Warped doors aren’t hard to detect.  You usually notice that the door is getting very hard to open or takes just a little more slamming before it will close.  Wood doors are the worst for getting warped but it does happen to metal doors too.

The wood door is usually sealed on all of its surfaces with a light coat of varnish or lacquer, but if that gets a scratch, or an area just rubs off, the next time it rains or at a change of seasons the moisture difference seeps into the door and causes it to warp.  The more the door warps, the harder it is to use the lock.  The metal door warping is typically the result of being kicked at the bottom during opening or someone using a wedge type door stop to hold it open.  In most cases I have seen, the top of the door contacts the jamb first when the door is warped.

When installed correctly a door should take very little pressure in the closing direction.  As a door warps either the top of the door or the bottom will contact the jamb before the other end of the door.  In order to get that other end aligned with the jamb and allow the lock to engage you need to push or pull harder on the door.

There is a way to fix it!

But it does require some patience.  You will need a wooden pencil.  Where the door contacts the jamb is a projecting part of it called the stop strip.  Sometimes the stop strip is part of the jamb and sometimes it is nailed or screwed in place.

Place the pencil between the stop strip and the door and pull hard on the knob or lever until the door latches in place.  The pencil is effectively bending the door to reverse the warping.  You need to leave the door closed with the pencil in place for around 15 minutes, that’s where the patience comes in.  After the 15 minutes, unlock the door, retrieve the pencil, and check the door. If there hasn’t been enough change, try it again with a thicker pencil.

If it is a wood door, find the place that is letting the moisture inside and refinish it once the door is straight again.  Note that in a few cases the door can become bowed and just the middle doesn’t touch the stop strip causing the need for slamming to latch.  In those cases you need to put a pencil or something a little thinner at the top and bottom to straighten it.

The same process can be applied to a metal door but they may require something larger than a pencil to have any lasting effect.