What You Need Before You Start

A patio door lock installs in about 30 minutes with basic hand tools you likely already own. Most residential patio doors use one of two lock types: a mortise lock that sits inside the door frame, or a surface-mounted lock that attaches to the outside of the door. The mortise lock requires drilling into the door itself; the surface lock straightforward screws on. Both are stronger than the flimsy latches that come standard on many sliding doors.

Before you buy, measure your door frame thickness — typically 1.375 inches for standard doors — and check whether your door swings inward or outward. Take a photo of your current latch to show the hardware store clerk, or bring the door measurement and frame material (aluminum, vinyl, or wood). Most locks cost between $25 and $80 and come with installation instructions specific to your door type.

Key Takeaways

  • Mortise locks require drilling a hole through the door frame and are stronger than surface locks, but surface locks install faster and need no drilling.
  • Measure your door frame thickness and take a photo of the existing latch before buying a replacement lock.
  • Most installations need only a drill, screwdriver, tape measure, and pencil — no special tools required.
  • Mark your hole locations with a pencil and drill slowly to avoid cracking the frame, especially on aluminum doors.
  • Test the lock with the door open before closing it, so you can adjust if the bolt does not line up with the strike plate.

Installing a Mortise Lock (Drilling Required)

A mortise lock sits inside a cavity routed into the edge of the door frame. Start by removing the old latch if one exists — usually two screws on the inside face. Mark the center line of the door edge with a pencil, then measure down from the top of the door to find the lock height. Standard height is 36 to 40 inches from the floor; check your new lock's instructions for the exact measurement.

Drill the bolt hole first. Use a drill bit that matches the bolt diameter listed in your lock instructions — typically 1 inch. Drill straight through the door frame from the outside, stopping when the bit breaks through the inside face. This hole is where the bolt will extend. Next, drill the smaller hole for the lock cylinder (usually 5/8 inch) perpendicular to the bolt hole, intersecting it at the center mark you made.

Insert the lock body into the mortise cavity and find it with the screws provided. Attach the strike plate to the door frame on the opposite side, aligning it so the bolt enters the strike opening when the door closes. Close the door gently and mark where the bolt touches the frame — the strike plate should sit directly over this mark. Drill pilot holes for the strike plate screws, then screw it down firmly.

Installing a Surface-Mounted Lock (No Drilling)

Surface locks mount on the inside face of the door and require no drilling through the frame. Remove the old latch hardware and clean the door surface with a dry cloth. Position the new lock at the standard height (36 to 40 inches from the floor, or as specified in the lock instructions) and mark the screw holes with a pencil.

For aluminum or vinyl doors, drill small pilot holes at each mark — this prevents the frame from cracking when you drive the screws. Use a drill bit slightly smaller than the screw diameter. For wood doors, pilot holes are optional but recommended. Screw the lock body to the door face, tightening firmly but not so hard that you strip the holes. Attach the strike plate to the frame using the same method, positioning it so the bolt catches cleanly when the door closes.

Aligning the Bolt and Strike Plate

The most common installation mistake is misaligning the bolt and strike. With the door open, close it slowly and watch where the bolt contacts the frame. Mark this spot with a pencil. The strike plate opening should sit directly over this mark. If the bolt hits above or below the strike opening, loosen the strike plate and shift it up or down slightly, then retighten.

Test the lock several times with the door open, then with it closed. The bolt should slide smoothly into the strike opening without binding or rattling. If the bolt sticks, the door frame may be slightly warped — loosen the strike plate, close the door, and tighten it again while the door is closed. This lets the frame settle into its natural position.

Choosing Between Mortise and Surface Locks

Mortise locks are stronger because the bolt is anchored deep inside the door frame, making them harder to force open. They also look cleaner because the lock body is hidden. The trade-off is that drilling through the frame takes more time and carries a small risk of cracking, especially on aluminum doors. If you are uncomfortable drilling, a surface lock is a valid alternative.

Surface locks are faster to install and work well on doors that are already damaged or where you want to avoid drilling. They are visible from the inside, which some people prefer for ease of use. The bolt is less protected, but a quality surface lock with a reinforced strike plate still provides meaningful security improvement over a standard latch.

Tools and Materials You Will Need

Gather these items before you start: a cordless drill with bits (1 inch for the bolt hole, 5/8 inch for the cylinder hole, and a small bit for pilot holes), a Phillips-head screwdriver, a tape measure, a pencil, and a level. If your door is aluminum, use a slow drill speed to avoid cracking the frame. Have a damp cloth nearby to wipe away metal shavings.

Your new lock kit should include the lock body, strike plate, all mounting screws, and installation instructions. Read the instructions before you start — lock designs vary, and some require steps in a different order. If your door is very old or the frame is damaged, you may need wood filler or shims to may support the strike plate sits flush against the frame.

Common Problems and How to Fix Them

If the bolt does not line up with the strike plate, the door frame is probably slightly warped or the lock is mounted at the wrong height. Loosen the strike plate, close the door, and mark where the bolt actually touches the frame. Move the strike plate to match this mark and retighten. If the bolt still binds, the door may need shimming — slide thin wood shims behind the strike plate until the bolt moves freely.

If the lock feels loose after installation, the screws may be stripped. Remove the lock, fill the old screw holes with wooden toothpicks and wood glue, let it dry, then drill new pilot holes slightly offset from the originals. Reinstall the lock with fresh screws. If you drilled the bolt hole off-center, you may need to install a new lock — attempting to fill and redrill usually results in a weak installation.

Frequently Asked Questions

Can I install a patio door lock on an aluminum frame?

Yes, but drill slowly and use a sharp bit to avoid cracking the aluminum. Aluminum is softer than wood but more brittle, so high-speed drilling can cause stress fractures. If you hit resistance, stop and let the drill cool for a minute. Pilot holes are especially important on aluminum doors.

What if my patio door is a double slider?

Install the lock on the stationary panel (the one that does not slide), not the moving one. If both panels slide, lock the one that closes last — usually the right panel on a standard slider. This prevents the door from being forced open from the outside.

Do I need to remove the door to install a lock?

No. You can install most locks with the door in place. Work with the door open and closed several times during installation to check alignment. Removing the door is only necessary if the frame is severely damaged or you need to access the back side of the frame.

How tight should I screw the strike plate?

Screw it firmly until it sits flush against the frame, but do not over-tighten — you can strip the holes or crack the frame. If you feel strong resistance, stop and check that the pilot hole is the right size. One full turn past snug is usually enough.

Will a new lock work on a vinyl patio door?

Yes, but use pilot holes to prevent cracking the vinyl. Vinyl is flexible and can split if you drive screws without pilot holes. Drill the pilot hole slightly smaller than the screw diameter, then drive the screw at a steady pace without forcing it.