DOORSWhy a door won't latch, and why it usually isn't the lock

First test: try the lock with the door open

Hold the door open. Turn the key or the thumb-turn on a deadbolt, or the handle on a latch. If the bolt slides in and out smoothly, or the latch pulls back and springs all the way out again, the lock is working. If it binds even with the door open, the problem is in the lock or how it's mounted: dirt, wear, or a lock that's loose, overtightened or sitting crooked in the door.

If it works with the door open but binds when you close the door, the bolt isn't lining up with the hole in the frame. That's alignment, and it's the most common cause.

Find where the latch is landing

Put a little lipstick, chalk or marker on the end of the latch or bolt. For a latch, hold the handle turned, close the door, then let go so the latch springs out against the plate. For a deadbolt, close the door and turn the key or thumb-turn until the bolt touches. The mark transfers to the strike plate and shows exactly where the latch is hitting, so you can see whether it's high, low or off to one side.

Common causes, most likely first

• Loose hinge screws. It's usually the top hinge that works loose. The latch side of the door drops a little and the latch lands low on the strike, under the opening. Tighten them first. If a screw on the frame side just spins, the hole is stripped, and a longer screw that reaches the framing behind the jamb often fixes it.
• A strike plate that's a little out of position. A small miss can be fixed by filing the opening in the strike plate slightly larger, or by moving the plate. File only a little. On a keyed knob or lever, the small plunger next to the latch has to stay pressed against the plate when the door is closed. If it drops into the opening with the latch, the latch is no longer held locked.
• A swollen door. Humidity can make a wood door swell and rub the frame, usually on the latch side or at the top. Look for shiny spots or worn paint on the edge of the door.
• New weatherstripping or a fresh coat of paint. Thick weatherstripping holds the door away from the stop, so the latch stops just short of the hole and only catches when you push hard on the door. Paint built up on the door edge or the stop does the same thing.
• A worn or damaged latch. A latch that doesn't spring all the way out has a problem of its own.

A deadbolt that won't throw

When a deadbolt only works if you lift or push on the door, the bolt is rubbing on the strike plate or the edge of the hole in the frame. The fix is the same: find out why the door moved, then adjust the strike or enlarge the hole slightly. Check the depth too. If the hole is too shallow or packed with paint or sawdust, the bolt stops before it's all the way out, and a bolt that isn't all the way out isn't fully locked. Forcing the key against a binding bolt wears the lock out and can snap a key off in it.

If you're thinking about a smart lock

A motorized lock has less muscle than your hand, and it can't lift or push on the door to help the bolt in. Against a binding bolt it strains, jams, runs its batteries down and wears out sooner. It can also stop short and leave the door unlocked. If your deadbolt sticks today, fix the door first and then buy the lock.

Want to know what's inside the lock when it does need work? How a pin tumbler lock works.