Doors are perhaps the most used, abused, and neglected parts of a house. We rely on doors to separate us from weather, people, and things. We slam them shut, whip them open, and use all parts of our body to open and shut them. They are designed to open and close with little effort, with either a light turn or flip, and ease shut with a light click.
There are many factors that can ruin a perfectly functioning door. First and most important, is proper installation. But over time things move and wear. A house dropping, failing hinges, a poor functioning lockset, conflicting weather stripping, and weather all contribute to poor function. So what stops you from tearing a door out and replacing it? Repairability.
Every part of a door can be repaired and replaced. A door consists of many parts; some static and some moving. A door consists of the slab, hardware, and frame. The door can be wood, metal, fiberglass, composite, or plastic. The hardware includes the lockset, deadbolt, hinges, and other accessories like a door sweep, peep holes,
or mail slots. On the frame is the lockset or deadbolt strike plate, other side of the hinge, jamb (where the door hits), and the threshold (piece that is stepped on), and weather stripping. First step is to observe the door and see where the problem is.
The well functioning door slab relies on the jamb to stay square, the threshold to remain flat, the hinges to stay tight, the lockset and strike plate to remain functioning, and most importantly, the slab must not warp. Many wood doors suffer from poor construction, so they slowly drop on the lockset side, or warp from poor wood selection. Wood doors also suffer from rot, either slight or catastrophic, or from cracks and splits that bring in water or air. Metal can rust but they tend to stay more stable. Fiberglass, composite, or plastic tend to have components that fail, or they just die off slowly, but they cannot be repaired very easily. We tend to save old wood doors, since old wood is better quality than any wood today, and they have proven they can last for decades. Parts can be replaced, or epoxy can be used to repair them. They can also be cut and fitted to fit the jamb better.
Hardware does wear, especially if is it used a lot. Check to see if the strike plate, or where the lock flapper slides into, it properly aligned. They can be moved or made a little bigger with a Dremel tool. Old mortise locks, or the rectangular locks that slide into the side of the door, can always be fixed, especially in exterior doors. Old mortise locks with side buttons are some of the best locksets ever. Many require tuning. They are removed, opened up, cleaned, oiled, and reinstalled to work beautifully. Newer locksets can wear and break and can be easily replaced.
Hinges do wear down over time and the screws tend to come loose. Typically a door will rub against the threshold or the jamb if hinges are the problem. Check to see if hinges are worn. The two leaves should be even with each other. Also, you can lift the door when it is half open to see if the hinges are loose.
The last resort is to trim the door if it continues to rub. Sanding the door works well if it is rubbing slightly. Lay a piece of 80 grit sandpaper on the threshold and open and close the door if the bottom rubs slightly. You can use a block and sandpaper, a belt sander, or other electric sanders to take more off. A sharp block plane or power planer can take more off. Remove the door and cut it if it needs even more.
If a door cannot be repaired, then replacing it is an option. Setting a proper door takes some experience, since clearance is key to long-term operation. Be sure to follow installation directions, and check numerous times for smooth function. It can be hard to find a technician to repair a door, but they are out there. Many carpenters can help tune them.