adjust the plate, or if the door binds when you try to close it after installing the new lock. A professional can assess whether the frame needs repair or replacement before a new lock will work properly.

If you are uncomfortable using a screwdriver or unsure which lock type you have after reading this guide, a locksmith or handyperson can replace the lock for you in under an hour. The cost is usually between $75 and $150 for labor, plus the price of the lock itself.

Frequently Asked Questions

Can I replace a patio door lock without removing the door?

Yes. The lock sits on the inside of the frame and is held by screws you can reach without moving the door. You do not need to lift the door off its track or disassemble anything except the lock itself.

What if I lose the key to my old lock?

You do not need the key to remove the old lock. straightforward unscrew it from the frame and pull it out. The new lock will come with its own keys.

Do I need to buy a lock that matches the old one exactly?

No, but you do need to match the backset (the distance from the frame edge to the lock hole center) and the lock type (mortise, surface-mounted, or handle-integrated). The finish and style can be different. Bring the old lock to the hardware store if you are unsure.

What if the new lock does not fit in the old hole?

This usually means the backset is different. Check your measurement against the new lock's specifications. If they do not match, return the lock and buy one with the correct backset. Do not try to force a lock with the wrong backset into the frame.

How do I know if my strike plate needs replacing too?

Most new locks work with the existing strike plate. Test the door after you install the lock — if the bolt slides smoothly into the strike plate opening and the door closes without binding, the old strike plate is fine. If the bolt does not align, loosen and adjust the strike plate first before considering a replacement.