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Installing Prehung Door

So you want to install a new door or in some cases the “want” is actually a “need” to install a new door.  This could be an exterior or an interior door but usually you will have an existing opening made for a specific size door.   This could be a new wall in an unfinished basement or some existing door needs to be replaced.   Either way using a pre-hung door is the recommended approach.   You may want to see “buying the correct door”.

Remove existing door and jamb

If you are replacing an existing door then the current door with jamb will need to be removed.  The jamb is best described as the frame immediately around the door.  This demolition is straight forward, especially if you have the proper tools.  First remove the hinge pins on the door and set door itself aside, then remove the trim around the door with as little damage as possible to the trim.   The trim will be attached around the door with finishing nails.   Use a pry bar to remove the trim in small steps to avoid breaking it, and then remove all nails with pliers.

Now the existing jamb can be cut out with a reciprocating saw that will cut through the nail/screws holding the door in place.  Cut close to the existing stud but not into it.  If you don’t have a reciprocating saw, you will need to use a hacksaw or pry bar to remove the door.  If you plan on doing much demolition work in the future, a reciprocating saw is a must and can be used to also cut PVC, metal pipes and many other uses for other jobs.

Door Installation Preparation

Now with the opening available with the studs showing, the new pre-hung door can be installed.  Check each side, top and floor with a four foot level to understand how far out of plumb the rough opening is prior to installation. This will tell you how much of a problem the installation will be.  The primary thing to keep in mind is that the door and jambs must be installed plumb, this is why the rough opening is slightly larger than the pre-hung door.   Plumb is being exactly
perpendicular or vertical, at right angles.  Use a four foot level to check and re-check the jambs during installation.  Also check the floor to make sure it is level.  The pre-hung door will be square and the trick is to keep it that way after the door is installed.  Usually interior doors will be installed above the floor far enough to allow opening without scraping the floor and any carpet or area rugs.

Before the pre-hung door is placed into position, typically you will need to trim the ends off both jambs on each side and potentially also the door bottom.   This could be up to an inch, again depending upon the rough opening.  To determine the over jamb length, measure the rough opening height from floor to header.   The pre-hung door side jambs should be less than an inch shorter than this rough opening, a ½ inch or less is good depending upon how square the opening is.   The door jambs will usually rest on the wood, tile or sub floor, assuming the floor is level.  The door itself should be ½ inch above the flooring.  If not you may need to have one jamb slightly longer, keeping in mind the door MUST be plumb so as to not scrape the floor when open or closed.

If needed, this will also be a good time to stain, paint and finish the door.   I would recommend to temporarily remove the door from the jambs by removing the hinges and any shipping material.  Then reassemble when finished.

Pre-hung door installation

Now you are ready to install the pre-hung door.  Use tapered, wood or composite, shims and 10-penny finishing nails to attach the hinge side of the door first; again and again making sure the jamb is plumb on all sides. This hinge side of the door is the most important since it will carry door weight.  Place the shims at each hinge point and at each place where the jamb will be nailed to the stud, about every two feet and top and bottom of side jambs.  If using standard finishing nails and hammer, leave ¼ inch of the nail showing just in case you need to make some adjustment and have to remove the nail.  The shims will need to be cut off evenly with the jambs once the door is completely installed.   If you have an air powered finish nail gun, this also works well.   Once the hinge side is attached to the studs, attach the latch side jamb in a similar manner and again making sure the jamb is plumb on all sides.   After a couple nails have been driven into the shims and stud, evaluate if the door opens and closes properly.  This includes checking the 1/8 inch space between the door and jambs is maintained around the door and the door is completely even with the jamb at both top and bottom.

On the hinge side, it is recommended that several short hinge screws, especially at the top hinge, be replaced with longer screws that actually go through the jamb and into the stud.  Most home improvement stores that carry hinge hardware will have these longer screws specifically designed to match the hinges.  Counter sink the nails with a hammer and small punch, then fill with matching wood putty.  Once the pre-hung door is solidly attached to the rough opening and checked again to be plumb, the trim can then be re-installed.

The last step is to install the knob hardware and take satisfaction in what you have accomplished.

Related Information:

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