No door takes more use and abuse than an exterior door. It takes a beating and can become warped and cracked. If the door is in bad shape, you're better off replacing it with a newer, more energy-efficient door. Most of the replacement doors on the market today are not too difficult to install.
The easiest approach is to purchase a new door that fits directly into your existing jamb. Since most doors are now manufactured in standard sizes, you will probably be able to find a replacement door the same size as your existing door. Unless your door is very unusual in size, you should be able to find a door close enough to your existing door. You can trim only about an inch or so off the door's width and 3 or 4 inches from its height before you weaken the door or cause problems when installing the locks.
Installing Your Door
It is not possible to cut down a steel, aluminum, or fiberglass-clad door, so if you are thinking about upgrading to one of these doors, you should look for a pre-hung door. Easiest to install are doors that come pre-hung in weather-stripped frames that can be inserted directly into your old door-jamb. They are nailed or screwed to the old jamb. All you have to do is insulate the cracks between the jambs and reinstall the old trim or purchase new trim and install it.
This is not a difficult project, but care must be taken to get everything square and see that the new jamb is properly attached to the old jamb for the door to function properly over the long haul. You should be aware that installing a new jamb over your old takes up a bit of the door opening. So the replacement door will be slightly smaller than the original door.
Step 1: Measure your existing jamb carefully. Take measurements in several places to determine the height, then use the smallest of these measurements. Do the same for the width. You can place wood shims behind the new jamb if the old opening is large, but there is not much you can do if the opening is too small. Most replacement doors give you an inch or so of adjustment, but that is all. Know the swing of the door you need to order. Viewed from inside, doors that have hinges on the left side are right-hand doors, those with hinges on the right are left-hand doors.
Step 2: Remove the old door. Then carefully remove the inside and outside trim. We have found that driving the finishing nails through the trim with a nail set is easier than trying to pull the nails out. Some doors require you to remove the threshold.
Step 3: Place the new door frame in the opening and square it up. Then place shims between the new jamb and the old frame. Use the recommended screws to fasten the new jamb in place. Constantly check that everything stays square as you tighten the screws. Open and close the door to see if there is any binding. It will get worse over time, not better, so take your time and get it right.
Step 4: The threshold, weather stripping, and insulation go on next. Be sure to fill any gaps between the jambs with insulation, and carefully caulk up any cracks. It's easiest to weather strip and caulk before the trim goes on.
Step 5: Reinstall the interior and exterior trim and caulk any cracks. Then install the door locksets. Most of these doors come predrilled.
Tip: Some replacement doors require you to remove the old jamb. This is a major job, and you should hire a carpenter to hang this type of door unless you have had some experience working with exterior doors. The one advantage to this type of door is that you will get a door that is at least as wide as your old door.
