How to Install Exterior Doors With Existing Frames

1. Measure Door Opening

   You will need these precise measurements for ordering the door. Take these steps:

   • With the prybar, remove the door's inner casing trim.
   • Use the tape measure to measure the height of the door's rough opening.
   • Measure both the left and the right sides.
   • Measure the width at the top, middle, and bottom.

2. Verify Door Opening Square

   • Run the tape measure diagonally from the top left down to the bottom right.
   • Similarly, run it in the other direction (top right to bottom left).
   • If both measurements are the same, the door opening is square.
   • With the bubble level, make sure that the vertical sides are plumb.
   • Use the level to make sure the header (top) is level.
   • Use the level once more to make sure the threshold (floor) is level.

   Tip

   Out-of-square tolerance for most exterior doors is 1/8-inch. For doors that are out of square beyond 1/8-inch or so, have a contractor rebuild the home's door frame.

3. Purchase Door

   • Most rough openings need to be 1/2-inch taller and 3/4-inch wider than the exterior door unit.
   • Purchase the door before removing the existing door.
   • All pre-hung doors come with specific handing: either left- or right-handed. The door swing direction cannot be reversed, so be sure to buy a door with the correct handing.

   Tip

   To find the door's handing, stand facing the door on the outside of the house.

   • If the hinges are on the left side of the door frame, this is a left-handed door.
   • If the hinges are on the right side, buy a right-handed door.

4. Prepare Door Opening

   • With the pry bar, remove exterior trim and any insulation tucked into the spaces around the door.
   • With a cordless drill, remove the existing door from the hinges.
   • Remove the hinges from the door jamb.
   • Unscrew the existing door sill.
   • Use the reciprocating saw to cut any nails that might be holding the existing door frame to the studs on each side.

5. Dry-Fit Door in Opening

   • Place the door unit in the opening and center it.
   • By measuring, make sure that the door, when swung open, will clear the top of the finished floor inside the house.
   • Test the door sill for level.

   Warning

   Do not open the door, as it may tip out of balance and fall. Keep the restraining shipment brackets in place for now.

6. Trace Around Door Trim

   • The new door's exterior trim, which typically comes pre-attached to the door unit, may fit perfectly into place where the previous molding had been.
   • If not, use a pencil to trace on the siding around the molding.

7. Cut Siding (Optional)

   • If necessary, use an oscillating multi-tool to cut the pencil outline that you made on the siding.
   • Remove the waste material.

8. Create Sill Pan

   • With your assistant, remove the door unit and set it aside safely.
   • Piece together the three sections of sill pan in the door frame.
   • Two pieces should be at each side.
   • The third piece should be in the middle.

9. Add Sill Pan

   • Caulk the three pieces of sill pan together.
   • Let the caulk harden.
   • Set the assembled sill pan aside. 
   • Add three parallel beads of caulk to the floor.
   • Set the sill pan in it.
   • Caulk the back and the seams of the sill pan.

10. Set Door in Place

    • Add caulk to the back of the door's pre-attached outside trim.
    • With your assistant, set the door unit back in place.

11. Add Shims

    • On the inside of the house, add a couple of shims at the top of the door frame to hold the door in place and plumb.
    • Use the bubble level to verify plumb.
    • Gently tap the shims into place as far as is needed to be able to tilt the door from left to right to bring it into plumb.
    • Place shims at each hinge, about 1 inch above.
    • The door lock side of the unit should have a gap of about 1/8-inch.
    • Make sure this gap is consistent from top to bottom.
    • Double up two shims from front to back to create a flat spacer.
    • Place this spacer in the gap on the lock side of the door unit.

12. Add Screws

    • After first making pilot holes, drive and countersink exterior screws at each of the three hinge locations through the door casing.
    • On the door lock side, add screws above and below the strike plate and at the top and bottom.
    • Screw the strike plate into place.

13. Test Door

    • Remove the door's restraining shipment brackets and test the opening and closing of the door.
    • Along the bottom, the door should fit the sill plate tightly but should have enough give that the door opens easily.
    • If necessary, adjust the sill plate's adjustable cap up or down with a Phillips screwdriver.

    Tip

    Place a sheet of paper on top of the sill plate, then close the door. You should be able to pull the paper out without tearing it, but the paper should not move freely on its own.

14. Cut off Shims

    • Remove the exposed sections of shims by scoring them with a utility knife and then snapping them off.
    • Alternatively, remove shims by cutting them with an oscillating multi-tool.

15. Add Foam Insulation

    • Apply minimally expanding foam insulation in the gap between the door unit and the door frame.
    • After the foam has expanded and dried, slice away any excess foam with a putty knife.

16. Add Interior Trim

    • Cut three pieces of interior trim on the electric miter saw.
    • Place the three pieces on the house interior, securing the pieces with finish nails or with the cordless nailer.

17. Add Doorknob and Deadbolt

    • Add the doorknob to the door.
    • If the door is bored for a deadbolt, add this as well.

18. Add Caulk

    On the house exterior, add caulk around the trim.

Tips for Installing Exterior Doors

• Start the project as early as possible in the morning to give yourself as much time as possible to work on this one-day project.
• To avoid running out to the hardware store and slowing down the process, make sure that you have all tools and materials on hand before you start.
• Identify an alternative way to get in and out of the house when dry fitting and installing the exterior door, since you will not be able to pass through that door for periods of time.

When to Call a Professional

If the door frame is unusually sized or requires significant repair work, call a door installation company or a contractor. 

Along with precision, speed is important. If you cannot remove the existing door and install the new door in one day, call a professional. French doors, double doors, sliders, and entry doors with sidelights are large, heavy, and have unique installation requirements. These also should be installed by professionals.