DEAR TIM: I need to know how to install a prehung door. This door is a regular interior door, not an exterior door. Is this job as easy as the salesman made it out to be? I am suspicious, as it seems that you need some good skills to install a prehung door. What can you share so that I might have a fighting chance as I wrestle with this door? Bill F., Greeneville, NC

DEAR BILL: Installing a prehung door is not a job I would entrust to a rookie carpenter. The job is fraught with all sorts of little challenges that are solved with tricks that come with lots of experience. To end up with a door that fits perfectly without rubbing, you need to know some of these tricks. In other words, the salesman either thought you were a seasoned carpenter or he needed to make a sale. No worries though, as I will try to give you as many tips as possible to help you survive this job.

Tapered wood shims are needed when you are installing a door. PHOTO CREDIT: Brent Walter

I am going to assume you purchased the right-sized prehung door and that it is handed correctly. It is a very common mistake to purchase a prehung door that swings the wrong way. To determine proper handing, always imagine looking at the door and be on the side where the door swings towards you. If the door knob is on the left side of the door, it is a left-handed door. If the knob is on the right, it would be a right-handed door.

Prehung doors often come from the factory with the jambs a little long. This is done on purpose so you can install the door in places where there will be carpet and not have to cut off the bottom of the new door. The goal is to have about one-half-inch of airspace between the bottom of the door and the top of the carpet or any finished flooring material. The door jambs don't have to touch the subfloor in carpeted situations, but they must be in contact with hardwood flooring, ceramic tile, cork or other finished floors that are typically smooth.

Before the door is installed, make sure the top and bottom of the door is painted. You can just lay the door flat on the ground and open it to check to see if this was done at the factory. It is important to have the top and bottom of doors sealed to prevent warping.

Prehung doors typically come from the factory with a one-eighth-inch gap between the edges of the door and the door jamb. The doors are almost always perfectly square, so the challenge is to square up the jamb within the rough opening so the gap between the door and the jamb stays consistent. The biggest challenge is cutting the lengths of the jambs correctly so the gap at the top of the door is correct. If one leg of the jamb is too long, you will end up with the door rubbing against the top jamb or a horribly large gap between the door and the jamb just above the door knob.

It is really important for the door jambs to be installed plumb. Use a four-foot level to see how plumb the rough opening is that is common to the hinge-side of the door. Use wood shims with the level and tack the shims against the jamb until the level is perfectly plumb. It is best to install shims at the same height as the door hinges.

Position the prehung door into position, slide it up against the shims, and see how things looks. You know you are in great shape if the gap along the hinge side of the door and the top of the door are the consistent one-eighth inch. Don't worry for now about the gap along side the door-knob side of the door. This is the least important jamb.

Use 10-penny finish nails to attach the hinge jamb of the door to the rough jamb. Carefully nail through the jamb and into the shims. Do not drive the nails completely into place. Leave at least one-half inch of the nails exposed in case you need to make adjustments to the length of the door jambs or you have to move the jambs slightly in and out so the door fits perfectly. Once all adjustments are made, all shims are in place and the door fits well, then you can drive the nails home recessing the nail heads with a nail-setting tool.

To really add a professional touch to your installation once complete, you will want to temporarily remove the top hinge from just the door jamb. Doors are heavy and over time the weight of the door can actually pull the jamb away from the finish nails. This will cause the door to rub and not fit well.

If you take a three-inch-long screw and drive it through the jamb into the rough jamb at this upper hinge, the door should stay put for many years. It is important to predrill the hole so you do not split the jamb and create a countersink hole so the head of the screw does not touch the back side of the hinge once the hinge is reinstalled.

Some carpenters just remove one of the regular short hinge screws and replace it with a longer one that will penetrate into the rough jamb. This method will also work.

When shimming the jambs, place the shims at about two-foot intervals and always nail through the jamb at a location where you have installed shims.