Installing Interior Prehung Doors

You will notice that the side door jambs come a few inches longer than necessary. This is to accommodate door openings that are not level across the width of the opening. You can use your 2’ level to determine if the door opening is not level.

If it is not, raise one end of the level up until the bubble inside the level is centered in the view glass. The distance from the bottom of the level to the floor is the amount of additional side door jamb material that must be cut away compared to the opposite side of the prehung door.

Once you have trimmed the bottom edges of the side door jambs you can separate the prehung door into two halves. One half includes the door itself along with casing trim. Do not disengage the prehung door from this section of casing trim. The other half of the interior prehung door is simply a door casing trim frame. Position the prehung door and casing section into the door opening. Make sure you center the door in the opening. Examine the prehung door trim to see how it sits along the drywall surface.

Also make sure to check the top of the door. If you need to raise the door up a little, place a shim underneath the hinge side door jamb.

Next, use your 4 foot level to see if the prehung door is plumb within the door frame. You probably will need to add some shims or small blocks of wood to square the interior prehung door in the opening. However, focus your efforts on the hinge side of the door first.

Make sure to install spacers or shims near each of the hinge areas of the interior prehung door.

Once you have confirmed that the prehung door is sitting plumb inside the door frame tack 8 penny finished nails into the hinge side door jamb using your hammer. Position the nails near each of the hinges.

Repeat the same process on the latch side door jamb, adding shims or spacers where necessary. I always like to include shims or spacers near the latch assembly to create a very rigid surface for the door latch. Once you have tacked up the latch side door jamb, again, use your level to make sure the interior prehung door is perfectly square and plumb. Make sure you also tack in 3 additional finish nails along the top of the door frame. Now install the door knob and latch assembly into the door knob bore hole, and attach the strike plate to the door frame.

Check to make sure the door opens and closes properly.

If the door opens and closes smoothly sink the finish nails using a nail set and hammer.

Next slide the other half of the door molding trim into the sleeve area of the already installed prehung door. Again, use 8 penny finish nails to fasten this half of the door to the perimeter of the door frame.

Finally, install finish nails every 2 feet or so along the outside perimeter edge of the interior prehung door case molding. Make sure you use a nail set to countersink all of the finish nails.

And with all the finish nails countersunk into the door trim, you have completed your first interior prehung door installation project.