DEAR TIM: My wife wants to change out the front door on our existing home. But the door is flanked on either side with sidelights. I am puzzled how to attack this project as the frames of the door and the sidelights seem to be permanently attached to one another. Can I easily separate them? Changing out the entire door seems as if it would be a nightmare as the brick touches up against the exterior door trim. What are my options? Jim L., Hickory, NC

DEAR JIM: You have a keen eye for the obvious. The frames of the sidelights and the actual door unit are connected and if they are like most, it will be a tough job to separate them. Perhaps the most challenging part is at your feet and you might have overlooked it. The threshold of the door typically is continuous under both sidelights as well as the door. You can't imagine the difficulty of disconnecting the door frame from the threshold with little or no damage to the threshold.

The task of replacing the entire door unit including the sidelights is not as hard as you might think. I realize you may be intimidated by the complexity since the brickwork is touching the exterior trim of the door. This is fairly normal.

The biggest challenge in replacing this door would be making sure the woodwork surrounding the door is placed back exactly where it is now. The wallpaper will look perfect if this happens.

First, the door itself should float in the opening. The weight of the structure should never be transmitted to the door or the door frame. By creating an opening larger than the door, the weight of the structure above is transferred to the framing material on either side of the door.

Secondly, the framing of the house can move as the house shrinks and it can move over time with changes in the weather. If the door and its frame were to be a tight fit, this movement would cause the door to bind on a regular basis. Finally, the rough opening might not be perfectly square and plumb. The finish carpenter needs some wiggle room to adjust the door as it is installed so it operates perfectly.

The good news is that many new replacement exterior doors with sidelights are usually the same size or within a fraction of an inch of your existing one. The only exceptions might be door units that are 50 or more years old. Purchasing a replacement door that will fit can be done by discovering the actual unit dimension of your existing door and sidelights. The unit dimension is the actual width and height of the combined door and sidelight frames without any interior or exterior trim applied to them. Remove the interior trim that surrounds the door to begin this measuring process.

Once the trim is removed, you should be able to see the side edges of the door unit. The sidelight and door jamb frames are typically three quarters of an inch thick. The interior trim often covers one half inch of this material. The next thing you need to discover is the actual bottom of the threshold. Using a tape measure, you should be able to get very accurate measurements side to side of the frame and the measurement from the bottom of the threshold to the top of the door frame.

Take these measurements and a photo of the inside and outside of the door with you to an exterior door store. They will have catalogs that list the actual unit dimensions of replacement doors. I am very confident you will find a door unit that will easily fit in your opening.

The exterior trim of the new door may need a small amount of adjustment to make it look as good as new. If you have a gap, simply add some smaller trim moldings that compliment the larger molding. If the factory molding is too large, remove the necessary wood to make the trim fit tightly against the brick.

Some carpenters make huge mistakes when installing exterior doors. They sit them directly upon the concrete slab or wood subfloor. When the interior finished flooring is installed months later, there is rarely enough room for a throw rug. The door bottom is simply too close to the finish floor and pushes the throw rug out of the way.

It is always best to install a special flashing pan in the rough opening first and then place a 5/8 inch thick piece of redwood or CCA treated lumber on top of the flashing. The door threshold sits on this rot resistant wood and is now high enough so that most finished floor materials either slide under the threshold or can be notched to fit under the threshold. This gives you plenty of room for a throw rug in the entrance hall.