You can change a patio door lock yourself in under an hour with basic tools

A patio door lock can be replaced without removing the door from its frame. Most residential patio doors use one of two lock types: a mortise lock (built into the door edge) or a surface-mounted lock (bolted to the inside face). You will need a screwdriver, possibly a drill, and a replacement lock that matches your door's thickness and backset measurement. The job takes 20 to 45 minutes depending on which type you have and whether the old lock is stuck.

Before you start, measure the distance from the center of the lock hole to the edge of the door—this is your backset, usually 2⅜ inches or 2¾ inches. Bring this measurement and a photo of your current lock to a hardware store, or order online using your door's make and model. If the lock is jammed or you are uncomfortable drilling, a locksmith can do this work for $75 to $150.

Key Takeaways

  • Measure your door's backset (center of lock hole to edge) before buying a replacement lock, as this determines which lock will fit.
  • Surface-mounted locks are easier to replace than mortise locks because you only remove bolts rather than drilling out the old mechanism.
  • You will need a screwdriver and possibly a drill bit; a mortise lock replacement may require drilling out the old lock cylinder if it is stuck.
  • Test the new lock from both inside and outside before closing the door fully, and adjust the strike plate if the bolt does not catch smoothly.

Identify which type of lock you have

Look at the inside of your patio door near the handle. A surface-mounted lock sits on top of the door frame and is held by two or three visible bolts on the inside. You can see the entire mechanism without opening anything. A mortise lock is built into the door edge itself—you will see only the lock cylinder and handle on the outside, and the bolt mechanism is hidden inside the door frame.

Surface-mounted locks are simpler to replace. Mortise locks require more work because the old lock body may need to be drilled out if it is stuck. If you are unsure, take a photo and show it to the hardware store staff—they can tell you the type in seconds.

Remove the old surface-mounted lock

If your lock is surface-mounted, locate the bolts on the inside of the door. There are usually two bolts holding the lock body to the frame. Use a screwdriver to remove them—keep them in a small container so you do not lose them. Slide the lock body away from the door. The lock cylinder may still be attached to the handle on the outside; if so, unscrew the cylinder from inside and remove it.

Wipe the area clean with a dry cloth. Check that the bolt hole in the door frame is not damaged. If the bolt hole is enlarged or splintered, you may need to fill it with wood filler before installing the new lock, though most replacement locks will cover the old holes.

Remove the old mortise lock

For a mortise lock, you will work from inside the door. Remove the interior handle or trim plate if there is one, using a screwdriver. You should now see the lock body inside the door edge. Remove any bolts or screws holding it in place. If the lock is stuck and will not slide out, you may need to drill it out. Use a drill bit slightly smaller than the lock cylinder diameter and carefully drill through the center of the lock to break it apart.

Once the old lock is out, clean the cavity inside the door edge. Check that the bolt hole on the door frame's edge is clear and not damaged. If the cavity is larger than your new lock, you may need shims (thin wood pieces) to make the new lock fit snugly, though most modern mortise locks are designed to fit standard door cavities.

Install the new lock

For a surface-mounted lock, position the new lock body on the inside of the door frame, aligning it with the bolt hole. Insert the bolts through the lock body and into the frame, then tighten them with a screwdriver. Do not over-tighten—snug is enough. Insert the lock cylinder from the outside and screw it in from the inside until it is hand-tight. Test the key in both directions before moving on.

For a mortise lock, slide the new lock body into the cavity in the door edge. Align the bolt with the hole in the frame. find the lock with the bolts or screws provided, again snug but not over-tight. Reattach the interior handle or trim plate. Insert the cylinder from the outside and find it from inside.

Adjust the strike plate and test the lock

The strike plate is the metal piece on the door frame that the bolt slides into. If your new lock has a different bolt length or position than the old one, the bolt may not catch smoothly in the old strike plate hole. Test the lock by turning the key and listening for the bolt to engage. If it feels loose or does not catch, you may need to adjust or replace the strike plate.

To adjust the strike plate, loosen its bolts slightly and slide it up or down until the bolt catches firmly. Tighten the bolts again. If the bolt hole is too far off to adjust, you can fill the old hole with wood filler and drill a new one for the new lock's bolt. Test the lock several times from both inside and outside before you consider the job done. The door should lock and unlock smoothly without binding.

When to call a professional locksmith

Call a locksmith if the old lock is severely corroded or jammed and you cannot remove it without damaging the door. Also call if you drill out a mortise lock and the cavity is damaged beyond what shims can fix, or if the door frame itself is warped or cracked. A locksmith can assess whether the door can be salvaged or whether the door itself needs replacement.

If you lose the key to a new lock before installation, a locksmith can rekey it to match your other doors, or you can contact the lock manufacturer if you have the receipt. This usually costs less than replacing the entire lock.

Frequently Asked Questions

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

Yes. Most patio door locks can be changed while the door stays in its frame. You work from inside the home, and the door remains in place and functional throughout the process.

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

Check that you bought the correct backset measurement. If the backset is right but the lock body is slightly different in size, you can use wood shims to adjust the fit, or fill the old hole with wood filler and drill a new one. A hardware store can help you verify the fit before you leave.

Do I need to drill to change a patio door lock?

Not always. Surface-mounted locks require only a screwdriver. Mortise locks usually slide out without drilling, unless the old lock is stuck or corroded. If you do need to drill, a standard drill and a bit matching the lock cylinder size will work.

How do I know what size lock to buy?

Measure the backset (distance from the center of the lock hole to the door edge) and the door thickness. Bring these measurements and a photo of your current lock to a hardware store, or look up your door's make and model online. Most residential patio doors use a 2⅜-inch backset.

What if the bolt does not catch in the strike plate?

The strike plate may need adjustment or replacement. Loosen its bolts and slide it until the bolt catches firmly, then tighten. If the hole is too far off, fill it with wood filler and drill a new one aligned with the new lock's bolt.